docs: Add supported versions and fix-backport policy

[lttng-tools.git] / src / bin / lttng-sessiond / notification-thread-events.cpp

/*
 * Copyright (C) 2017 Jérémie Galarneau <jeremie.galarneau@efficios.com>
 *
 * SPDX-License-Identifier: GPL-2.0-only
 *
 */

#include "lttng/action/action.h"
#include "lttng/trigger/trigger-internal.hpp"
#define _LGPL_SOURCE
#include "condition-internal.hpp"
#include "event-notifier-error-accounting.hpp"
#include "kernel.hpp"
#include "lttng-sessiond.hpp"
#include "notification-thread-commands.hpp"
#include "notification-thread-events.hpp"
#include "notification-thread.hpp"

#include <common/defaults.hpp>
#include <common/dynamic-buffer.hpp>
#include <common/error.hpp>
#include <common/futex.hpp>
#include <common/hashtable/utils.hpp>
#include <common/macros.hpp>
#include <common/sessiond-comm/sessiond-comm.hpp>
#include <common/unix.hpp>
#include <common/urcu.hpp>

#include <lttng/action/action-internal.hpp>
#include <lttng/action/list-internal.hpp>
#include <lttng/condition/buffer-usage-internal.hpp>
#include <lttng/condition/condition-internal.hpp>
#include <lttng/condition/condition.h>
#include <lttng/condition/event-rule-matches-internal.hpp>
#include <lttng/condition/session-consumed-size-internal.hpp>
#include <lttng/condition/session-rotation-internal.hpp>
#include <lttng/domain-internal.hpp>
#include <lttng/event-rule/event-rule-internal.hpp>
#include <lttng/location-internal.hpp>
#include <lttng/notification/channel-internal.hpp>
#include <lttng/notification/notification-internal.hpp>
#include <lttng/trigger/trigger-internal.hpp>

#include <fcntl.h>
#include <inttypes.h>
#include <time.h>
#include <unistd.h>
#include <urcu.h>
#include <urcu/rculfhash.h>

#define CLIENT_POLL_EVENTS_IN     (LPOLLIN | LPOLLRDHUP)
#define CLIENT_POLL_EVENTS_IN_OUT (CLIENT_POLL_EVENTS_IN | LPOLLOUT)

/* The tracers currently limit the capture size to PIPE_BUF (4kb on linux). */
#define MAX_CAPTURE_SIZE (PIPE_BUF)

enum lttng_object_type {
        LTTNG_OBJECT_TYPE_UNKNOWN,
        LTTNG_OBJECT_TYPE_NONE,
        LTTNG_OBJECT_TYPE_CHANNEL,
        LTTNG_OBJECT_TYPE_SESSION,
};

struct lttng_channel_trigger_list {
        struct channel_key channel_key;
        /* List of struct lttng_trigger_list_element. */
        struct cds_list_head list;
        /* Node in the channel_triggers_ht */
        struct cds_lfht_node channel_triggers_ht_node;
        /* call_rcu delayed reclaim. */
        struct rcu_head rcu_node;
};

/*
 * List of triggers applying to a given session.
 *
 * See:
 *   - lttng_session_trigger_list_create()
 *   - lttng_session_trigger_list_build()
 *   - lttng_session_trigger_list_destroy()
 *   - lttng_session_trigger_list_add()
 */
struct lttng_session_trigger_list {
        char *session_name;
        /* List of struct lttng_trigger_list_element. */
        struct cds_list_head list;
        /* Node in the session_triggers_ht */
        struct cds_lfht_node session_triggers_ht_node;
        /*
         * Weak reference to the notification system's session triggers
         * hashtable.
         *
         * The session trigger list structure structure is owned by
         * the session's session_info.
         *
         * The session_info is kept alive the the channel_infos holding a
         * reference to it (reference counting). When those channels are
         * destroyed (at runtime or on teardown), the reference they hold
         * to the session_info are released. On destruction of session_info,
         * session_info_destroy() will remove the list of triggers applying
         * to this session from the notification system's state.
         *
         * This implies that the session_triggers_ht must be destroyed
         * after the channels.
         */
        struct cds_lfht *session_triggers_ht;
        /* Used for delayed RCU reclaim. */
        struct rcu_head rcu_node;
};

namespace {
struct lttng_trigger_list_element {
        /* No ownership of the trigger object is assumed. */
        struct lttng_trigger *trigger;
        struct cds_list_head node;
};

struct lttng_trigger_ht_element {
        struct lttng_trigger *trigger;
        struct cds_lfht_node node;
        struct cds_lfht_node node_by_name_uid;
        struct cds_list_head client_list_trigger_node;
        /* call_rcu delayed reclaim. */
        struct rcu_head rcu_node;
};

struct lttng_condition_list_element {
        struct lttng_condition *condition;
        struct cds_list_head node;
};

struct channel_state_sample {
        struct channel_key key;
        struct cds_lfht_node channel_state_ht_node;
        uint64_t highest_usage;
        uint64_t lowest_usage;
        /* call_rcu delayed reclaim. */
        struct rcu_head rcu_node;
};
} /* namespace */

static unsigned long hash_channel_key(struct channel_key *key);
static int evaluate_buffer_condition(const struct lttng_condition *condition,
                                     struct lttng_evaluation **evaluation,
                                     const struct notification_thread_state *state,
                                     const struct channel_state_sample *previous_sample,
                                     const struct channel_state_sample *latest_sample,
                                     struct channel_info *channel_info);
static int send_evaluation_to_clients(const struct lttng_trigger *trigger,
                                      const struct lttng_evaluation *evaluation,
                                      struct notification_client_list *client_list,
                                      struct notification_thread_state *state,
                                      uid_t channel_uid,
                                      gid_t channel_gid);

/* session_info API */
static void session_info_destroy(void *_data);
static void session_info_get(struct session_info *session_info);
static void session_info_put(struct session_info *session_info);
static struct session_info *session_info_create(uint64_t id,
                                                const char *name,
                                                uid_t uid,
                                                gid_t gid,
                                                struct lttng_session_trigger_list *trigger_list,
                                                struct cds_lfht *sessions_ht);
static void session_info_add_channel(struct session_info *session_info,
                                     struct channel_info *channel_info);
static void session_info_remove_channel(struct session_info *session_info,
                                        struct channel_info *channel_info);

/* lttng_session_trigger_list API */
static struct lttng_session_trigger_list *
lttng_session_trigger_list_create(const char *session_name, struct cds_lfht *session_triggers_ht);
static struct lttng_session_trigger_list *
lttng_session_trigger_list_build(const struct notification_thread_state *state,
                                 const char *session_name);
static void lttng_session_trigger_list_destroy(struct lttng_session_trigger_list *list);
static int lttng_session_trigger_list_add(struct lttng_session_trigger_list *list,
                                          struct lttng_trigger *trigger);

static int client_handle_transmission_status(struct notification_client *client,
                                             enum client_transmission_status transmission_status,
                                             struct notification_thread_state *state);

static int handle_one_event_notifier_notification(struct notification_thread_state *state,
                                                  int pipe,
                                                  enum lttng_domain_type domain);

static void free_lttng_trigger_ht_element_rcu(struct rcu_head *node);

static int match_client_socket(struct cds_lfht_node *node, const void *key)
{
        /* This double-cast is intended to supress pointer-to-cast warning. */
        const int socket = (int) (intptr_t) key;
        const struct notification_client *client =
                caa_container_of(node, struct notification_client, client_socket_ht_node);

        return client->socket == socket;
}

static int match_client_id(struct cds_lfht_node *node, const void *key)
{
        /* This double-cast is intended to supress pointer-to-cast warning. */
        const notification_client_id id = *((notification_client_id *) key);
        const struct notification_client *client =
                lttng::utils::container_of(node, &notification_client::client_id_ht_node);

        return client->id == id;
}

static int match_channel_trigger_list(struct cds_lfht_node *node, const void *key)
{
        struct channel_key *channel_key = (struct channel_key *) key;
        struct lttng_channel_trigger_list *trigger_list;

        trigger_list =
                caa_container_of(node, struct lttng_channel_trigger_list, channel_triggers_ht_node);

        return !!((channel_key->key == trigger_list->channel_key.key) &&
                  (channel_key->domain == trigger_list->channel_key.domain));
}

static int match_session_trigger_list(struct cds_lfht_node *node, const void *key)
{
        const char *session_name = (const char *) key;
        struct lttng_session_trigger_list *trigger_list;

        trigger_list =
                caa_container_of(node, struct lttng_session_trigger_list, session_triggers_ht_node);

        return !!(strcmp(trigger_list->session_name, session_name) == 0);
}

static int match_channel_state_sample(struct cds_lfht_node *node, const void *key)
{
        struct channel_key *channel_key = (struct channel_key *) key;
        struct channel_state_sample *sample;

        sample = caa_container_of(node, struct channel_state_sample, channel_state_ht_node);

        return !!((channel_key->key == sample->key.key) &&
                  (channel_key->domain == sample->key.domain));
}

static int match_channel_info(struct cds_lfht_node *node, const void *key)
{
        struct channel_key *channel_key = (struct channel_key *) key;
        struct channel_info *channel_info;

        channel_info = caa_container_of(node, struct channel_info, channels_ht_node);

        return !!((channel_key->key == channel_info->key.key) &&
                  (channel_key->domain == channel_info->key.domain));
}

static int match_trigger(struct cds_lfht_node *node, const void *key)
{
        struct lttng_trigger *trigger_key = (struct lttng_trigger *) key;
        struct lttng_trigger_ht_element *trigger_ht_element;

        trigger_ht_element = caa_container_of(node, struct lttng_trigger_ht_element, node);

        return !!lttng_trigger_is_equal(trigger_key, trigger_ht_element->trigger);
}

static int match_trigger_token(struct cds_lfht_node *node, const void *key)
{
        const uint64_t *_key = (uint64_t *) key;
        struct notification_trigger_tokens_ht_element *element;

        element = caa_container_of(node, struct notification_trigger_tokens_ht_element, node);
        return *_key == element->token;
}

static int match_client_list_condition(struct cds_lfht_node *node, const void *key)
{
        struct lttng_condition *condition_key = (struct lttng_condition *) key;
        struct notification_client_list *client_list;
        const struct lttng_condition *condition;

        LTTNG_ASSERT(condition_key);

        client_list = caa_container_of(
                node, struct notification_client_list, notification_trigger_clients_ht_node);
        condition = client_list->condition;

        return !!lttng_condition_is_equal(condition_key, condition);
}

static int match_session_info(struct cds_lfht_node *node, const void *key)
{
        const auto session_id = *((uint64_t *) key);
        const auto *session_info =
                lttng::utils::container_of(node, &session_info::sessions_ht_node);

        return session_id == session_info->id;
}

static unsigned long hash_session_info_id(uint64_t id)
{
        return hash_key_u64(&id, lttng_ht_seed);
}

static unsigned long hash_session_info(const struct session_info *session_info)
{
        return hash_session_info_id(session_info->id);
}

static struct session_info *get_session_info_by_id(const struct notification_thread_state *state,
                                                   uint64_t id)
{
        struct cds_lfht_iter iter;
        struct cds_lfht_node *node;
        lttng::urcu::read_lock_guard read_lock_guard;

        cds_lfht_lookup(
                state->sessions_ht, hash_session_info_id(id), match_session_info, &id, &iter);
        node = cds_lfht_iter_get_node(&iter);

        if (node) {
                auto session_info =
                        lttng::utils::container_of(node, &session_info::sessions_ht_node);

                session_info_get(session_info);
                return session_info;
        }

        return nullptr;
}

static struct session_info *get_session_info_by_name(const struct notification_thread_state *state,
                                                     const char *name)
{
        uint64_t session_id;
        const auto found = sample_session_id_by_name(name, &session_id);

        return found ? get_session_info_by_id(state, session_id) : nullptr;
}

static const char *notification_command_type_str(enum notification_thread_command_type type)
{
        switch (type) {
        case NOTIFICATION_COMMAND_TYPE_REGISTER_TRIGGER:
                return "REGISTER_TRIGGER";
        case NOTIFICATION_COMMAND_TYPE_UNREGISTER_TRIGGER:
                return "UNREGISTER_TRIGGER";
        case NOTIFICATION_COMMAND_TYPE_ADD_CHANNEL:
                return "ADD_CHANNEL";
        case NOTIFICATION_COMMAND_TYPE_REMOVE_CHANNEL:
                return "REMOVE_CHANNEL";
        case NOTIFICATION_COMMAND_TYPE_ADD_SESSION:
                return "ADD_SESSION";
        case NOTIFICATION_COMMAND_TYPE_REMOVE_SESSION:
                return "REMOVE_SESSION";
        case NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_ONGOING:
                return "SESSION_ROTATION_ONGOING";
        case NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_COMPLETED:
                return "SESSION_ROTATION_COMPLETED";
        case NOTIFICATION_COMMAND_TYPE_ADD_TRACER_EVENT_SOURCE:
                return "ADD_TRACER_EVENT_SOURCE";
        case NOTIFICATION_COMMAND_TYPE_REMOVE_TRACER_EVENT_SOURCE:
                return "REMOVE_TRACER_EVENT_SOURCE";
        case NOTIFICATION_COMMAND_TYPE_LIST_TRIGGERS:
                return "LIST_TRIGGERS";
        case NOTIFICATION_COMMAND_TYPE_GET_TRIGGER:
                return "GET_TRIGGER";
        case NOTIFICATION_COMMAND_TYPE_QUIT:
                return "QUIT";
        case NOTIFICATION_COMMAND_TYPE_CLIENT_COMMUNICATION_UPDATE:
                return "CLIENT_COMMUNICATION_UPDATE";
        default:
                abort();
        }
}

/*
 * Match trigger based on name and credentials only.
 * Name duplication is NOT allowed for the same uid.
 */
static int match_trigger_by_name_uid(struct cds_lfht_node *node, const void *key)
{
        bool match = false;
        const char *element_trigger_name;
        const char *key_name;
        enum lttng_trigger_status status;
        const struct lttng_credentials *key_creds;
        const struct lttng_credentials *node_creds;
        const struct lttng_trigger *trigger_key = (const struct lttng_trigger *) key;
        const struct lttng_trigger_ht_element *trigger_ht_element =
                caa_container_of(node, struct lttng_trigger_ht_element, node_by_name_uid);

        status = lttng_trigger_get_name(trigger_ht_element->trigger, &element_trigger_name);
        element_trigger_name = status == LTTNG_TRIGGER_STATUS_OK ? element_trigger_name : nullptr;

        status = lttng_trigger_get_name(trigger_key, &key_name);
        key_name = status == LTTNG_TRIGGER_STATUS_OK ? key_name : nullptr;

        /*
         * Compare the names.
         * Consider null names as not equal. This is to maintain backwards
         * compatibility with pre-2.13 anonymous triggers. Multiples anonymous
         * triggers are allowed for a given user.
         */
        if (!element_trigger_name || !key_name) {
                goto end;
        }

        if (strcmp(element_trigger_name, key_name) != 0) {
                goto end;
        }

        /* Compare the owners' UIDs. */
        key_creds = lttng_trigger_get_credentials(trigger_key);
        node_creds = lttng_trigger_get_credentials(trigger_ht_element->trigger);

        match = lttng_credentials_is_equal_uid(key_creds, node_creds);

end:
        return match;
}

/*
 * Hash trigger based on name and credentials only.
 */
static unsigned long hash_trigger_by_name_uid(const struct lttng_trigger *trigger)
{
        unsigned long hash = 0;
        const struct lttng_credentials *trigger_creds;
        const char *trigger_name;
        enum lttng_trigger_status status;

        status = lttng_trigger_get_name(trigger, &trigger_name);
        if (status == LTTNG_TRIGGER_STATUS_OK) {
                hash = hash_key_str(trigger_name, lttng_ht_seed);
        }

        trigger_creds = lttng_trigger_get_credentials(trigger);
        hash ^= hash_key_ulong((void *) (unsigned long) LTTNG_OPTIONAL_GET(trigger_creds->uid),
                               lttng_ht_seed);

        return hash;
}

static unsigned long hash_channel_key(struct channel_key *key)
{
        unsigned long key_hash = hash_key_u64(&key->key, lttng_ht_seed);
        unsigned long domain_hash =
                hash_key_ulong((void *) (unsigned long) key->domain, lttng_ht_seed);

        return key_hash ^ domain_hash;
}

static unsigned long hash_client_socket(int socket)
{
        return hash_key_ulong((void *) (unsigned long) socket, lttng_ht_seed);
}

static unsigned long hash_client_id(notification_client_id id)
{
        return hash_key_u64(&id, lttng_ht_seed);
}

/*
 * Get the type of object to which a given condition applies. Bindings let
 * the notification system evaluate a trigger's condition when a given
 * object's state is updated.
 *
 * For instance, a condition bound to a channel will be evaluated everytime
 * the channel's state is changed by a channel monitoring sample.
 */
static enum lttng_object_type get_condition_binding_object(const struct lttng_condition *condition)
{
        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
                return LTTNG_OBJECT_TYPE_CHANNEL;
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING:
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED:
        case LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE:
                return LTTNG_OBJECT_TYPE_SESSION;
        case LTTNG_CONDITION_TYPE_EVENT_RULE_MATCHES:
                return LTTNG_OBJECT_TYPE_NONE;
        default:
                return LTTNG_OBJECT_TYPE_UNKNOWN;
        }
}

static void free_channel_info_rcu(struct rcu_head *node)
{
        free(lttng::utils::container_of(node, &channel_info::rcu_node));
}

static void channel_info_destroy(struct channel_info *channel_info)
{
        if (!channel_info) {
                return;
        }

        if (channel_info->session_info) {
                session_info_remove_channel(channel_info->session_info, channel_info);
                session_info_put(channel_info->session_info);
        }
        if (channel_info->name) {
                free(channel_info->name);
        }
        call_rcu(&channel_info->rcu_node, free_channel_info_rcu);
}

static void free_session_info_rcu(struct rcu_head *node)
{
        free(lttng::utils::container_of(node, &session_info::rcu_node));
}

/* Don't call directly, use the ref-counting mechanism. */
static void session_info_destroy(void *_data)
{
        struct session_info *session_info = (struct session_info *) _data;
        int ret;

        LTTNG_ASSERT(session_info);
        if (session_info->channel_infos_ht) {
                ret = cds_lfht_destroy(session_info->channel_infos_ht, nullptr);
                if (ret) {
                        ERR("Failed to destroy channel information hash table");
                }
        }
        lttng_session_trigger_list_destroy(session_info->trigger_list);

        lttng::urcu::read_lock_guard read_lock;
        cds_lfht_del(session_info->sessions_ht, &session_info->sessions_ht_node);
        free(session_info->name);
        lttng_trace_archive_location_put(session_info->last_state_sample.rotation.location);
        call_rcu(&session_info->rcu_node, free_session_info_rcu);
}

static void session_info_get(struct session_info *session_info)
{
        if (!session_info) {
                return;
        }
        lttng_ref_get(&session_info->ref);
}

static void session_info_put(struct session_info *session_info)
{
        if (!session_info) {
                return;
        }
        lttng_ref_put(&session_info->ref);
}

static struct session_info *session_info_create(uint64_t id,
                                                const char *name,
                                                uid_t uid,
                                                gid_t gid,
                                                struct lttng_session_trigger_list *trigger_list,
                                                struct cds_lfht *sessions_ht)
{
        struct session_info *session_info;

        LTTNG_ASSERT(name);

        session_info = zmalloc<struct session_info>();
        if (!session_info) {
                goto end;
        }

        lttng_ref_init(&session_info->ref, session_info_destroy);

        session_info->channel_infos_ht = cds_lfht_new(
                DEFAULT_HT_SIZE, 1, 0, CDS_LFHT_AUTO_RESIZE | CDS_LFHT_ACCOUNTING, nullptr);
        if (!session_info->channel_infos_ht) {
                goto error;
        }

        cds_lfht_node_init(&session_info->sessions_ht_node);
        session_info->id = id;
        session_info->name = strdup(name);
        if (!session_info->name) {
                goto error;
        }

        session_info->uid = uid;
        session_info->gid = gid;
        session_info->trigger_list = trigger_list;
        session_info->sessions_ht = sessions_ht;
end:
        return session_info;
error:
        session_info_put(session_info);
        return nullptr;
}

static void session_info_add_channel(struct session_info *session_info,
                                     struct channel_info *channel_info)
{
        lttng::urcu::read_lock_guard read_lock;
        cds_lfht_add(session_info->channel_infos_ht,
                     hash_channel_key(&channel_info->key),
                     &channel_info->session_info_channels_ht_node);
}

static void session_info_remove_channel(struct session_info *session_info,
                                        struct channel_info *channel_info)
{
        lttng::urcu::read_lock_guard read_lock;
        cds_lfht_del(session_info->channel_infos_ht, &channel_info->session_info_channels_ht_node);
}

static struct channel_info *channel_info_create(const char *channel_name,
                                                struct channel_key *channel_key,
                                                uint64_t channel_capacity,
                                                struct session_info *session_info)
{
        struct channel_info *channel_info = zmalloc<struct channel_info>();

        if (!channel_info) {
                goto end;
        }

        cds_lfht_node_init(&channel_info->channels_ht_node);
        cds_lfht_node_init(&channel_info->session_info_channels_ht_node);
        memcpy(&channel_info->key, channel_key, sizeof(*channel_key));
        channel_info->capacity = channel_capacity;

        channel_info->name = strdup(channel_name);
        if (!channel_info->name) {
                goto error;
        }

        /*
         * Set the references between session and channel infos:
         *   - channel_info holds a strong reference to session_info
         *   - session_info holds a weak reference to channel_info
         */
        session_info_get(session_info);
        session_info_add_channel(session_info, channel_info);
        channel_info->session_info = session_info;
end:
        return channel_info;
error:
        channel_info_destroy(channel_info);
        return nullptr;
}

bool notification_client_list_get(struct notification_client_list *list)
{
        return urcu_ref_get_unless_zero(&list->ref);
}

static void free_notification_client_list_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct notification_client_list, rcu_node));
}

static void notification_client_list_release(struct urcu_ref *list_ref)
{
        struct notification_client_list *list =
                lttng::utils::container_of(list_ref, &notification_client_list::ref);
        struct notification_client_list_element *client_list_element, *tmp;

        lttng_condition_put(list->condition);

        if (list->notification_trigger_clients_ht) {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_del(list->notification_trigger_clients_ht,
                             &list->notification_trigger_clients_ht_node);
                list->notification_trigger_clients_ht = nullptr;
        }
        cds_list_for_each_entry_safe (client_list_element, tmp, &list->clients_list, node) {
                free(client_list_element);
        }

        LTTNG_ASSERT(cds_list_empty(&list->triggers_list));

        pthread_mutex_destroy(&list->lock);
        call_rcu(&list->rcu_node, free_notification_client_list_rcu);
}

static bool condition_applies_to_client(const struct lttng_condition *condition,
                                        struct notification_client *client)
{
        bool applies = false;
        struct lttng_condition_list_element *condition_list_element;

        cds_list_for_each_entry (condition_list_element, &client->condition_list, node) {
                applies = lttng_condition_is_equal(condition_list_element->condition, condition);
                if (applies) {
                        break;
                }
        }

        return applies;
}

static struct notification_client_list *
notification_client_list_create(struct notification_thread_state *state,
                                const struct lttng_condition *condition)
{
        struct notification_client *client;
        struct cds_lfht_iter iter;
        struct notification_client_list *client_list;

        client_list = zmalloc<notification_client_list>();
        if (!client_list) {
                PERROR("Failed to allocate notification client list");
                goto end;
        }

        pthread_mutex_init(&client_list->lock, nullptr);
        /*
         * The trigger that owns the condition has the first reference to this
         * client list.
         */
        urcu_ref_init(&client_list->ref);
        cds_lfht_node_init(&client_list->notification_trigger_clients_ht_node);
        CDS_INIT_LIST_HEAD(&client_list->clients_list);
        CDS_INIT_LIST_HEAD(&client_list->triggers_list);

        /*
         * Create a copy of the condition so that it's independent of any
         * trigger. The client list may outlive the trigger object (which owns
         * the condition) that is used to create it.
         */
        client_list->condition = lttng_condition_copy(condition);

        {
                /* Build a list of clients to which this new condition applies. */
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (
                        state->client_socket_ht, &iter, client, client_socket_ht_node) {
                        struct notification_client_list_element *client_list_element;

                        if (!condition_applies_to_client(condition, client)) {
                                continue;
                        }

                        client_list_element = zmalloc<notification_client_list_element>();
                        if (!client_list_element) {
                                goto error_put_client_list;
                        }

                        CDS_INIT_LIST_HEAD(&client_list_element->node);
                        client_list_element->client = client;
                        cds_list_add(&client_list_element->node, &client_list->clients_list);
                }
        }

        client_list->notification_trigger_clients_ht = state->notification_trigger_clients_ht;

        /*
         * Add the client list to the global list of client list.
         */
        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_add_unique(state->notification_trigger_clients_ht,
                                    lttng_condition_hash(client_list->condition),
                                    match_client_list_condition,
                                    client_list->condition,
                                    &client_list->notification_trigger_clients_ht_node);
        }
        goto end;

error_put_client_list:
        notification_client_list_put(client_list);
        client_list = nullptr;

end:
        return client_list;
}

void notification_client_list_put(struct notification_client_list *list)
{
        if (!list) {
                return;
        }
        return urcu_ref_put(&list->ref, notification_client_list_release);
}

/* Provides a reference to the returned list. */
static struct notification_client_list *
get_client_list_from_condition(struct notification_thread_state *state,
                               const struct lttng_condition *condition)
{
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;
        struct notification_client_list *list = nullptr;

        lttng::urcu::read_lock_guard read_lock;
        cds_lfht_lookup(state->notification_trigger_clients_ht,
                        lttng_condition_hash(condition),
                        match_client_list_condition,
                        condition,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (node) {
                list = lttng::utils::container_of(
                        node, &notification_client_list::notification_trigger_clients_ht_node);
                list = notification_client_list_get(list) ? list : nullptr;
        }

        return list;
}

static int evaluate_channel_condition_for_client(const struct lttng_condition *condition,
                                                 struct notification_thread_state *state,
                                                 struct lttng_evaluation **evaluation,
                                                 uid_t *session_uid,
                                                 gid_t *session_gid)
{
        int ret;
        struct cds_lfht_iter iter;
        struct cds_lfht_node *node;
        struct channel_info *channel_info = nullptr;
        struct channel_key *channel_key = nullptr;
        struct channel_state_sample *last_sample = nullptr;
        struct lttng_channel_trigger_list *channel_trigger_list = nullptr;

        lttng::urcu::read_lock_guard read_lock;

        /* Find the channel associated with the condition. */
        cds_lfht_for_each_entry (
                state->channel_triggers_ht, &iter, channel_trigger_list, channel_triggers_ht_node) {
                struct lttng_trigger_list_element *element;

                cds_list_for_each_entry (element, &channel_trigger_list->list, node) {
                        const struct lttng_condition *current_condition =
                                lttng_trigger_get_const_condition(element->trigger);

                        LTTNG_ASSERT(current_condition);
                        if (!lttng_condition_is_equal(condition, current_condition)) {
                                continue;
                        }

                        /* Found the trigger, save the channel key. */
                        channel_key = &channel_trigger_list->channel_key;
                        break;
                }
                if (channel_key) {
                        /* The channel key was found stop iteration. */
                        break;
                }
        }

        if (!channel_key) {
                /* No channel found; normal exit. */
                DBG("No known channel associated with newly subscribed-to condition");
                ret = 0;
                goto end;
        }

        /* Fetch channel info for the matching channel. */
        cds_lfht_lookup(state->channels_ht,
                        hash_channel_key(channel_key),
                        match_channel_info,
                        channel_key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        LTTNG_ASSERT(node);
        channel_info = caa_container_of(node, struct channel_info, channels_ht_node);

        /* Retrieve the channel's last sample, if it exists. */
        cds_lfht_lookup(state->channel_state_ht,
                        hash_channel_key(channel_key),
                        match_channel_state_sample,
                        channel_key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (node) {
                last_sample =
                        caa_container_of(node, struct channel_state_sample, channel_state_ht_node);
        } else {
                /* Nothing to evaluate, no sample was ever taken. Normal exit */
                DBG("No channel sample associated with newly subscribed-to condition");
                ret = 0;
                goto end;
        }

        ret = evaluate_buffer_condition(
                condition, evaluation, state, nullptr, last_sample, channel_info);
        if (ret) {
                WARN("Fatal error occurred while evaluating a newly subscribed-to condition");
                goto end;
        }

        *session_uid = channel_info->session_info->uid;
        *session_gid = channel_info->session_info->gid;
end:
        return ret;
}

static const char *get_condition_session_name(const struct lttng_condition *condition)
{
        const char *session_name = nullptr;
        enum lttng_condition_status status;

        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
                status = lttng_condition_buffer_usage_get_session_name(condition, &session_name);
                break;
        case LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE:
                status = lttng_condition_session_consumed_size_get_session_name(condition,
                                                                                &session_name);
                break;
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING:
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED:
                status =
                        lttng_condition_session_rotation_get_session_name(condition, &session_name);
                break;
        default:
                abort();
        }
        if (status != LTTNG_CONDITION_STATUS_OK) {
                ERR("Failed to retrieve session rotation condition's session name");
                goto end;
        }
end:
        return session_name;
}

static bool evaluate_session_rotation_ongoing_condition(const struct lttng_condition *condition
                                                        __attribute__((unused)),
                                                        const struct session_state_sample *sample)
{
        return sample->rotation.ongoing;
}

static bool evaluate_session_consumed_size_condition(const struct lttng_condition *condition,
                                                     const struct session_state_sample *sample)
{
        uint64_t threshold;
        const struct lttng_condition_session_consumed_size *size_condition =
                lttng::utils::container_of(condition,
                                           &lttng_condition_session_consumed_size::parent);

        threshold = size_condition->consumed_threshold_bytes.value;
        DBG("Session consumed size condition being evaluated: threshold = %" PRIu64
            ", current size = %" PRIu64,
            threshold,
            sample->consumed_data_size);
        return sample->consumed_data_size >= threshold;
}

/*
 * `new_state` can be NULL to indicate that we are not evaluating a
 * state transition. A client subscribed or a trigger was registered and
 * we wish to perform an initial evaluation.
 */
static int evaluate_session_condition(const struct lttng_condition *condition,
                                      const struct session_info *session_info,
                                      const struct session_state_sample *new_state,
                                      struct lttng_evaluation **evaluation)
{
        int ret;
        bool previous_result, newest_result;

        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING:
                if (new_state) {
                        previous_result = evaluate_session_rotation_ongoing_condition(
                                condition, &session_info->last_state_sample);
                        newest_result =
                                evaluate_session_rotation_ongoing_condition(condition, new_state);
                } else {
                        previous_result = false;
                        newest_result = evaluate_session_rotation_ongoing_condition(
                                condition, &session_info->last_state_sample);
                }
                break;
        case LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE:
                if (new_state) {
                        previous_result = evaluate_session_consumed_size_condition(
                                condition, &session_info->last_state_sample);
                        newest_result =
                                evaluate_session_consumed_size_condition(condition, new_state);
                } else {
                        previous_result = false;
                        newest_result = evaluate_session_consumed_size_condition(
                                condition, &session_info->last_state_sample);
                }
                break;
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED:
                /*
                 * Note that LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED is
                 * evaluated differently to only consider state transitions without regard for the
                 * initial state. This is a deliberate choice as it is unlikely that a user would
                 * expect an action to occur for a rotation that occurred long before the trigger or
                 * subscription occurred.
                 */
                if (!new_state) {
                        ret = 0;
                        goto end;
                }

                previous_result = !session_info->last_state_sample.rotation.ongoing;
                newest_result = !new_state->rotation.ongoing;
                break;
        default:
                ret = 0;
                goto end;
        }

        if (!newest_result || (previous_result == newest_result)) {
                /* Not a state transition, evaluate to false. */
                ret = 0;
                goto end;
        }

        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING:
        {
                const auto rotation_id = new_state ? new_state->rotation.id :
                                                     session_info->last_state_sample.rotation.id;

                *evaluation = lttng_evaluation_session_rotation_ongoing_create(rotation_id);
                break;
        }
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED:
        {
                const auto& sample = new_state ? *new_state : session_info->last_state_sample;
                const auto rotation_id = sample.rotation.id;

                /* Callee acquires a reference to location. */
                *evaluation = lttng_evaluation_session_rotation_completed_create(
                        rotation_id, sample.rotation.location);
                break;
        }
        case LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE:
        {
                const auto latest_session_consumed_total = new_state ?
                        new_state->consumed_data_size :
                        session_info->last_state_sample.consumed_data_size;

                *evaluation = lttng_evaluation_session_consumed_size_create(
                        latest_session_consumed_total);
                break;
        }
        default:
                abort();
        }

        if (!*evaluation) {
                /* Fatal error. */
                ERR("Failed to create session condition evaluation: session name = `%s`",
                    session_info->name);
                ret = -1;
                goto end;
        }

        ret = 0;
end:
        return ret;
}

static int evaluate_condition_for_client(const struct lttng_trigger *trigger,
                                         const struct lttng_condition *condition,
                                         struct notification_client *client,
                                         struct notification_thread_state *state)
{
        int ret;
        struct lttng_evaluation *evaluation = nullptr;
        struct notification_client_list client_list = {
                .lock = PTHREAD_MUTEX_INITIALIZER,
                .ref = {},
                .condition = nullptr,
                .triggers_list = {},
                .clients_list = {},
                .notification_trigger_clients_ht = nullptr,
                .notification_trigger_clients_ht_node = {},
                .rcu_node = {},
        };
        struct notification_client_list_element client_list_element = {};
        uid_t object_uid = 0;
        gid_t object_gid = 0;

        LTTNG_ASSERT(trigger);
        LTTNG_ASSERT(condition);
        LTTNG_ASSERT(client);
        LTTNG_ASSERT(state);

        switch (get_condition_binding_object(condition)) {
        case LTTNG_OBJECT_TYPE_SESSION:
        {
                /* Find the session associated with the condition. */
                const auto *session_name = get_condition_session_name(condition);
                auto session_info = get_session_info_by_name(state, session_name);
                if (!session_info) {
                        /* Not an error, the session doesn't exist yet. */
                        DBG("Session not found while evaluating session condition for client: session name = `%s`",
                            session_name);
                        ret = 0;
                        goto end;
                }

                object_uid = session_info->uid;
                object_gid = session_info->gid;

                ret = evaluate_session_condition(condition, session_info, nullptr, &evaluation);
                session_info_put(session_info);
                break;
        }
        case LTTNG_OBJECT_TYPE_CHANNEL:
                ret = evaluate_channel_condition_for_client(
                        condition, state, &evaluation, &object_uid, &object_gid);
                break;
        case LTTNG_OBJECT_TYPE_NONE:
                DBG("Newly subscribed-to condition not bound to object, nothing to evaluate");
                ret = 0;
                goto end;
        case LTTNG_OBJECT_TYPE_UNKNOWN:
        default:
                ret = -1;
                goto end;
        }
        if (ret) {
                /* Fatal error. */
                goto end;
        }
        if (!evaluation) {
                /* Evaluation yielded nothing. Normal exit. */
                DBG("Newly subscribed-to condition evaluated to false, nothing to report to client");
                ret = 0;
                goto end;
        }

        /*
         * Create a temporary client list with the client currently
         * subscribing.
         */
        cds_lfht_node_init(&client_list.notification_trigger_clients_ht_node);
        CDS_INIT_LIST_HEAD(&client_list.clients_list);

        CDS_INIT_LIST_HEAD(&client_list_element.node);
        client_list_element.client = client;
        cds_list_add(&client_list_element.node, &client_list.clients_list);

        /* Send evaluation result to the newly-subscribed client. */
        DBG("Newly subscribed-to condition evaluated to true, notifying client");
        ret = send_evaluation_to_clients(
                trigger, evaluation, &client_list, state, object_uid, object_gid);

end:
        return ret;
}

static int notification_thread_client_subscribe(struct notification_client *client,
                                                struct lttng_condition *condition,
                                                struct notification_thread_state *state,
                                                enum lttng_notification_channel_status *_status)
{
        int ret = 0;
        struct notification_client_list *client_list = nullptr;
        struct lttng_condition_list_element *condition_list_element = nullptr;
        struct notification_client_list_element *client_list_element = nullptr;
        struct lttng_trigger_ht_element *trigger_ht_element;
        enum lttng_notification_channel_status status = LTTNG_NOTIFICATION_CHANNEL_STATUS_OK;

        /*
         * Ensure that the client has not already subscribed to this condition
         * before.
         */
        cds_list_for_each_entry (condition_list_element, &client->condition_list, node) {
                if (lttng_condition_is_equal(condition_list_element->condition, condition)) {
                        status = LTTNG_NOTIFICATION_CHANNEL_STATUS_ALREADY_SUBSCRIBED;
                        goto end;
                }
        }

        condition_list_element = zmalloc<lttng_condition_list_element>();
        if (!condition_list_element) {
                ret = -1;
                goto error;
        }
        client_list_element = zmalloc<notification_client_list_element>();
        if (!client_list_element) {
                ret = -1;
                goto error;
        }

        /*
         * Add the newly-subscribed condition to the client's subscription list.
         */
        CDS_INIT_LIST_HEAD(&condition_list_element->node);
        condition_list_element->condition = condition;
        condition = nullptr;
        cds_list_add(&condition_list_element->node, &client->condition_list);

        client_list = get_client_list_from_condition(state, condition_list_element->condition);
        if (!client_list) {
                /*
                 * No notification-emiting trigger registered with this
                 * condition. We don't evaluate the condition right away
                 * since this trigger is not registered yet.
                 */
                free(client_list_element);
                goto end;
        }

        /*
         * The condition to which the client just subscribed is evaluated
         * at this point so that conditions that are already TRUE result
         * in a notification being sent out.
         *
         * Note the iteration on all triggers which share an identical
         * `condition` than the one to which the client is registering. This is
         * done to ensure that the client receives a distinct notification for
         * all triggers that have a `notify` action that have this condition.
         */
        pthread_mutex_lock(&client_list->lock);
        cds_list_for_each_entry (
                trigger_ht_element, &client_list->triggers_list, client_list_trigger_node) {
                if (evaluate_condition_for_client(trigger_ht_element->trigger,
                                                  condition_list_element->condition,
                                                  client,
                                                  state)) {
                        WARN("Evaluation of a condition on client subscription failed, aborting.");
                        ret = -1;
                        free(client_list_element);
                        pthread_mutex_unlock(&client_list->lock);
                        goto end;
                }
        }
        pthread_mutex_unlock(&client_list->lock);

        /*
         * Add the client to the list of clients interested in a given trigger
         * if a "notification" trigger with a corresponding condition was
         * added prior.
         */
        client_list_element->client = client;
        CDS_INIT_LIST_HEAD(&client_list_element->node);

        pthread_mutex_lock(&client_list->lock);
        cds_list_add(&client_list_element->node, &client_list->clients_list);
        pthread_mutex_unlock(&client_list->lock);
end:
        if (_status) {
                *_status = status;
        }
        if (client_list) {
                notification_client_list_put(client_list);
        }
        lttng_condition_destroy(condition);
        return ret;
error:
        free(condition_list_element);
        free(client_list_element);
        lttng_condition_destroy(condition);
        return ret;
}

static int notification_thread_client_unsubscribe(struct notification_client *client,
                                                  struct lttng_condition *condition,
                                                  struct notification_thread_state *state,
                                                  enum lttng_notification_channel_status *_status)
{
        struct notification_client_list *client_list;
        struct lttng_condition_list_element *condition_list_element, *condition_tmp;
        struct notification_client_list_element *client_list_element, *client_tmp;
        bool condition_found = false;
        enum lttng_notification_channel_status status = LTTNG_NOTIFICATION_CHANNEL_STATUS_OK;

        /* Remove the condition from the client's condition list. */
        cds_list_for_each_entry_safe (
                condition_list_element, condition_tmp, &client->condition_list, node) {
                if (!lttng_condition_is_equal(condition_list_element->condition, condition)) {
                        continue;
                }

                cds_list_del(&condition_list_element->node);
                /*
                 * The caller may be iterating on the client's conditions to
                 * tear down a client's connection. In this case, the condition
                 * will be destroyed at the end.
                 */
                if (condition != condition_list_element->condition) {
                        lttng_condition_destroy(condition_list_element->condition);
                }
                free(condition_list_element);
                condition_found = true;
                break;
        }

        if (!condition_found) {
                status = LTTNG_NOTIFICATION_CHANNEL_STATUS_UNKNOWN_CONDITION;
                goto end;
        }

        /*
         * Remove the client from the list of clients interested the trigger
         * matching the condition.
         */
        client_list = get_client_list_from_condition(state, condition);
        if (!client_list) {
                goto end;
        }

        pthread_mutex_lock(&client_list->lock);
        cds_list_for_each_entry_safe (
                client_list_element, client_tmp, &client_list->clients_list, node) {
                if (client_list_element->client->id != client->id) {
                        continue;
                }
                cds_list_del(&client_list_element->node);
                free(client_list_element);
                break;
        }
        pthread_mutex_unlock(&client_list->lock);
        notification_client_list_put(client_list);
        client_list = nullptr;
end:
        lttng_condition_destroy(condition);
        if (_status) {
                *_status = status;
        }
        return 0;
}

static void free_notification_client_rcu(struct rcu_head *node)
{
        free(lttng::utils::container_of(node, &notification_client::rcu_node));
}

static void notification_client_destroy(struct notification_client *client)
{
        if (!client) {
                return;
        }

        /*
         * The client object is not reachable by other threads, no need to lock
         * the client here.
         */
        if (client->socket >= 0) {
                (void) lttcomm_close_unix_sock(client->socket);
                client->socket = -1;
        }
        client->communication.active = false;
        lttng_payload_reset(&client->communication.inbound.payload);
        lttng_payload_reset(&client->communication.outbound.payload);
        pthread_mutex_destroy(&client->lock);
        call_rcu(&client->rcu_node, free_notification_client_rcu);
}

/*
 * Call with rcu_read_lock held (and hold for the lifetime of the returned
 * client pointer).
 */
static struct notification_client *get_client_from_socket(int socket,
                                                          struct notification_thread_state *state)
{
        struct cds_lfht_iter iter;
        struct cds_lfht_node *node;
        struct notification_client *client = nullptr;

        ASSERT_RCU_READ_LOCKED();

        cds_lfht_lookup(state->client_socket_ht,
                        hash_client_socket(socket),
                        match_client_socket,
                        (void *) (unsigned long) socket,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (!node) {
                goto end;
        }

        client = caa_container_of(node, struct notification_client, client_socket_ht_node);
end:
        return client;
}

/*
 * Call with rcu_read_lock held (and hold for the lifetime of the returned
 * client pointer).
 */
static struct notification_client *get_client_from_id(notification_client_id id,
                                                      struct notification_thread_state *state)
{
        struct cds_lfht_iter iter;
        struct cds_lfht_node *node;
        struct notification_client *client = nullptr;

        ASSERT_RCU_READ_LOCKED();

        cds_lfht_lookup(state->client_id_ht, hash_client_id(id), match_client_id, &id, &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (!node) {
                goto end;
        }

        client = caa_container_of(node, struct notification_client, client_id_ht_node);
end:
        return client;
}

static bool buffer_usage_condition_applies_to_channel(const struct lttng_condition *condition,
                                                      const struct channel_info *channel_info)
{
        enum lttng_condition_status status;
        enum lttng_domain_type condition_domain;
        const char *condition_session_name = nullptr;
        const char *condition_channel_name = nullptr;

        status = lttng_condition_buffer_usage_get_domain_type(condition, &condition_domain);
        LTTNG_ASSERT(status == LTTNG_CONDITION_STATUS_OK);
        if (channel_info->key.domain != condition_domain) {
                goto fail;
        }

        status = lttng_condition_buffer_usage_get_session_name(condition, &condition_session_name);
        LTTNG_ASSERT((status == LTTNG_CONDITION_STATUS_OK) && condition_session_name);

        status = lttng_condition_buffer_usage_get_channel_name(condition, &condition_channel_name);
        LTTNG_ASSERT((status == LTTNG_CONDITION_STATUS_OK) && condition_channel_name);

        if (strcmp(channel_info->session_info->name, condition_session_name) != 0) {
                goto fail;
        }
        if (strcmp(channel_info->name, condition_channel_name) != 0) {
                goto fail;
        }

        return true;
fail:
        return false;
}

static bool trigger_applies_to_channel(const struct lttng_trigger *trigger,
                                       const struct channel_info *channel_info)
{
        const struct lttng_condition *condition;
        bool trigger_applies;

        condition = lttng_trigger_get_const_condition(trigger);
        if (!condition) {
                goto fail;
        }

        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
                trigger_applies =
                        buffer_usage_condition_applies_to_channel(condition, channel_info);
                break;
        default:
                goto fail;
        }

        return trigger_applies;
fail:
        return false;
}

/* Must be called with RCU read lock held. */
static struct lttng_session_trigger_list *
get_session_trigger_list(struct notification_thread_state *state, const char *session_name)
{
        struct lttng_session_trigger_list *list = nullptr;
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;

        ASSERT_RCU_READ_LOCKED();

        cds_lfht_lookup(state->session_triggers_ht,
                        hash_key_str(session_name, lttng_ht_seed),
                        match_session_trigger_list,
                        session_name,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (!node) {
                /*
                 * Not an error, the list of triggers applying to that session
                 * will be initialized when the session is created.
                 */
                DBG("No trigger list found for session \"%s\" as it is not yet known to the notification system",
                    session_name);
                goto end;
        }

        list = caa_container_of(node, struct lttng_session_trigger_list, session_triggers_ht_node);
end:
        return list;
}

/*
 * Allocate an empty lttng_session_trigger_list for the session named
 * 'session_name'.
 */
static struct lttng_session_trigger_list *
lttng_session_trigger_list_create(const char *session_name, struct cds_lfht *session_triggers_ht)
{
        struct lttng_session_trigger_list *list = nullptr;
        char *session_name_copy = strdup(session_name);

        if (!session_name_copy) {
                PERROR("Failed to allocate session name while building trigger list");
                goto end;
        }

        list = zmalloc<lttng_session_trigger_list>();
        if (!list) {
                PERROR("Failed to allocate session trigger list while building trigger list");
                goto end;
        }

        list->session_name = session_name_copy;
        CDS_INIT_LIST_HEAD(&list->list);
        cds_lfht_node_init(&list->session_triggers_ht_node);
        list->session_triggers_ht = session_triggers_ht;

        /* Publish the list through the session_triggers_ht. */
        {
                lttng::urcu::read_lock_guard read_lock;
                cds_lfht_add(session_triggers_ht,
                             hash_key_str(session_name, lttng_ht_seed),
                             &list->session_triggers_ht_node);
        }
end:
        return list;
}

static void free_session_trigger_list_rcu(struct rcu_head *node)
{
        struct lttng_session_trigger_list *list =
                caa_container_of(node, struct lttng_session_trigger_list, rcu_node);

        free(list->session_name);
        free(list);
}

static void lttng_session_trigger_list_destroy(struct lttng_session_trigger_list *list)
{
        struct lttng_trigger_list_element *trigger_list_element, *tmp;

        /* Empty the list element by element, and then free the list itself. */
        cds_list_for_each_entry_safe (trigger_list_element, tmp, &list->list, node) {
                cds_list_del(&trigger_list_element->node);
                free(trigger_list_element);
        }
        lttng::urcu::read_lock_guard read_lock;
        /* Unpublish the list from the session_triggers_ht. */
        cds_lfht_del(list->session_triggers_ht, &list->session_triggers_ht_node);
        call_rcu(&list->rcu_node, free_session_trigger_list_rcu);
}

static int lttng_session_trigger_list_add(struct lttng_session_trigger_list *list,
                                          struct lttng_trigger *trigger)
{
        int ret = 0;
        struct lttng_trigger_list_element *new_element = zmalloc<lttng_trigger_list_element>();

        if (!new_element) {
                ret = -1;
                goto end;
        }
        CDS_INIT_LIST_HEAD(&new_element->node);
        new_element->trigger = trigger;
        cds_list_add(&new_element->node, &list->list);
end:
        return ret;
}

static bool trigger_applies_to_session(const struct lttng_trigger *trigger,
                                       const char *session_name)
{
        bool applies = false;
        const struct lttng_condition *condition;

        condition = lttng_trigger_get_const_condition(trigger);
        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_ONGOING:
        case LTTNG_CONDITION_TYPE_SESSION_ROTATION_COMPLETED:
        case LTTNG_CONDITION_TYPE_SESSION_CONSUMED_SIZE:
        {
                const char *condition_session_name;

                condition_session_name = get_condition_session_name(condition);
                LTTNG_ASSERT(condition_session_name);
                applies = !strcmp(condition_session_name, session_name);
                break;
        }
        default:
                goto end;
        }
end:
        return applies;
}

/*
 * Allocate and initialize an lttng_session_trigger_list which contains
 * all triggers that apply to the session named 'session_name'.
 */
static struct lttng_session_trigger_list *
lttng_session_trigger_list_build(const struct notification_thread_state *state,
                                 const char *session_name)
{
        int trigger_count = 0;
        struct lttng_session_trigger_list *session_trigger_list = nullptr;
        struct lttng_trigger_ht_element *trigger_ht_element = nullptr;
        struct cds_lfht_iter iter;

        session_trigger_list =
                lttng_session_trigger_list_create(session_name, state->session_triggers_ht);

        {
                /* Add all triggers applying to the session named 'session_name'. */
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (state->triggers_ht, &iter, trigger_ht_element, node) {
                        int ret;

                        if (!trigger_applies_to_session(trigger_ht_element->trigger,
                                                        session_name)) {
                                continue;
                        }

                        ret = lttng_session_trigger_list_add(session_trigger_list,
                                                             trigger_ht_element->trigger);
                        if (ret) {
                                goto error;
                        }

                        trigger_count++;
                }
        }

        DBG("Found %i triggers that apply to newly created session", trigger_count);
        return session_trigger_list;
error:
        lttng_session_trigger_list_destroy(session_trigger_list);
        return nullptr;
}

static struct session_info *create_and_publish_session_info(struct notification_thread_state *state,
                                                            uint64_t id,
                                                            const char *name,
                                                            uid_t uid,
                                                            gid_t gid)
{
        struct session_info *session = nullptr;
        struct lttng_session_trigger_list *trigger_list;

        lttng::urcu::read_lock_guard read_lock;
        trigger_list = lttng_session_trigger_list_build(state, name);
        if (!trigger_list) {
                goto error;
        }

        session = session_info_create(id, name, uid, gid, trigger_list, state->sessions_ht);
        if (!session) {
                ERR("Failed to allocation session info for session \"%s\" (uid = %i, gid = %i)",
                    name,
                    uid,
                    gid);
                lttng_session_trigger_list_destroy(trigger_list);
                goto error;
        }

        /* Transferred ownership to the new session. */
        trigger_list = nullptr;

        if (cds_lfht_add_unique(state->sessions_ht,
                                hash_session_info(session),
                                match_session_info,
                                &id,
                                &session->sessions_ht_node) != &session->sessions_ht_node) {
                ERR("Duplicate session found: name = `%s`, id = %" PRIu64, name, id);
                goto error;
        }

        return session;
error:
        session_info_put(session);
        return nullptr;
}

static int handle_notification_thread_command_add_channel(struct notification_thread_state *state,
                                                          uint64_t session_id,
                                                          const char *channel_name,
                                                          enum lttng_domain_type channel_domain,
                                                          uint64_t channel_key_int,
                                                          uint64_t channel_capacity,
                                                          enum lttng_error_code *cmd_result)
{
        struct cds_list_head trigger_list;
        struct channel_info *new_channel_info = nullptr;
        struct channel_key channel_key = {
                .key = channel_key_int,
                .domain = channel_domain,
        };
        struct lttng_channel_trigger_list *channel_trigger_list = nullptr;
        struct lttng_trigger_ht_element *trigger_ht_element = nullptr;
        int trigger_count = 0;
        struct cds_lfht_iter iter;
        struct session_info *session_info = nullptr;
        lttng::urcu::read_lock_guard read_lock;

        DBG("Adding channel: channel name = `%s`, session id = %" PRIu64 ", channel key = %" PRIu64
            ", domain = %s",
            channel_name,
            session_id,
            channel_key_int,
            lttng_domain_type_str(channel_domain));

        CDS_INIT_LIST_HEAD(&trigger_list);

        session_info = get_session_info_by_id(state, session_id);
        if (!session_info) {
                /* Fatal logic error. */
                ERR("Failed to find session while adding channel: session id = %" PRIu64,
                    session_id);
                goto error;
        }

        new_channel_info =
                channel_info_create(channel_name, &channel_key, channel_capacity, session_info);
        if (!new_channel_info) {
                goto error;
        }

        /* Build a list of all triggers applying to the new channel. */
        cds_lfht_for_each_entry (state->triggers_ht, &iter, trigger_ht_element, node) {
                struct lttng_trigger_list_element *new_element;

                if (!trigger_applies_to_channel(trigger_ht_element->trigger, new_channel_info)) {
                        continue;
                }

                new_element = zmalloc<lttng_trigger_list_element>();
                if (!new_element) {
                        goto error;
                }
                CDS_INIT_LIST_HEAD(&new_element->node);
                new_element->trigger = trigger_ht_element->trigger;
                cds_list_add(&new_element->node, &trigger_list);
                trigger_count++;
        }

        DBG("Found %i triggers that apply to newly added channel", trigger_count);
        channel_trigger_list = zmalloc<lttng_channel_trigger_list>();
        if (!channel_trigger_list) {
                goto error;
        }
        channel_trigger_list->channel_key = new_channel_info->key;
        CDS_INIT_LIST_HEAD(&channel_trigger_list->list);
        cds_lfht_node_init(&channel_trigger_list->channel_triggers_ht_node);
        cds_list_splice(&trigger_list, &channel_trigger_list->list);

        /* Add channel to the channel_ht which owns the channel_infos. */
        cds_lfht_add(state->channels_ht,
                     hash_channel_key(&new_channel_info->key),
                     &new_channel_info->channels_ht_node);
        /*
         * Add the list of triggers associated with this channel to the
         * channel_triggers_ht.
         */
        cds_lfht_add(state->channel_triggers_ht,
                     hash_channel_key(&new_channel_info->key),
                     &channel_trigger_list->channel_triggers_ht_node);
        session_info_put(session_info);
        *cmd_result = LTTNG_OK;
        return 0;
error:
        channel_info_destroy(new_channel_info);
        session_info_put(session_info);
        return 1;
}

static int handle_notification_thread_command_add_session(struct notification_thread_state *state,
                                                          uint64_t session_id,
                                                          const char *session_name,
                                                          uid_t session_uid,
                                                          gid_t session_gid,
                                                          enum lttng_error_code *cmd_result)
{
        int ret;

        DBG("Adding session: session name = `%s`, session id = %" PRIu64
            ", session uid = %d, session gid = %d",
            session_name,
            session_id,
            session_uid,
            session_gid);

        auto session = create_and_publish_session_info(
                state, session_id, session_name, session_uid, session_gid);
        if (!session) {
                PERROR("Failed to add session: session name = `%s`, session id = %" PRIu64
                       ", session uid = %d, session gid = %d",
                       session_name,
                       session_id,
                       session_uid,
                       session_gid);
                ret = -1;
                *cmd_result = LTTNG_ERR_NOMEM;
                goto end;
        }

        /*
         * Note that the reference to `session` is not released; this reference is
         * the "global" reference that is used to allow look-ups. This reference will
         * only be released when the session is removed. See
         * handle_notification_thread_command_remove_session.
         */
        ret = 0;
        *cmd_result = LTTNG_OK;
end:
        return ret;
}

static int
handle_notification_thread_command_remove_session(struct notification_thread_state *state,
                                                  uint64_t session_id,
                                                  enum lttng_error_code *cmd_result)
{
        int ret;

        DBG("Removing session: session id = %" PRIu64, session_id);

        auto session = get_session_info_by_id(state, session_id);
        if (!session) {
                ERR("Failed to remove session: session id = %" PRIu64, session_id);
                ret = -1;
                *cmd_result = LTTNG_ERR_NO_SESSION;
                goto end;
        }

        /* Release the reference returned by the look-up, and then release the global reference. */
        session_info_put(session);
        session_info_put(session);
        ret = 0;
        *cmd_result = LTTNG_OK;
end:
        return ret;
}

static void free_channel_trigger_list_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct lttng_channel_trigger_list, rcu_node));
}

static void free_channel_state_sample_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct channel_state_sample, rcu_node));
}

static int
handle_notification_thread_command_remove_channel(struct notification_thread_state *state,
                                                  uint64_t channel_key,
                                                  enum lttng_domain_type domain,
                                                  enum lttng_error_code *cmd_result)
{
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;
        struct lttng_channel_trigger_list *trigger_list;
        struct lttng_trigger_list_element *trigger_list_element, *tmp;
        struct channel_key key = { .key = channel_key, .domain = domain };
        struct channel_info *channel_info;

        DBG("Removing channel key = %" PRIu64 " in %s domain",
            channel_key,
            lttng_domain_type_str(domain));

        lttng::urcu::read_lock_guard read_lock;

        cds_lfht_lookup(state->channel_triggers_ht,
                        hash_channel_key(&key),
                        match_channel_trigger_list,
                        &key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        /*
         * There is a severe internal error if we are being asked to remove a
         * channel that doesn't exist.
         */
        if (!node) {
                ERR("Channel being removed is unknown to the notification thread");
                goto end;
        }

        /* Free the list of triggers associated with this channel. */
        trigger_list =
                caa_container_of(node, struct lttng_channel_trigger_list, channel_triggers_ht_node);
        cds_list_for_each_entry_safe (trigger_list_element, tmp, &trigger_list->list, node) {
                cds_list_del(&trigger_list_element->node);
                free(trigger_list_element);
        }
        cds_lfht_del(state->channel_triggers_ht, node);
        call_rcu(&trigger_list->rcu_node, free_channel_trigger_list_rcu);

        /* Free sampled channel state. */
        cds_lfht_lookup(state->channel_state_ht,
                        hash_channel_key(&key),
                        match_channel_state_sample,
                        &key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        /*
         * This is expected to be NULL if the channel is destroyed before we
         * received a sample.
         */
        if (node) {
                struct channel_state_sample *sample =
                        caa_container_of(node, struct channel_state_sample, channel_state_ht_node);

                cds_lfht_del(state->channel_state_ht, node);
                call_rcu(&sample->rcu_node, free_channel_state_sample_rcu);
        }

        /* Remove the channel from the channels_ht and free it. */
        cds_lfht_lookup(
                state->channels_ht, hash_channel_key(&key), match_channel_info, &key, &iter);
        node = cds_lfht_iter_get_node(&iter);
        LTTNG_ASSERT(node);
        channel_info = caa_container_of(node, struct channel_info, channels_ht_node);
        cds_lfht_del(state->channels_ht, node);
        channel_info_destroy(channel_info);
end:
        *cmd_result = LTTNG_OK;
        return 0;
}

static int
handle_notification_thread_command_session_rotation(struct notification_thread_state *state,
                                                    enum notification_thread_command_type cmd_type,
                                                    uint64_t session_id,
                                                    uint64_t trace_archive_chunk_id,
                                                    struct lttng_trace_archive_location *location,
                                                    enum lttng_error_code *_cmd_result)
{
        int ret = 0;
        enum lttng_error_code cmd_result = LTTNG_OK;
        struct lttng_session_trigger_list *trigger_list;
        struct lttng_trigger_list_element *trigger_list_element;
        struct session_info *session_info;
        struct lttng_credentials session_creds;
        struct session_state_sample new_session_state;

        lttng::urcu::read_lock_guard read_lock;

        session_info = get_session_info_by_id(state, session_id);
        if (!session_info) {
                /* Fatal logic error. */
                ERR("Failed to find session while handling rotation state change: session id = %" PRIu64,
                    session_id);
                ret = -1;
                cmd_result = LTTNG_ERR_FATAL;
                goto end;
        }

        new_session_state = session_info->last_state_sample;
        if (location) {
                lttng_trace_archive_location_get(location);
                new_session_state.rotation.location = location;
        } else {
                new_session_state.rotation.location = nullptr;
        }

        session_creds = {
                .uid = LTTNG_OPTIONAL_INIT_VALUE(session_info->uid),
                .gid = LTTNG_OPTIONAL_INIT_VALUE(session_info->gid),
        };

        new_session_state.rotation.ongoing = cmd_type ==
                NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_ONGOING;
        new_session_state.rotation.id = trace_archive_chunk_id;

        trigger_list = get_session_trigger_list(state, session_info->name);
        LTTNG_ASSERT(trigger_list);

        cds_list_for_each_entry (trigger_list_element, &trigger_list->list, node) {
                const struct lttng_condition *condition;
                struct lttng_trigger *trigger;
                struct notification_client_list *client_list;
                struct lttng_evaluation *evaluation = nullptr;
                enum action_executor_status executor_status;

                trigger = trigger_list_element->trigger;
                condition = lttng_trigger_get_const_condition(trigger);
                LTTNG_ASSERT(condition);

                ret = evaluate_session_condition(
                        condition, session_info, &new_session_state, &evaluation);
                if (ret) {
                        ret = -1;
                        cmd_result = LTTNG_ERR_NOMEM;
                        goto end;
                }

                if (!evaluation) {
                        continue;
                }

                /*
                 * Ownership of `evaluation` transferred to the action executor
                 * no matter the result. The callee acquires a reference to the
                 * client list: we can release our own.
                 */
                client_list = get_client_list_from_condition(state, condition);
                executor_status = action_executor_enqueue_trigger(
                        state->executor, trigger, evaluation, &session_creds, client_list);
                notification_client_list_put(client_list);
                evaluation = nullptr;
                switch (executor_status) {
                case ACTION_EXECUTOR_STATUS_OK:
                        break;
                case ACTION_EXECUTOR_STATUS_ERROR:
                case ACTION_EXECUTOR_STATUS_INVALID:
                        /*
                         * TODO Add trigger identification (name/id) when
                         * it is added to the API.
                         */
                        ERR("Fatal error occurred while enqueuing action associated with session rotation trigger");
                        ret = -1;
                        goto end;
                case ACTION_EXECUTOR_STATUS_OVERFLOW:
                        /*
                         * TODO Add trigger identification (name/id) when
                         * it is added to the API.
                         *
                         * Not a fatal error.
                         */
                        WARN("No space left when enqueuing action associated with session rotation trigger");
                        ret = 0;
                        goto end;
                default:
                        abort();
                }
        }

end:
        if (session_info) {
                /* Ownership of new_session_state::location is transferred. */
                lttng_trace_archive_location_put(session_info->last_state_sample.rotation.location);
                session_info->last_state_sample = new_session_state;
        }

        session_info_put(session_info);
        *_cmd_result = cmd_result;
        return ret;
}

static int
handle_notification_thread_command_add_tracer_event_source(struct notification_thread_state *state,
                                                           int tracer_event_source_fd,
                                                           enum lttng_domain_type domain_type,
                                                           enum lttng_error_code *_cmd_result)
{
        int ret = 0;
        enum lttng_error_code cmd_result = LTTNG_OK;
        struct notification_event_tracer_event_source_element *element = nullptr;

        element = zmalloc<notification_event_tracer_event_source_element>();
        if (!element) {
                cmd_result = LTTNG_ERR_NOMEM;
                ret = -1;
                goto end;
        }

        element->fd = tracer_event_source_fd;
        element->domain = domain_type;

        cds_list_add(&element->node, &state->tracer_event_sources_list);

        DBG3("Adding tracer event source fd to poll set: tracer_event_source_fd = %d, domain = '%s'",
             tracer_event_source_fd,
             lttng_domain_type_str(domain_type));

        /* Adding the read side pipe to the event poll. */
        ret = lttng_poll_add(&state->events, tracer_event_source_fd, LPOLLPRI | LPOLLIN);
        if (ret < 0) {
                ERR("Failed to add tracer event source to poll set: tracer_event_source_fd = %d, domain = '%s'",
                    tracer_event_source_fd,
                    lttng_domain_type_str(element->domain));
                cds_list_del(&element->node);
                free(element);
                goto end;
        }

        element->is_fd_in_poll_set = true;

end:
        *_cmd_result = cmd_result;
        return ret;
}

static int drain_event_notifier_notification_pipe(struct notification_thread_state *state,
                                                  int pipe,
                                                  enum lttng_domain_type domain)
{
        struct lttng_poll_event events = {};
        int ret;

        ret = lttng_poll_create(&events, 1, LTTNG_CLOEXEC);
        if (ret < 0) {
                ERR("Error creating lttng_poll_event");
                goto end;
        }

        ret = lttng_poll_add(&events, pipe, LPOLLIN);
        if (ret < 0) {
                ERR("Error adding fd event notifier notification pipe to lttng_poll_event: fd = %d",
                    pipe);
                goto end;
        }

        while (true) {
                /*
                 * Continue to consume notifications as long as there are new
                 * ones coming in. The tracer has been asked to stop producing
                 * them.
                 *
                 * LPOLLIN is explicitly checked since LPOLLHUP is implicitly
                 * monitored (on Linux, at least) and will be returned when
                 * the pipe is closed but empty.
                 */
                ret = lttng_poll_wait_interruptible(&events, 0);
                if (ret == 0 || (LTTNG_POLL_GETEV(&events, 0) & LPOLLIN) == 0) {
                        /* No more notification to be read on this pipe. */
                        ret = 0;
                        goto end;
                } else if (ret < 0) {
                        PERROR("Failed on lttng_poll_wait_interruptible() call");
                        ret = -1;
                        goto end;
                }

                ret = handle_one_event_notifier_notification(state, pipe, domain);
                if (ret) {
                        ERR("Error consuming an event notifier notification from pipe: fd = %d",
                            pipe);
                }
        }
end:
        lttng_poll_clean(&events);
        return ret;
}

static struct notification_event_tracer_event_source_element *
find_tracer_event_source_element(struct notification_thread_state *state,
                                 int tracer_event_source_fd)
{
        struct notification_event_tracer_event_source_element *source_element;

        cds_list_for_each_entry (source_element, &state->tracer_event_sources_list, node) {
                if (source_element->fd == tracer_event_source_fd) {
                        goto end;
                }
        }

        source_element = nullptr;
end:
        return source_element;
}

static int remove_tracer_event_source_from_pollset(
        struct notification_thread_state *state,
        struct notification_event_tracer_event_source_element *source_element)
{
        int ret = 0;

        LTTNG_ASSERT(source_element->is_fd_in_poll_set);

        DBG3("Removing tracer event source from poll set: tracer_event_source_fd = %d, domain = '%s'",
             source_element->fd,
             lttng_domain_type_str(source_element->domain));

        /* Removing the fd from the event poll set. */
        ret = lttng_poll_del(&state->events, source_element->fd);
        if (ret < 0) {
                ERR("Failed to remove tracer event source from poll set: tracer_event_source_fd = %d, domain = '%s'",
                    source_element->fd,
                    lttng_domain_type_str(source_element->domain));
                ret = -1;
                goto end;
        }

        source_element->is_fd_in_poll_set = false;

        /*
         * Force the notification thread to restart the poll() loop to ensure
         * that any events from the removed fd are removed.
         */
        state->restart_poll = true;

        ret = drain_event_notifier_notification_pipe(
                state, source_element->fd, source_element->domain);
        if (ret) {
                ERR("Error draining event notifier notification: tracer_event_source_fd = %d, domain = %s",
                    source_element->fd,
                    lttng_domain_type_str(source_element->domain));
                ret = -1;
                goto end;
        }

end:
        return ret;
}

int handle_notification_thread_tracer_event_source_died(struct notification_thread_state *state,
                                                        int tracer_event_source_fd)
{
        int ret = 0;
        struct notification_event_tracer_event_source_element *source_element;

        source_element = find_tracer_event_source_element(state, tracer_event_source_fd);

        LTTNG_ASSERT(source_element);

        ret = remove_tracer_event_source_from_pollset(state, source_element);
        if (ret) {
                ERR("Failed to remove dead tracer event source from poll set");
        }

        return ret;
}

static int handle_notification_thread_command_remove_tracer_event_source(
        struct notification_thread_state *state,
        int tracer_event_source_fd,
        enum lttng_error_code *_cmd_result)
{
        int ret = 0;
        enum lttng_error_code cmd_result = LTTNG_OK;
        struct notification_event_tracer_event_source_element *source_element = nullptr;

        source_element = find_tracer_event_source_element(state, tracer_event_source_fd);

        LTTNG_ASSERT(source_element);

        /* Remove the tracer source from the list. */
        cds_list_del(&source_element->node);

        if (!source_element->is_fd_in_poll_set) {
                /* Skip the poll set removal. */
                goto end;
        }

        ret = remove_tracer_event_source_from_pollset(state, source_element);
        if (ret) {
                ERR("Failed to remove tracer event source from poll set");
                cmd_result = LTTNG_ERR_FATAL;
        }

end:
        free(source_element);
        *_cmd_result = cmd_result;
        return ret;
}

static int
handle_notification_thread_command_list_triggers(struct notification_thread_handle *handle
                                                 __attribute__((unused)),
                                                 struct notification_thread_state *state,
                                                 uid_t client_uid,
                                                 struct lttng_triggers **triggers,
                                                 enum lttng_error_code *_cmd_result)
{
        int ret = 0;
        enum lttng_error_code cmd_result = LTTNG_OK;
        struct cds_lfht_iter iter;
        struct lttng_trigger_ht_element *trigger_ht_element;
        struct lttng_triggers *local_triggers = nullptr;
        const struct lttng_credentials *creds;

        lttng::urcu::read_lock_guard read_lock;

        local_triggers = lttng_triggers_create();
        if (!local_triggers) {
                /* Not a fatal error. */
                cmd_result = LTTNG_ERR_NOMEM;
                goto end;
        }

        cds_lfht_for_each_entry (state->triggers_ht, &iter, trigger_ht_element, node) {
                /*
                 * Only return the triggers to which the client has access.
                 * The root user has visibility over all triggers.
                 */
                creds = lttng_trigger_get_credentials(trigger_ht_element->trigger);
                if (client_uid != lttng_credentials_get_uid(creds) && client_uid != 0) {
                        continue;
                }

                ret = lttng_triggers_add(local_triggers, trigger_ht_element->trigger);
                if (ret < 0) {
                        /* Not a fatal error. */
                        ret = 0;
                        cmd_result = LTTNG_ERR_NOMEM;
                        goto end;
                }
        }

        /* Transferring ownership to the caller. */
        *triggers = local_triggers;
        local_triggers = nullptr;

end:
        lttng_triggers_destroy(local_triggers);
        *_cmd_result = cmd_result;
        return ret;
}

static inline void get_trigger_info_for_log(const struct lttng_trigger *trigger,
                                            const char **trigger_name,
                                            uid_t *trigger_owner_uid)
{
        enum lttng_trigger_status trigger_status;

        trigger_status = lttng_trigger_get_name(trigger, trigger_name);
        switch (trigger_status) {
        case LTTNG_TRIGGER_STATUS_OK:
                break;
        case LTTNG_TRIGGER_STATUS_UNSET:
                *trigger_name = "(anonymous)";
                break;
        default:
                abort();
        }

        trigger_status = lttng_trigger_get_owner_uid(trigger, trigger_owner_uid);
        LTTNG_ASSERT(trigger_status == LTTNG_TRIGGER_STATUS_OK);
}

static int handle_notification_thread_command_get_trigger(struct notification_thread_state *state,
                                                          const struct lttng_trigger *trigger,
                                                          struct lttng_trigger **registered_trigger,
                                                          enum lttng_error_code *_cmd_result)
{
        int ret = -1;
        struct cds_lfht_iter iter;
        struct lttng_trigger_ht_element *trigger_ht_element;
        enum lttng_error_code cmd_result = LTTNG_ERR_TRIGGER_NOT_FOUND;
        const char *trigger_name;
        uid_t trigger_owner_uid;

        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (state->triggers_ht, &iter, trigger_ht_element, node) {
                        if (lttng_trigger_is_equal(trigger, trigger_ht_element->trigger)) {
                                /* Take one reference on the return trigger. */
                                *registered_trigger = trigger_ht_element->trigger;
                                lttng_trigger_get(*registered_trigger);
                                ret = 0;
                                cmd_result = LTTNG_OK;
                                goto end;
                        }
                }
        }

        /* Not a fatal error if the trigger is not found. */
        get_trigger_info_for_log(trigger, &trigger_name, &trigger_owner_uid);
        DBG("Failed to retrieve registered version of trigger: trigger name = '%s', trigger owner uid = %d",
            trigger_name,
            (int) trigger_owner_uid);

        ret = 0;

end:
        *_cmd_result = cmd_result;
        return ret;
}

static bool condition_is_supported(struct lttng_condition *condition)
{
        bool is_supported;

        switch (lttng_condition_get_type(condition)) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
        {
                int ret;
                enum lttng_domain_type domain;

                ret = lttng_condition_buffer_usage_get_domain_type(condition, &domain);
                LTTNG_ASSERT(ret == 0);

                if (domain != LTTNG_DOMAIN_KERNEL) {
                        is_supported = true;
                        goto end;
                }

                /*
                 * Older kernel tracers don't expose the API to monitor their
                 * buffers. Therefore, we reject triggers that require that
                 * mechanism to be available to be evaluated.
                 *
                 * Assume unsupported on error.
                 */
                is_supported = kernel_supports_ring_buffer_snapshot_sample_positions() == 1;
                break;
        }
        case LTTNG_CONDITION_TYPE_EVENT_RULE_MATCHES:
        {
                const struct lttng_event_rule *event_rule;
                enum lttng_domain_type domain;
                const enum lttng_condition_status status =
                        lttng_condition_event_rule_matches_get_rule(condition, &event_rule);

                LTTNG_ASSERT(status == LTTNG_CONDITION_STATUS_OK);

                domain = lttng_event_rule_get_domain_type(event_rule);
                if (domain != LTTNG_DOMAIN_KERNEL) {
                        is_supported = true;
                        goto end;
                }

                /*
                 * Older kernel tracers can't emit notification. Therefore, we
                 * reject triggers that require that mechanism to be available
                 * to be evaluated.
                 *
                 * Assume unsupported on error.
                 */
                is_supported = kernel_supports_event_notifiers() == 1;
                break;
        }
        default:
                is_supported = true;
        }
end:
        return is_supported;
}

/* Must be called with RCU read lock held. */
static int bind_trigger_to_matching_session(struct lttng_trigger *trigger,
                                            struct notification_thread_state *state)
{
        int ret = 0;
        const struct lttng_condition *condition;
        const char *session_name;
        struct lttng_session_trigger_list *trigger_list;

        ASSERT_RCU_READ_LOCKED();

        condition = lttng_trigger_get_const_condition(trigger);
        session_name = get_condition_session_name(condition);

        trigger_list = get_session_trigger_list(state, session_name);
        if (!trigger_list) {
                DBG("Unable to bind trigger applying to session \"%s\" as it is not yet known to the notification system",
                    session_name);
                goto end;
        }

        DBG("Newly registered trigger bound to session \"%s\"", session_name);
        ret = lttng_session_trigger_list_add(trigger_list, trigger);
end:
        return ret;
}

/* Must be called with RCU read lock held. */
static int bind_trigger_to_matching_channels(struct lttng_trigger *trigger,
                                             struct notification_thread_state *state)
{
        int ret = 0;
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;
        struct channel_info *channel;

        ASSERT_RCU_READ_LOCKED();

        cds_lfht_for_each_entry (state->channels_ht, &iter, channel, channels_ht_node) {
                struct lttng_trigger_list_element *trigger_list_element;
                struct lttng_channel_trigger_list *trigger_list;
                struct cds_lfht_iter lookup_iter;

                if (!trigger_applies_to_channel(trigger, channel)) {
                        continue;
                }

                cds_lfht_lookup(state->channel_triggers_ht,
                                hash_channel_key(&channel->key),
                                match_channel_trigger_list,
                                &channel->key,
                                &lookup_iter);
                node = cds_lfht_iter_get_node(&lookup_iter);
                LTTNG_ASSERT(node);
                trigger_list = caa_container_of(
                        node, struct lttng_channel_trigger_list, channel_triggers_ht_node);

                trigger_list_element = zmalloc<lttng_trigger_list_element>();
                if (!trigger_list_element) {
                        ret = -1;
                        goto end;
                }
                CDS_INIT_LIST_HEAD(&trigger_list_element->node);
                trigger_list_element->trigger = trigger;
                cds_list_add(&trigger_list_element->node, &trigger_list->list);
                DBG("Newly registered trigger bound to channel \"%s\"", channel->name);
        }
end:
        return ret;
}

static bool is_trigger_action_notify(const struct lttng_trigger *trigger)
{
        bool is_notify = false;
        unsigned int i, count;
        enum lttng_action_status action_status;
        const struct lttng_action *action = lttng_trigger_get_const_action(trigger);
        enum lttng_action_type action_type;

        LTTNG_ASSERT(action);
        action_type = lttng_action_get_type(action);
        if (action_type == LTTNG_ACTION_TYPE_NOTIFY) {
                is_notify = true;
                goto end;
        } else if (action_type != LTTNG_ACTION_TYPE_LIST) {
                goto end;
        }

        action_status = lttng_action_list_get_count(action, &count);
        LTTNG_ASSERT(action_status == LTTNG_ACTION_STATUS_OK);

        for (i = 0; i < count; i++) {
                const struct lttng_action *inner_action = lttng_action_list_get_at_index(action, i);

                action_type = lttng_action_get_type(inner_action);
                if (action_type == LTTNG_ACTION_TYPE_NOTIFY) {
                        is_notify = true;
                        goto end;
                }
        }

end:
        return is_notify;
}

static bool trigger_name_taken(struct notification_thread_state *state,
                               const struct lttng_trigger *trigger)
{
        struct cds_lfht_iter iter;

        /*
         * No duplicata is allowed in the triggers_by_name_uid_ht.
         * The match is done against the trigger name and uid.
         */
        cds_lfht_lookup(state->triggers_by_name_uid_ht,
                        hash_trigger_by_name_uid(trigger),
                        match_trigger_by_name_uid,
                        trigger,
                        &iter);
        return !!cds_lfht_iter_get_node(&iter);
}

static enum lttng_error_code generate_trigger_name(struct notification_thread_state *state,
                                                   struct lttng_trigger *trigger,
                                                   const char **name)
{
        enum lttng_error_code ret_code = LTTNG_OK;
        bool taken = false;
        enum lttng_trigger_status status;

        do {
                const int ret =
                        lttng_trigger_generate_name(trigger, state->trigger_id.name_offset++);
                if (ret) {
                        /* The only reason this can fail right now. */
                        ret_code = LTTNG_ERR_NOMEM;
                        break;
                }

                status = lttng_trigger_get_name(trigger, name);
                LTTNG_ASSERT(status == LTTNG_TRIGGER_STATUS_OK);

                taken = trigger_name_taken(state, trigger);
        } while (taken || state->trigger_id.name_offset == UINT64_MAX);

        return ret_code;
}

static inline void
notif_thread_state_remove_trigger_ht_elem(struct notification_thread_state *state,
                                          struct lttng_trigger_ht_element *trigger_ht_element)
{
        LTTNG_ASSERT(state);
        LTTNG_ASSERT(trigger_ht_element);

        cds_lfht_del(state->triggers_ht, &trigger_ht_element->node);
        cds_lfht_del(state->triggers_by_name_uid_ht, &trigger_ht_element->node_by_name_uid);
}

static enum lttng_error_code setup_tracer_notifier(struct notification_thread_state *state,
                                                   struct lttng_trigger *trigger)
{
        enum lttng_error_code ret;
        enum event_notifier_error_accounting_status error_accounting_status;
        struct cds_lfht_node *node;
        uint64_t error_counter_index = 0;
        struct lttng_condition *condition = lttng_trigger_get_condition(trigger);
        struct notification_trigger_tokens_ht_element *trigger_tokens_ht_element = nullptr;

        trigger_tokens_ht_element = zmalloc<notification_trigger_tokens_ht_element>();
        if (!trigger_tokens_ht_element) {
                ret = LTTNG_ERR_NOMEM;
                goto end;
        }

        /* Add trigger token to the trigger_tokens_ht. */
        cds_lfht_node_init(&trigger_tokens_ht_element->node);
        trigger_tokens_ht_element->token = LTTNG_OPTIONAL_GET(trigger->tracer_token);
        trigger_tokens_ht_element->trigger = trigger;

        node = cds_lfht_add_unique(state->trigger_tokens_ht,
                                   hash_key_u64(&trigger_tokens_ht_element->token, lttng_ht_seed),
                                   match_trigger_token,
                                   &trigger_tokens_ht_element->token,
                                   &trigger_tokens_ht_element->node);
        if (node != &trigger_tokens_ht_element->node) {
                ret = LTTNG_ERR_TRIGGER_EXISTS;
                goto error_free_ht_element;
        }

        error_accounting_status = event_notifier_error_accounting_register_event_notifier(
                trigger, &error_counter_index);
        if (error_accounting_status != EVENT_NOTIFIER_ERROR_ACCOUNTING_STATUS_OK) {
                if (error_accounting_status ==
                    EVENT_NOTIFIER_ERROR_ACCOUNTING_STATUS_NO_INDEX_AVAILABLE) {
                        DBG("Trigger list error accounting counter full.");
                        ret = LTTNG_ERR_EVENT_NOTIFIER_ERROR_ACCOUNTING_FULL;
                } else {
                        ERR("Error registering trigger for error accounting");
                        ret = LTTNG_ERR_EVENT_NOTIFIER_REGISTRATION;
                }

                goto error_remove_ht_element;
        }

        lttng_condition_event_rule_matches_set_error_counter_index(condition, error_counter_index);

        ret = LTTNG_OK;
        goto end;

error_remove_ht_element:
        cds_lfht_del(state->trigger_tokens_ht, &trigger_tokens_ht_element->node);
error_free_ht_element:
        free(trigger_tokens_ht_element);
end:
        return ret;
}

/*
 * FIXME A client's credentials are not checked when registering a trigger.
 *
 * The effects of this are benign since:
 *     - The client will succeed in registering the trigger, as it is valid,
 *     - The trigger will, internally, be bound to the channel/session,
 *     - The notifications will not be sent since the client's credentials
 *       are checked against the channel at that moment.
 *
 * If this function returns a non-zero value, it means something is
 * fundamentally broken and the whole subsystem/thread will be torn down.
 *
 * If a non-fatal error occurs, just set the cmd_result to the appropriate
 * error code.
 */
static int
handle_notification_thread_command_register_trigger(struct notification_thread_state *state,
                                                    struct lttng_trigger *trigger,
                                                    bool is_trigger_anonymous,
                                                    enum lttng_error_code *cmd_result)
{
        int ret = 0;
        struct lttng_condition *condition;
        struct notification_client_list *client_list = nullptr;
        struct lttng_trigger_ht_element *trigger_ht_element = nullptr;
        struct cds_lfht_node *node;
        const char *trigger_name;
        bool free_trigger = true;
        struct lttng_evaluation *evaluation = nullptr;
        struct lttng_credentials object_creds;
        uid_t object_uid;
        gid_t object_gid;
        enum action_executor_status executor_status;
        const uint64_t trigger_tracer_token = state->trigger_id.next_tracer_token++;

        lttng::urcu::read_lock_guard read_lock;

        /* Set the trigger's tracer token. */
        lttng_trigger_set_tracer_token(trigger, trigger_tracer_token);

        if (!is_trigger_anonymous) {
                if (lttng_trigger_get_name(trigger, &trigger_name) == LTTNG_TRIGGER_STATUS_UNSET) {
                        const enum lttng_error_code ret_code =
                                generate_trigger_name(state, trigger, &trigger_name);

                        if (ret_code != LTTNG_OK) {
                                /* Fatal error. */
                                ret = -1;
                                *cmd_result = ret_code;
                                goto error;
                        }
                } else if (trigger_name_taken(state, trigger)) {
                        /* Not a fatal error. */
                        *cmd_result = LTTNG_ERR_TRIGGER_EXISTS;
                        ret = 0;
                        goto error;
                }
        } else {
                trigger_name = "(anonymous)";
        }

        condition = lttng_trigger_get_condition(trigger);
        LTTNG_ASSERT(condition);

        /* Some conditions require tracers to implement a minimal ABI version. */
        if (!condition_is_supported(condition)) {
                *cmd_result = LTTNG_ERR_NOT_SUPPORTED;
                goto error;
        }

        trigger_ht_element = zmalloc<lttng_trigger_ht_element>();
        if (!trigger_ht_element) {
                ret = -1;
                goto error;
        }

        /* Add trigger to the trigger_ht. */
        cds_lfht_node_init(&trigger_ht_element->node);
        cds_lfht_node_init(&trigger_ht_element->node_by_name_uid);
        trigger_ht_element->trigger = trigger;

        node = cds_lfht_add_unique(state->triggers_ht,
                                   lttng_condition_hash(condition),
                                   match_trigger,
                                   trigger,
                                   &trigger_ht_element->node);
        if (node != &trigger_ht_element->node) {
                /* Not a fatal error, simply report it to the client. */
                *cmd_result = LTTNG_ERR_TRIGGER_EXISTS;
                goto error_free_ht_element;
        }

        node = cds_lfht_add_unique(state->triggers_by_name_uid_ht,
                                   hash_trigger_by_name_uid(trigger),
                                   match_trigger_by_name_uid,
                                   trigger,
                                   &trigger_ht_element->node_by_name_uid);
        if (node != &trigger_ht_element->node_by_name_uid) {
                /* Internal error: add to triggers_ht should have failed. */
                ret = -1;
                goto error_free_ht_element;
        }

        /* From this point consider the trigger registered. */
        lttng_trigger_set_as_registered(trigger);

        /*
         * Some triggers might need a tracer notifier depending on its
         * condition and actions.
         */
        if (lttng_trigger_needs_tracer_notifier(trigger)) {
                enum lttng_error_code error_code;

                error_code = setup_tracer_notifier(state, trigger);
                if (error_code != LTTNG_OK) {
                        notif_thread_state_remove_trigger_ht_elem(state, trigger_ht_element);
                        if (error_code == LTTNG_ERR_NOMEM) {
                                ret = -1;
                        } else {
                                *cmd_result = error_code;
                                ret = 0;
                        }

                        goto error_free_ht_element;
                }
        }

        /*
         * The rest only applies to triggers that have a "notify" action.
         * It is not skipped as this is the only action type currently
         * supported.
         */
        if (is_trigger_action_notify(trigger)) {
                /*
                 * Find or create the client list of this condition. It may
                 * already be present if another trigger is already registered
                 * with the same condition.
                 */
                client_list = get_client_list_from_condition(state, condition);
                if (!client_list) {
                        /*
                         * No client list for this condition yet. We create new
                         * one and build it up.
                         */
                        client_list = notification_client_list_create(state, condition);
                        if (!client_list) {
                                ERR("Error creating notification client list for trigger %s",
                                    trigger->name);
                                ret = -1;
                                goto error_free_ht_element;
                        }
                }

                CDS_INIT_LIST_HEAD(&trigger_ht_element->client_list_trigger_node);

                pthread_mutex_lock(&client_list->lock);
                cds_list_add(&trigger_ht_element->client_list_trigger_node,
                             &client_list->triggers_list);
                pthread_mutex_unlock(&client_list->lock);
        }

        /*
         * Ownership of the trigger and of its wrapper was transfered to
         * the triggers_ht. Same for token ht element if necessary.
         */
        trigger_ht_element = nullptr;
        free_trigger = false;

        switch (get_condition_binding_object(condition)) {
        case LTTNG_OBJECT_TYPE_SESSION:
                /* Add the trigger to the list if it matches a known session. */
                ret = bind_trigger_to_matching_session(trigger, state);
                if (ret) {
                        goto error_free_ht_element;
                }
                break;
        case LTTNG_OBJECT_TYPE_CHANNEL:
                /*
                 * Add the trigger to list of triggers bound to the channels
                 * currently known.
                 */
                ret = bind_trigger_to_matching_channels(trigger, state);
                if (ret) {
                        goto error_free_ht_element;
                }
                break;
        case LTTNG_OBJECT_TYPE_NONE:
                break;
        default:
                ERR("Unknown object type on which to bind a newly registered trigger was encountered");
                ret = -1;
                goto error_free_ht_element;
        }

        /*
         * The new trigger's condition must be evaluated against the current
         * state.
         *
         * In the case of `notify` action, nothing preventing clients from
         * subscribing to a condition before the corresponding trigger is
         * registered, we have to evaluate this new condition right away.
         *
         * At some point, we were waiting for the next "evaluation" (e.g. on
         * reception of a channel sample) to evaluate this new condition, but
         * that was broken.
         *
         * The reason it was broken is that waiting for the next sample
         * does not allow us to properly handle transitions for edge-triggered
         * conditions.
         *
         * Consider this example: when we handle a new channel sample, we
         * evaluate each conditions twice: once with the previous state, and
         * again with the newest state. We then use those two results to
         * determine whether a state change happened: a condition was false and
         * became true. If a state change happened, we have to notify clients.
         *
         * Now, if a client subscribes to a given notification and registers
         * a trigger *after* that subscription, we have to make sure the
         * condition is evaluated at this point while considering only the
         * current state. Otherwise, the next evaluation cycle may only see
         * that the evaluations remain the same (true for samples n-1 and n) and
         * the client will never know that the condition has been met.
         */
        switch (get_condition_binding_object(condition)) {
        case LTTNG_OBJECT_TYPE_SESSION:
        {
                /* Find the session associated with the condition. */
                const auto *session_name = get_condition_session_name(condition);
                auto session_info = get_session_info_by_name(state, session_name);
                if (!session_info) {
                        /* Not an error, the session doesn't exist yet. */
                        DBG("Session not found while evaluating session condition during registration of trigger: session name = `%s`",
                            session_name);
                        ret = 0;
                        goto success;
                }

                LTTNG_OPTIONAL_SET(&object_creds.uid, session_info->uid);
                LTTNG_OPTIONAL_SET(&object_creds.gid, session_info->gid);

                ret = evaluate_session_condition(condition, session_info, nullptr, &evaluation);
                session_info_put(session_info);
                break;
        }
        case LTTNG_OBJECT_TYPE_CHANNEL:
                ret = evaluate_channel_condition_for_client(
                        condition, state, &evaluation, &object_uid, &object_gid);
                LTTNG_OPTIONAL_SET(&object_creds.uid, object_uid);
                LTTNG_OPTIONAL_SET(&object_creds.gid, object_gid);
                break;
        case LTTNG_OBJECT_TYPE_NONE:
                ret = 0;
                break;
        case LTTNG_OBJECT_TYPE_UNKNOWN:
        default:
                ret = -1;
                break;
        }

        if (ret) {
                /* Fatal error. */
                goto error_free_ht_element;
        }

        DBG("Newly registered trigger's condition evaluated to %s", evaluation ? "true" : "false");
        if (!evaluation) {
                /* Evaluation yielded nothing. Normal exit. */
                ret = 0;
                goto success;
        }

        /*
         * Ownership of `evaluation` transferred to the action executor
         * no matter the result.
         */
        executor_status = action_executor_enqueue_trigger(
                state->executor, trigger, evaluation, &object_creds, client_list);
        evaluation = nullptr;
        switch (executor_status) {
        case ACTION_EXECUTOR_STATUS_OK:
                break;
        case ACTION_EXECUTOR_STATUS_ERROR:
        case ACTION_EXECUTOR_STATUS_INVALID:
                /*
                 * TODO Add trigger identification (name/id) when
                 * it is added to the API.
                 */
                ERR("Fatal error occurred while enqueuing action associated to newly registered trigger");
                ret = -1;
                goto error_free_ht_element;
        case ACTION_EXECUTOR_STATUS_OVERFLOW:
                /*
                 * TODO Add trigger identification (name/id) when
                 * it is added to the API.
                 *
                 * Not a fatal error.
                 */
                WARN("No space left when enqueuing action associated to newly registered trigger");
                ret = 0;
                goto success;
        default:
                abort();
        }

success:
        *cmd_result = LTTNG_OK;
        DBG("Registered trigger: name = `%s`, tracer token = %" PRIu64,
            trigger_name,
            trigger_tracer_token);
        goto end;

error_free_ht_element:
        if (trigger_ht_element) {
                /* Delayed removal due to RCU constraint on delete. */
                call_rcu(&trigger_ht_element->rcu_node, free_lttng_trigger_ht_element_rcu);
        }
error:
        if (free_trigger) {
                /*
                 * Other objects might have a reference to the trigger, mark it
                 * as unregistered.
                 */
                lttng_trigger_set_as_unregistered(trigger);
                lttng_trigger_destroy(trigger);
        }
end:
        return ret;
}

static void free_lttng_trigger_ht_element_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct lttng_trigger_ht_element, rcu_node));
}

static void free_notification_trigger_tokens_ht_element_rcu(struct rcu_head *node)
{
        free(caa_container_of(node, struct notification_trigger_tokens_ht_element, rcu_node));
}

static void teardown_tracer_notifier(struct notification_thread_state *state,
                                     const struct lttng_trigger *trigger)
{
        struct cds_lfht_iter iter;
        struct notification_trigger_tokens_ht_element *trigger_tokens_ht_element;

        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (
                        state->trigger_tokens_ht, &iter, trigger_tokens_ht_element, node) {
                        if (!lttng_trigger_is_equal(trigger, trigger_tokens_ht_element->trigger)) {
                                continue;
                        }

                        event_notifier_error_accounting_unregister_event_notifier(
                                trigger_tokens_ht_element->trigger);

                        /* TODO talk to all app and remove it */
                        DBG("Removed trigger from tokens_ht");
                        cds_lfht_del(state->trigger_tokens_ht, &trigger_tokens_ht_element->node);

                        call_rcu(&trigger_tokens_ht_element->rcu_node,
                                 free_notification_trigger_tokens_ht_element_rcu);

                        break;
                }
        }
}

static void
remove_trigger_from_session_trigger_list(struct lttng_session_trigger_list *trigger_list,
                                         const struct lttng_trigger *trigger)
{
        bool found = false;
        struct lttng_trigger_list_element *trigger_element, *tmp;

        cds_list_for_each_entry_safe (trigger_element, tmp, &trigger_list->list, node) {
                if (!lttng_trigger_is_equal(trigger, trigger_element->trigger)) {
                        continue;
                }

                DBG("Removed trigger from session_triggers_ht");
                cds_list_del(&trigger_element->node);
                free(trigger_element);
                /* A trigger can only appear once per session. */
                found = true;
                break;
        }

        if (!found) {
                ERR("Failed to find trigger associated with session: session name = `%s`",
                    trigger_list->session_name);
        }

        LTTNG_ASSERT(found);
}

static int
handle_notification_thread_command_unregister_trigger(struct notification_thread_state *state,
                                                      const struct lttng_trigger *trigger,
                                                      enum lttng_error_code *_cmd_reply)
{
        struct cds_lfht_iter iter;
        struct cds_lfht_node *triggers_ht_node;
        struct notification_client_list *client_list;
        struct lttng_trigger_ht_element *trigger_ht_element = nullptr;
        const struct lttng_condition *condition = lttng_trigger_get_const_condition(trigger);
        enum lttng_error_code cmd_reply;

        lttng::urcu::read_lock_guard read_lock;

        cds_lfht_lookup(
                state->triggers_ht, lttng_condition_hash(condition), match_trigger, trigger, &iter);
        triggers_ht_node = cds_lfht_iter_get_node(&iter);
        if (!triggers_ht_node) {
                cmd_reply = LTTNG_ERR_TRIGGER_NOT_FOUND;
                goto end;
        } else {
                cmd_reply = LTTNG_OK;
        }

        trigger_ht_element =
                caa_container_of(triggers_ht_node, struct lttng_trigger_ht_element, node);

        switch (get_condition_binding_object(condition)) {
        case LTTNG_OBJECT_TYPE_CHANNEL:
        {
                struct lttng_channel_trigger_list *trigger_list;

                /*
                 * Remove trigger from channel_triggers_ht.
                 *
                 * Note that multiple channels may have matched the trigger's
                 * condition (e.g. all instances of a given channel in per-pid buffering
                 * mode).
                 *
                 * Iterate on all lists since we don't know the target channels' keys.
                 */
                cds_lfht_for_each_entry (
                        state->channel_triggers_ht, &iter, trigger_list, channel_triggers_ht_node) {
                        struct lttng_trigger_list_element *trigger_element, *tmp;

                        cds_list_for_each_entry_safe (
                                trigger_element, tmp, &trigger_list->list, node) {
                                if (!lttng_trigger_is_equal(trigger, trigger_element->trigger)) {
                                        continue;
                                }

                                DBG("Removed trigger from channel_triggers_ht");
                                cds_list_del(&trigger_element->node);
                                free(trigger_element);
                                /* A trigger can only appear once per channel */
                                break;
                        }
                }
                break;
        }
        case LTTNG_OBJECT_TYPE_SESSION:
        {
                auto session =
                        get_session_info_by_name(state, get_condition_session_name(condition));

                /* Session doesn't exist, no trigger to remove. */
                if (!session) {
                        break;
                }

                auto session_trigger_list = get_session_trigger_list(state, session->name);
                remove_trigger_from_session_trigger_list(session_trigger_list, trigger);
                session_info_put(session);
        }
        case LTTNG_OBJECT_TYPE_NONE:
                break;
        default:
                abort();
        }

        if (lttng_trigger_needs_tracer_notifier(trigger)) {
                teardown_tracer_notifier(state, trigger);
        }

        if (is_trigger_action_notify(trigger)) {
                /*
                 * Remove and release the client list from
                 * notification_trigger_clients_ht.
                 */
                client_list = get_client_list_from_condition(state, condition);
                LTTNG_ASSERT(client_list);

                pthread_mutex_lock(&client_list->lock);
                cds_list_del(&trigger_ht_element->client_list_trigger_node);
                pthread_mutex_unlock(&client_list->lock);

                /* Put new reference and the hashtable's reference. */
                notification_client_list_put(client_list);
                notification_client_list_put(client_list);
                client_list = nullptr;
        }

        /* Remove trigger from triggers_ht. */
        notif_thread_state_remove_trigger_ht_elem(state, trigger_ht_element);

        /* Release the ownership of the trigger. */
        lttng_trigger_destroy(trigger_ht_element->trigger);
        call_rcu(&trigger_ht_element->rcu_node, free_lttng_trigger_ht_element_rcu);
end:
        if (_cmd_reply) {
                *_cmd_reply = cmd_reply;
        }
        return 0;
}

static int pop_cmd_queue(struct notification_thread_handle *handle,
                         struct notification_thread_command **cmd)
{
        int ret;
        uint64_t counter;

        pthread_mutex_lock(&handle->cmd_queue.lock);
        ret = lttng_read(handle->cmd_queue.event_fd, &counter, sizeof(counter));
        if (ret != sizeof(counter)) {
                ret = -1;
                goto error_unlock;
        }

        *cmd = cds_list_first_entry(
                &handle->cmd_queue.list, struct notification_thread_command, cmd_list_node);
        cds_list_del(&((*cmd)->cmd_list_node));
        ret = 0;

error_unlock:
        pthread_mutex_unlock(&handle->cmd_queue.lock);
        return ret;
}

/* Returns 0 on success, 1 on exit requested, negative value on error. */
int handle_notification_thread_command(struct notification_thread_handle *handle,
                                       struct notification_thread_state *state)
{
        int ret;
        struct notification_thread_command *cmd;

        ret = pop_cmd_queue(handle, &cmd);
        if (ret) {
                goto error;
        }

        DBG("Received `%s` command", notification_command_type_str(cmd->type));
        switch (cmd->type) {
        case NOTIFICATION_COMMAND_TYPE_REGISTER_TRIGGER:
                ret = handle_notification_thread_command_register_trigger(
                        state,
                        cmd->parameters.register_trigger.trigger,
                        cmd->parameters.register_trigger.is_trigger_anonymous,
                        &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_UNREGISTER_TRIGGER:
                ret = handle_notification_thread_command_unregister_trigger(
                        state, cmd->parameters.unregister_trigger.trigger, &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_ADD_CHANNEL:
                ret = handle_notification_thread_command_add_channel(
                        state,
                        cmd->parameters.add_channel.session.id,
                        cmd->parameters.add_channel.channel.name,
                        cmd->parameters.add_channel.channel.domain,
                        cmd->parameters.add_channel.channel.key,
                        cmd->parameters.add_channel.channel.capacity,
                        &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_REMOVE_CHANNEL:
                ret = handle_notification_thread_command_remove_channel(
                        state,
                        cmd->parameters.remove_channel.key,
                        cmd->parameters.remove_channel.domain,
                        &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_ADD_SESSION:
                ret = handle_notification_thread_command_add_session(
                        state,
                        cmd->parameters.add_session.session_id,
                        cmd->parameters.add_session.session_name,
                        cmd->parameters.add_session.session_uid,
                        cmd->parameters.add_session.session_gid,
                        &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_REMOVE_SESSION:
                ret = handle_notification_thread_command_remove_session(
                        state, cmd->parameters.remove_session.session_id, &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_ONGOING:
        case NOTIFICATION_COMMAND_TYPE_SESSION_ROTATION_COMPLETED:
                ret = handle_notification_thread_command_session_rotation(
                        state,
                        cmd->type,
                        cmd->parameters.session_rotation.session_id,
                        cmd->parameters.session_rotation.trace_archive_chunk_id,
                        cmd->parameters.session_rotation.location,
                        &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_ADD_TRACER_EVENT_SOURCE:
                ret = handle_notification_thread_command_add_tracer_event_source(
                        state,
                        cmd->parameters.tracer_event_source.tracer_event_source_fd,
                        cmd->parameters.tracer_event_source.domain,
                        &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_REMOVE_TRACER_EVENT_SOURCE:
                ret = handle_notification_thread_command_remove_tracer_event_source(
                        state,
                        cmd->parameters.tracer_event_source.tracer_event_source_fd,
                        &cmd->reply_code);
                break;
        case NOTIFICATION_COMMAND_TYPE_LIST_TRIGGERS:
        {
                struct lttng_triggers *triggers = nullptr;

                ret = handle_notification_thread_command_list_triggers(
                        handle,
                        state,
                        cmd->parameters.list_triggers.uid,
                        &triggers,
                        &cmd->reply_code);
                cmd->reply.list_triggers.triggers = triggers;
                ret = 0;
                break;
        }
        case NOTIFICATION_COMMAND_TYPE_QUIT:
                cmd->reply_code = LTTNG_OK;
                ret = 1;
                goto end;
        case NOTIFICATION_COMMAND_TYPE_GET_TRIGGER:
        {
                struct lttng_trigger *trigger = nullptr;

                ret = handle_notification_thread_command_get_trigger(
                        state, cmd->parameters.get_trigger.trigger, &trigger, &cmd->reply_code);
                cmd->reply.get_trigger.trigger = trigger;
                break;
        }
        case NOTIFICATION_COMMAND_TYPE_CLIENT_COMMUNICATION_UPDATE:
        {
                const enum client_transmission_status client_status =
                        cmd->parameters.client_communication_update.status;
                const notification_client_id client_id =
                        cmd->parameters.client_communication_update.id;
                struct notification_client *client;

                lttng::urcu::read_lock_guard read_lock;
                client = get_client_from_id(client_id, state);

                if (!client) {
                        /*
                         * Client error was probably already picked-up by the
                         * notification thread or it has disconnected
                         * gracefully while this command was queued.
                         */
                        DBG("Failed to find notification client to update communication status, client id = %" PRIu64,
                            client_id);
                        ret = 0;
                } else {
                        ret = client_handle_transmission_status(client, client_status, state);
                }
                break;
        }
        default:
                ERR("Unknown internal command received");
                goto error_unlock;
        }

        if (ret) {
                goto error_unlock;
        }
end:
        if (cmd->is_async) {
                free(cmd);
                cmd = nullptr;
        } else {
                lttng_waiter_wake_up(&cmd->reply_waiter);
        }
        return ret;
error_unlock:
        /* Wake-up and return a fatal error to the calling thread. */
        lttng_waiter_wake_up(&cmd->reply_waiter);
        cmd->reply_code = LTTNG_ERR_FATAL;
error:
        /* Indicate a fatal error to the caller. */
        return -1;
}

static int socket_set_non_blocking(int socket)
{
        int ret, flags;

        /* Set the pipe as non-blocking. */
        ret = fcntl(socket, F_GETFL, 0);
        if (ret == -1) {
                PERROR("fcntl get socket flags");
                goto end;
        }
        flags = ret;

        ret = fcntl(socket, F_SETFL, flags | O_NONBLOCK);
        if (ret == -1) {
                PERROR("fcntl set O_NONBLOCK socket flag");
                goto end;
        }
        DBG("Client socket (fd = %i) set as non-blocking", socket);
end:
        return ret;
}

static int client_reset_inbound_state(struct notification_client *client)
{
        int ret;

        lttng_payload_clear(&client->communication.inbound.payload);

        client->communication.inbound.bytes_to_receive =
                sizeof(struct lttng_notification_channel_message);
        client->communication.inbound.msg_type = LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_UNKNOWN;
        LTTNG_SOCK_SET_UID_CRED(&client->communication.inbound.creds, -1);
        LTTNG_SOCK_SET_GID_CRED(&client->communication.inbound.creds, -1);
        ret = lttng_dynamic_buffer_set_size(&client->communication.inbound.payload.buffer,
                                            client->communication.inbound.bytes_to_receive);

        return ret;
}

int handle_notification_thread_client_connect(struct notification_thread_state *state)
{
        int ret;
        struct notification_client *client;

        DBG("Handling new notification channel client connection");

        client = zmalloc<notification_client>();
        if (!client) {
                /* Fatal error. */
                ret = -1;
                goto error;
        }

        pthread_mutex_init(&client->lock, nullptr);
        client->id = state->next_notification_client_id++;
        CDS_INIT_LIST_HEAD(&client->condition_list);
        lttng_payload_init(&client->communication.inbound.payload);
        lttng_payload_init(&client->communication.outbound.payload);
        client->communication.inbound.expect_creds = true;

        ret = client_reset_inbound_state(client);
        if (ret) {
                ERR("Failed to reset client communication's inbound state");
                ret = 0;
                goto error;
        }

        ret = lttcomm_accept_unix_sock(state->notification_channel_socket);
        if (ret < 0) {
                ERR("Failed to accept new notification channel client connection");
                ret = 0;
                goto error;
        }

        client->socket = ret;

        ret = socket_set_non_blocking(client->socket);
        if (ret) {
                ERR("Failed to set new notification channel client connection socket as non-blocking");
                goto error;
        }

        ret = lttcomm_setsockopt_creds_unix_sock(client->socket);
        if (ret < 0) {
                ERR("Failed to set socket options on new notification channel client socket");
                ret = 0;
                goto error;
        }

        client->communication.current_poll_events = CLIENT_POLL_EVENTS_IN;
        ret = lttng_poll_add(
                &state->events, client->socket, client->communication.current_poll_events);
        if (ret < 0) {
                ERR("Failed to add notification channel client socket to poll set");
                ret = 0;
                goto error;
        }
        DBG("Added new notification channel client socket (%i) to poll set", client->socket);

        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_add(state->client_socket_ht,
                             hash_client_socket(client->socket),
                             &client->client_socket_ht_node);
                cds_lfht_add(state->client_id_ht,
                             hash_client_id(client->id),
                             &client->client_id_ht_node);
        }

        return ret;

error:
        notification_client_destroy(client);
        return ret;
}

/*
 * RCU read-lock must be held by the caller.
 * Client lock must _not_ be held by the caller.
 */
static int notification_thread_client_disconnect(struct notification_client *client,
                                                 struct notification_thread_state *state)
{
        int ret;
        struct lttng_condition_list_element *condition_list_element, *tmp;

        ASSERT_RCU_READ_LOCKED();

        /* Acquire the client lock to disable its communication atomically. */
        pthread_mutex_lock(&client->lock);
        client->communication.active = false;
        cds_lfht_del(state->client_socket_ht, &client->client_socket_ht_node);
        cds_lfht_del(state->client_id_ht, &client->client_id_ht_node);
        pthread_mutex_unlock(&client->lock);

        ret = lttng_poll_del(&state->events, client->socket);
        if (ret) {
                ERR("Failed to remove client socket %d from poll set", client->socket);
        }

        /* Release all conditions to which the client was subscribed. */
        cds_list_for_each_entry_safe (condition_list_element, tmp, &client->condition_list, node) {
                (void) notification_thread_client_unsubscribe(
                        client, condition_list_element->condition, state, nullptr);
        }

        /*
         * Client no longer accessible to other threads (through the
         * client lists).
         */
        notification_client_destroy(client);
        return ret;
}

int handle_notification_thread_client_disconnect(int client_socket,
                                                 struct notification_thread_state *state)
{
        int ret = 0;
        struct notification_client *client;

        lttng::urcu::read_lock_guard read_lock;

        DBG("Closing client connection (socket fd = %i)", client_socket);
        client = get_client_from_socket(client_socket, state);
        if (!client) {
                /* Internal state corruption, fatal error. */
                ERR("Unable to find client (socket fd = %i)", client_socket);
                ret = -1;
                goto end;
        }

        ret = notification_thread_client_disconnect(client, state);
end:
        return ret;
}

int handle_notification_thread_client_disconnect_all(struct notification_thread_state *state)
{
        struct cds_lfht_iter iter;
        struct notification_client *client;
        bool error_encoutered = false;

        DBG("Closing all client connections");

        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (
                        state->client_socket_ht, &iter, client, client_socket_ht_node) {
                        int ret;

                        ret = notification_thread_client_disconnect(client, state);
                        if (ret) {
                                error_encoutered = true;
                        }
                }
        }

        return error_encoutered ? 1 : 0;
}

int handle_notification_thread_trigger_unregister_all(struct notification_thread_state *state)
{
        bool error_occurred = false;
        struct cds_lfht_iter iter;
        struct lttng_trigger_ht_element *trigger_ht_element;

        lttng::urcu::read_lock_guard read_lock;
        cds_lfht_for_each_entry (state->triggers_ht, &iter, trigger_ht_element, node) {
                int ret = handle_notification_thread_command_unregister_trigger(
                        state, trigger_ht_element->trigger, nullptr);
                if (ret) {
                        error_occurred = true;
                }
        }
        return error_occurred ? -1 : 0;
}

static bool client_has_outbound_data_left(const struct notification_client *client)
{
        const struct lttng_payload_view pv =
                lttng_payload_view_from_payload(&client->communication.outbound.payload, 0, -1);
        const bool has_data = pv.buffer.size != 0;
        const bool has_fds = lttng_payload_view_get_fd_handle_count(&pv);

        return has_data || has_fds;
}

static int client_handle_transmission_status(struct notification_client *client,
                                             enum client_transmission_status transmission_status,
                                             struct notification_thread_state *state)
{
        int ret = 0;

        switch (transmission_status) {
        case CLIENT_TRANSMISSION_STATUS_COMPLETE:
        case CLIENT_TRANSMISSION_STATUS_QUEUED:
        {
                int current_poll_events;
                int new_poll_events;
                /*
                 * We want to be notified whenever there is buffer space
                 * available to send the rest of the payload if we are
                 * waiting to send data to the client.
                 *
                 * The state of the outbound queue being sampled here is
                 * fine since:
                 *   - it is okay to wake-up "for nothing" in case we see
                 *     that data is left, but another thread succeeds in
                 *     flushing it before us when handling the client "out"
                 *     event. We will simply stop monitoring that event the next
                 *     time it wakes us up and we see no data left to be sent,
                 *   - if another thread fails to flush the entire client
                 *     outgoing queue, it will issue a "communication update"
                 *     command and cause the client's (e)poll mask to be
                 *     re-evaluated.
                 *
                 * The situation we seek to avoid would be to disable the
                 * monitoring of "out" client events indefinitely when there is
                 * data to be sent, which can't happen because of the
                 * aforementioned "communication update" mechanism.
                 */
                pthread_mutex_lock(&client->lock);
                current_poll_events = client->communication.current_poll_events;
                new_poll_events = client_has_outbound_data_left(client) ?
                        CLIENT_POLL_EVENTS_IN_OUT :
                        CLIENT_POLL_EVENTS_IN;
                client->communication.current_poll_events = new_poll_events;
                pthread_mutex_unlock(&client->lock);

                /* Update the monitored event set only if it changed. */
                if (current_poll_events != new_poll_events) {
                        ret = lttng_poll_mod(&state->events, client->socket, new_poll_events);
                        if (ret) {
                                goto end;
                        }
                }

                break;
        }
        case CLIENT_TRANSMISSION_STATUS_FAIL:
                ret = notification_thread_client_disconnect(client, state);
                if (ret) {
                        goto end;
                }
                break;
        case CLIENT_TRANSMISSION_STATUS_ERROR:
                ret = -1;
                goto end;
        default:
                abort();
        }
end:
        return ret;
}

/* Client lock must be acquired by caller. */
static enum client_transmission_status
client_flush_outgoing_queue(struct notification_client *client)
{
        ssize_t ret;
        size_t to_send_count;
        enum client_transmission_status status;
        struct lttng_payload_view pv =
                lttng_payload_view_from_payload(&client->communication.outbound.payload, 0, -1);
        const int fds_to_send_count = lttng_payload_view_get_fd_handle_count(&pv);

        ASSERT_LOCKED(client->lock);

        if (!client->communication.active) {
                status = CLIENT_TRANSMISSION_STATUS_FAIL;
                goto end;
        }

        if (pv.buffer.size == 0) {
                /*
                 * If both data and fds are equal to zero, we are in an invalid
                 * state.
                 */
                LTTNG_ASSERT(fds_to_send_count != 0);
                goto send_fds;
        }

        /* Send data. */
        to_send_count = pv.buffer.size;
        DBG("Flushing client (socket fd = %i) outgoing queue", client->socket);

        ret = lttcomm_send_unix_sock_non_block(client->socket, pv.buffer.data, to_send_count);
        if ((ret >= 0 && ret < to_send_count)) {
                DBG("Client (socket fd = %i) outgoing queue could not be completely flushed",
                    client->socket);
                to_send_count -= std::max(ret, (ssize_t) 0);

                memmove(client->communication.outbound.payload.buffer.data,
                        pv.buffer.data + pv.buffer.size - to_send_count,
                        to_send_count);
                ret = lttng_dynamic_buffer_set_size(&client->communication.outbound.payload.buffer,
                                                    to_send_count);
                if (ret) {
                        goto error;
                }

                status = CLIENT_TRANSMISSION_STATUS_QUEUED;
                goto end;
        } else if (ret < 0) {
                /* Generic error, disable the client's communication. */
                ERR("Failed to flush outgoing queue, disconnecting client (socket fd = %i)",
                    client->socket);
                client->communication.active = false;
                status = CLIENT_TRANSMISSION_STATUS_FAIL;
                goto end;
        } else {
                /*
                 * No error and flushed the queue completely.
                 *
                 * The payload buffer size is used later to
                 * check if there is notifications queued. So albeit that the
                 * direct caller knows that the transmission is complete, we
                 * need to set the buffer size to zero.
                 */
                ret = lttng_dynamic_buffer_set_size(&client->communication.outbound.payload.buffer,
                                                    0);
                if (ret) {
                        goto error;
                }
        }

send_fds:
        /* No fds to send, transmission is complete. */
        if (fds_to_send_count == 0) {
                status = CLIENT_TRANSMISSION_STATUS_COMPLETE;
                goto end;
        }

        ret = lttcomm_send_payload_view_fds_unix_sock_non_block(client->socket, &pv);
        if (ret < 0) {
                /* Generic error, disable the client's communication. */
                ERR("Failed to flush outgoing fds queue, disconnecting client (socket fd = %i)",
                    client->socket);
                client->communication.active = false;
                status = CLIENT_TRANSMISSION_STATUS_FAIL;
                goto end;
        } else if (ret == 0) {
                /* Nothing could be sent. */
                status = CLIENT_TRANSMISSION_STATUS_QUEUED;
        } else {
                /* Fd passing is an all or nothing kind of thing. */
                status = CLIENT_TRANSMISSION_STATUS_COMPLETE;
                /*
                 * The payload _fd_array count is used later to
                 * check if there is notifications queued. So although the
                 * direct caller knows that the transmission is complete, we
                 * need to clear the _fd_array for the queuing check.
                 */
                lttng_dynamic_pointer_array_clear(
                        &client->communication.outbound.payload._fd_handles);
        }

end:
        if (status == CLIENT_TRANSMISSION_STATUS_COMPLETE) {
                client->communication.outbound.queued_command_reply = false;
                client->communication.outbound.dropped_notification = false;
                lttng_payload_clear(&client->communication.outbound.payload);
        }

        return status;
error:
        return CLIENT_TRANSMISSION_STATUS_ERROR;
}

/* Client lock must _not_ be held by the caller. */
static int client_send_command_reply(struct notification_client *client,
                                     struct notification_thread_state *state,
                                     enum lttng_notification_channel_status status)
{
        int ret;
        struct lttng_notification_channel_command_reply reply = {
                .status = (int8_t) status,
        };
        struct lttng_notification_channel_message msg;
        char buffer[sizeof(msg) + sizeof(reply)];
        enum client_transmission_status transmission_status;

        msg.type = (int8_t) LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_COMMAND_REPLY;
        msg.size = sizeof(reply);
        msg.fds = 0;

        memcpy(buffer, &msg, sizeof(msg));
        memcpy(buffer + sizeof(msg), &reply, sizeof(reply));
        DBG("Send command reply (%i)", (int) status);

        pthread_mutex_lock(&client->lock);
        if (client->communication.outbound.queued_command_reply) {
                /* Protocol error. */
                goto error_unlock;
        }

        /* Enqueue buffer to outgoing queue and flush it. */
        ret = lttng_dynamic_buffer_append(
                &client->communication.outbound.payload.buffer, buffer, sizeof(buffer));
        if (ret) {
                goto error_unlock;
        }

        transmission_status = client_flush_outgoing_queue(client);

        if (client_has_outbound_data_left(client)) {
                /* Queue could not be emptied. */
                client->communication.outbound.queued_command_reply = true;
        }

        pthread_mutex_unlock(&client->lock);
        ret = client_handle_transmission_status(client, transmission_status, state);
        if (ret) {
                goto error;
        }

        return 0;
error_unlock:
        pthread_mutex_unlock(&client->lock);
error:
        return -1;
}

static int client_handle_message_unknown(struct notification_client *client,
                                         struct notification_thread_state *state
                                         __attribute__((unused)))
{
        int ret;
        /*
         * Receiving message header. The function will be called again
         * once the rest of the message as been received and can be
         * interpreted.
         */
        const struct lttng_notification_channel_message *msg;

        LTTNG_ASSERT(sizeof(*msg) == client->communication.inbound.payload.buffer.size);
        msg = (const struct lttng_notification_channel_message *)
                      client->communication.inbound.payload.buffer.data;

        if (msg->size == 0 || msg->size > DEFAULT_MAX_NOTIFICATION_CLIENT_MESSAGE_PAYLOAD_SIZE) {
                ERR("Invalid notification channel message: length = %u", msg->size);
                ret = -1;
                goto end;
        }

        switch (msg->type) {
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_SUBSCRIBE:
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_UNSUBSCRIBE:
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_HANDSHAKE:
                break;
        default:
                ret = -1;
                ERR("Invalid notification channel message: unexpected message type");
                goto end;
        }

        client->communication.inbound.bytes_to_receive = msg->size;
        client->communication.inbound.fds_to_receive = msg->fds;
        client->communication.inbound.msg_type =
                (enum lttng_notification_channel_message_type) msg->type;
        ret = lttng_dynamic_buffer_set_size(&client->communication.inbound.payload.buffer,
                                            msg->size);

        /* msg is not valid anymore due to lttng_dynamic_buffer_set_size. */
        msg = nullptr;
end:
        return ret;
}

static int client_handle_message_handshake(struct notification_client *client,
                                           struct notification_thread_state *state)
{
        int ret;
        struct lttng_notification_channel_command_handshake *handshake_client;
        const struct lttng_notification_channel_command_handshake handshake_reply = {
                .major = LTTNG_NOTIFICATION_CHANNEL_VERSION_MAJOR,
                .minor = LTTNG_NOTIFICATION_CHANNEL_VERSION_MINOR,
        };
        struct lttng_notification_channel_message msg_header;
        enum lttng_notification_channel_status status = LTTNG_NOTIFICATION_CHANNEL_STATUS_OK;
        char send_buffer[sizeof(msg_header) + sizeof(handshake_reply)];

        msg_header.type = LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_HANDSHAKE;
        msg_header.size = sizeof(handshake_reply);
        msg_header.fds = 0;

        memcpy(send_buffer, &msg_header, sizeof(msg_header));
        memcpy(send_buffer + sizeof(msg_header), &handshake_reply, sizeof(handshake_reply));

        handshake_client = (struct lttng_notification_channel_command_handshake *)
                                   client->communication.inbound.payload.buffer.data;
        client->major = handshake_client->major;
        client->minor = handshake_client->minor;
        if (!client->communication.inbound.creds_received) {
                ERR("No credentials received from client");
                ret = -1;
                goto end;
        }

        client->uid = LTTNG_SOCK_GET_UID_CRED(&client->communication.inbound.creds);
        client->gid = LTTNG_SOCK_GET_GID_CRED(&client->communication.inbound.creds);
        client->is_sessiond = LTTNG_SOCK_GET_PID_CRED(&client->communication.inbound.creds) ==
                getpid();
        DBG("Received handshake from client: uid = %u, gid = %u, protocol version = %i.%i, client is sessiond = %s",
            client->uid,
            client->gid,
            (int) client->major,
            (int) client->minor,
            client->is_sessiond ? "true" : "false");

        if (handshake_client->major != LTTNG_NOTIFICATION_CHANNEL_VERSION_MAJOR) {
                status = LTTNG_NOTIFICATION_CHANNEL_STATUS_UNSUPPORTED_VERSION;
        }

        pthread_mutex_lock(&client->lock);
        /* Outgoing queue will be flushed when the command reply is sent. */
        ret = lttng_dynamic_buffer_append(
                &client->communication.outbound.payload.buffer, send_buffer, sizeof(send_buffer));
        if (ret) {
                ERR("Failed to send protocol version to notification channel client");
                goto end_unlock;
        }

        client->validated = true;
        client->communication.active = true;
        pthread_mutex_unlock(&client->lock);

        /* Set reception state to receive the next message header. */
        ret = client_reset_inbound_state(client);
        if (ret) {
                ERR("Failed to reset client communication's inbound state");
                goto end;
        }

        /* Flushes the outgoing queue. */
        ret = client_send_command_reply(client, state, status);
        if (ret) {
                ERR("Failed to send reply to notification channel client");
                goto end;
        }

        goto end;
end_unlock:
        pthread_mutex_unlock(&client->lock);
end:
        return ret;
}

static int client_handle_message_subscription(struct notification_client *client,
                                              enum lttng_notification_channel_message_type msg_type,
                                              struct notification_thread_state *state)
{
        int ret;
        struct lttng_condition *condition;
        enum lttng_notification_channel_status status = LTTNG_NOTIFICATION_CHANNEL_STATUS_OK;
        struct lttng_payload_view condition_view =
                lttng_payload_view_from_payload(&client->communication.inbound.payload, 0, -1);
        size_t expected_condition_size;

        /*
         * No need to lock client to sample the inbound state as the only
         * other thread accessing clients (action executor) only uses the
         * outbound state.
         */
        expected_condition_size = client->communication.inbound.payload.buffer.size;
        ret = lttng_condition_create_from_payload(&condition_view, &condition);
        if (ret != expected_condition_size) {
                ERR("Malformed condition received from client");
                goto end;
        }

        /* Ownership of condition is always transferred. */
        if (msg_type == LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_SUBSCRIBE) {
                ret = notification_thread_client_subscribe(client, condition, state, &status);
        } else {
                ret = notification_thread_client_unsubscribe(client, condition, state, &status);
        }

        if (ret) {
                goto end;
        }

        /* Set reception state to receive the next message header. */
        ret = client_reset_inbound_state(client);
        if (ret) {
                ERR("Failed to reset client communication's inbound state");
                goto end;
        }

        ret = client_send_command_reply(client, state, status);
        if (ret) {
                ERR("Failed to send reply to notification channel client");
                goto end;
        }

end:
        return ret;
}

static int client_dispatch_message(struct notification_client *client,
                                   struct notification_thread_state *state)
{
        int ret = 0;

        if (client->communication.inbound.msg_type !=
                    LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_HANDSHAKE &&
            client->communication.inbound.msg_type !=
                    LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_UNKNOWN &&
            !client->validated) {
                WARN("client attempted a command before handshake");
                ret = -1;
                goto end;
        }

        switch (client->communication.inbound.msg_type) {
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_UNKNOWN:
        {
                ret = client_handle_message_unknown(client, state);
                break;
        }
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_HANDSHAKE:
        {
                ret = client_handle_message_handshake(client, state);
                break;
        }
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_SUBSCRIBE:
        case LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_UNSUBSCRIBE:
        {
                ret = client_handle_message_subscription(
                        client, client->communication.inbound.msg_type, state);
                break;
        }
        default:
                abort();
        }
end:
        return ret;
}

/* Incoming data from client. */
int handle_notification_thread_client_in(struct notification_thread_state *state, int socket)
{
        int ret = 0;
        struct notification_client *client;
        ssize_t recv_ret;
        size_t offset;

        lttng::urcu::read_lock_guard read_lock;
        client = get_client_from_socket(socket, state);
        if (!client) {
                /* Internal error, abort. */
                ret = -1;
                goto end;
        }

        if (client->communication.inbound.bytes_to_receive == 0 &&
            client->communication.inbound.fds_to_receive != 0) {
                /* Only FDs left to receive. */
                goto receive_fds;
        }

        offset = client->communication.inbound.payload.buffer.size -
                client->communication.inbound.bytes_to_receive;
        if (client->communication.inbound.expect_creds) {
                recv_ret = lttcomm_recv_creds_unix_sock(
                        socket,
                        client->communication.inbound.payload.buffer.data + offset,
                        client->communication.inbound.bytes_to_receive,
                        &client->communication.inbound.creds);
                if (recv_ret > 0) {
                        client->communication.inbound.expect_creds = false;
                        client->communication.inbound.creds_received = true;
                }
        } else {
                recv_ret = lttcomm_recv_unix_sock_non_block(
                        socket,
                        client->communication.inbound.payload.buffer.data + offset,
                        client->communication.inbound.bytes_to_receive);
        }
        if (recv_ret >= 0) {
                client->communication.inbound.bytes_to_receive -= recv_ret;
        } else {
                goto error_disconnect_client;
        }

        if (client->communication.inbound.bytes_to_receive != 0) {
                /* Message incomplete wait for more data. */
                ret = 0;
                goto end;
        }

receive_fds:
        LTTNG_ASSERT(client->communication.inbound.bytes_to_receive == 0);

        /* Receive fds. */
        if (client->communication.inbound.fds_to_receive != 0) {
                ret = lttcomm_recv_payload_fds_unix_sock_non_block(
                        client->socket,
                        client->communication.inbound.fds_to_receive,
                        &client->communication.inbound.payload);
                if (ret > 0) {
                        /*
                         * Fds received. non blocking fds passing is all
                         * or nothing.
                         */
                        ssize_t expected_size;

                        expected_size = sizeof(int) * client->communication.inbound.fds_to_receive;
                        LTTNG_ASSERT(ret == expected_size);
                        client->communication.inbound.fds_to_receive = 0;
                } else if (ret == 0) {
                        /* Received nothing. */
                        ret = 0;
                        goto end;
                } else {
                        goto error_disconnect_client;
                }
        }

        /* At this point the message is complete.*/
        LTTNG_ASSERT(client->communication.inbound.bytes_to_receive == 0 &&
                     client->communication.inbound.fds_to_receive == 0);
        ret = client_dispatch_message(client, state);
        if (ret) {
                /*
                 * Only returns an error if this client must be
                 * disconnected.
                 */
                goto error_disconnect_client;
        }

end:
        return ret;

error_disconnect_client:
        ret = notification_thread_client_disconnect(client, state);
        goto end;
}

/* Client ready to receive outgoing data. */
int handle_notification_thread_client_out(struct notification_thread_state *state, int socket)
{
        int ret;
        struct notification_client *client;
        enum client_transmission_status transmission_status;

        lttng::urcu::read_lock_guard read_lock;
        client = get_client_from_socket(socket, state);
        if (!client) {
                /* Internal error, abort. */
                ret = -1;
                goto end;
        }

        pthread_mutex_lock(&client->lock);
        if (!client_has_outbound_data_left(client)) {
                /*
                 * A client "out" event can be received when no payload is left
                 * to send under some circumstances.
                 *
                 * Many threads can flush a client's outgoing queue and, if they
                 * had to queue their message (socket was full), will use the
                 * "communication update" command to signal the (e)poll thread
                 * to monitor for space being made available in the socket.
                 *
                 * Commands are sent over an internal pipe serviced by the same
                 * thread as the client sockets.
                 *
                 * When space is made available in the socket, there is a race
                 * between the (e)poll thread and the other threads that may
                 * wish to use the client's socket to flush its outgoing queue.
                 *
                 * A non-(e)poll thread may attempt (and succeed) in flushing
                 * the queue before the (e)poll thread gets a chance to service
                 * the client's "out" event.
                 *
                 * In this situation, the (e)poll thread processing the client
                 * out event will see an empty payload: there is nothing to do
                 * except unsubscribing (e)poll "out" events.
                 *
                 * Note that this thread is the (e)poll thread so it can modify
                 * the (e)poll mask directly without using a communication
                 * update command. Other threads that flush the outgoing queue
                 * will use the "communication update" command to wake up this
                 * thread and force it to monitor "out" events.
                 *
                 * When other threads succeed in emptying the outgoing queue,
                 * they don't need to update the (e)poll mask: if the "out"
                 * event is monitored, it will fire once and the (e)poll
                 * thread will reach this condition, causing the event to
                 * stop being monitored.
                 */
                transmission_status = CLIENT_TRANSMISSION_STATUS_COMPLETE;
        } else {
                transmission_status = client_flush_outgoing_queue(client);
        }
        pthread_mutex_unlock(&client->lock);

        ret = client_handle_transmission_status(client, transmission_status, state);
        if (ret) {
                goto end;
        }
end:
        return ret;
}

static bool evaluate_buffer_usage_condition(const struct lttng_condition *condition,
                                            const struct channel_state_sample *sample,
                                            uint64_t buffer_capacity)
{
        bool result = false;
        uint64_t threshold;
        enum lttng_condition_type condition_type;
        const struct lttng_condition_buffer_usage *use_condition =
                lttng::utils::container_of(condition, &lttng_condition_buffer_usage::parent);

        if (use_condition->threshold_bytes.set) {
                threshold = use_condition->threshold_bytes.value;
        } else {
                /*
                 * Threshold was expressed as a ratio.
                 *
                 * TODO the threshold (in bytes) of conditions expressed
                 * as a ratio of total buffer size could be cached to
                 * forego this double-multiplication or it could be performed
                 * as fixed-point math.
                 *
                 * Note that caching should accommodates the case where the
                 * condition applies to multiple channels (i.e. don't assume
                 * that all channels matching my_chann* have the same size...)
                 */
                threshold = (uint64_t) (use_condition->threshold_ratio.value *
                                        (double) buffer_capacity);
        }

        condition_type = lttng_condition_get_type(condition);
        if (condition_type == LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW) {
                DBG("Low buffer usage condition being evaluated: threshold = %" PRIu64
                    ", highest usage = %" PRIu64,
                    threshold,
                    sample->highest_usage);

                /*
                 * The low condition should only be triggered once _all_ of the
                 * streams in a channel have gone below the "low" threshold.
                 */
                if (sample->highest_usage <= threshold) {
                        result = true;
                }
        } else {
                DBG("High buffer usage condition being evaluated: threshold = %" PRIu64
                    ", highest usage = %" PRIu64,
                    threshold,
                    sample->highest_usage);

                /*
                 * For high buffer usage scenarios, we want to trigger whenever
                 * _any_ of the streams has reached the "high" threshold.
                 */
                if (sample->highest_usage >= threshold) {
                        result = true;
                }
        }

        return result;
}

static int evaluate_buffer_condition(const struct lttng_condition *condition,
                                     struct lttng_evaluation **evaluation,
                                     const struct notification_thread_state *state
                                     __attribute__((unused)),
                                     const struct channel_state_sample *previous_sample,
                                     const struct channel_state_sample *latest_sample,
                                     struct channel_info *channel_info)
{
        int ret = 0;
        enum lttng_condition_type condition_type;
        const bool previous_sample_available = !!previous_sample;
        bool previous_sample_result = false;
        bool latest_sample_result;

        condition_type = lttng_condition_get_type(condition);

        switch (condition_type) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
                if (caa_likely(previous_sample_available)) {
                        previous_sample_result = evaluate_buffer_usage_condition(
                                condition, previous_sample, channel_info->capacity);
                }
                latest_sample_result = evaluate_buffer_usage_condition(
                        condition, latest_sample, channel_info->capacity);
                break;
        default:
                /* Unknown condition type; internal error. */
                abort();
        }

        if (!latest_sample_result || (previous_sample_result == latest_sample_result)) {
                /*
                 * Only trigger on a condition evaluation transition.
                 *
                 * NOTE: This edge-triggered logic may not be appropriate for
                 * future condition types.
                 */
                goto end;
        }

        if (!evaluation || !latest_sample_result) {
                goto end;
        }

        switch (condition_type) {
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_LOW:
        case LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH:
                *evaluation = lttng_evaluation_buffer_usage_create(
                        condition_type, latest_sample->highest_usage, channel_info->capacity);
                break;
        default:
                abort();
        }

        if (!*evaluation) {
                ret = -1;
                goto end;
        }
end:
        return ret;
}

static int client_notification_overflow(struct notification_client *client)
{
        int ret = 0;
        struct lttng_notification_channel_message msg;

        msg.type = (int8_t) LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_NOTIFICATION_DROPPED;
        msg.size = 0;
        msg.fds = 0;

        ASSERT_LOCKED(client->lock);

        DBG("Dropping notification addressed to client (socket fd = %i)", client->socket);
        if (client->communication.outbound.dropped_notification) {
                /*
                 * The client already has a "notification dropped" message
                 * in its outgoing queue. Nothing to do since all
                 * of those messages are coalesced.
                 */
                goto end;
        }

        client->communication.outbound.dropped_notification = true;
        ret = lttng_dynamic_buffer_append(
                &client->communication.outbound.payload.buffer, &msg, sizeof(msg));
        if (ret) {
                PERROR("Failed to enqueue \"dropped notification\" message in client's (socket fd = %i) outgoing queue",
                       client->socket);
        }
end:
        return ret;
}

static int client_handle_transmission_status_wrapper(struct notification_client *client,
                                                     enum client_transmission_status status,
                                                     void *user_data)
{
        return client_handle_transmission_status(
                client, status, (struct notification_thread_state *) user_data);
}

static int send_evaluation_to_clients(const struct lttng_trigger *trigger,
                                      const struct lttng_evaluation *evaluation,
                                      struct notification_client_list *client_list,
                                      struct notification_thread_state *state,
                                      uid_t object_uid,
                                      gid_t object_gid)
{
        const struct lttng_credentials creds = {
                .uid = LTTNG_OPTIONAL_INIT_VALUE(object_uid),
                .gid = LTTNG_OPTIONAL_INIT_VALUE(object_gid),
        };

        return notification_client_list_send_evaluation(client_list,
                                                        trigger,
                                                        evaluation,
                                                        &creds,
                                                        client_handle_transmission_status_wrapper,
                                                        state);
}

/*
 * Permission checks relative to notification channel clients are performed
 * here. Notice how object, client, and trigger credentials are involved in
 * this check.
 *
 * The `object` credentials are the credentials associated with the "subject"
 * of a condition. For instance, a `rotation completed` condition applies
 * to a session. When that condition is met, it will produce an evaluation
 * against a session. Hence, in this case, the `object` credentials are the
 * credentials of the "subject" session.
 *
 * The `trigger` credentials are the credentials of the user that registered the
 * trigger.
 *
 * The `client` credentials are the credentials of the user that created a given
 * notification channel.
 *
 * In terms of visibility, it is expected that non-privilieged users can only
 * register triggers against "their" objects (their own sessions and
 * applications they are allowed to interact with). They can then open a
 * notification channel and subscribe to notifications associated with those
 * triggers.
 *
 * As for privilieged users, they can register triggers against the objects of
 * other users. They can then subscribe to the notifications associated to their
 * triggers. Privilieged users _can't_ subscribe to the notifications of
 * triggers owned by other users; they must create their own triggers.
 *
 * This is more a concern of usability than security. It would be difficult for
 * a root user reliably subscribe to a specific set of conditions without
 * interference from external users (those could, for instance, unregister
 * their triggers).
 */
int notification_client_list_send_evaluation(struct notification_client_list *client_list,
                                             const struct lttng_trigger *trigger,
                                             const struct lttng_evaluation *evaluation,
                                             const struct lttng_credentials *source_object_creds,
                                             report_client_transmission_result_cb client_report,
                                             void *user_data)
{
        int ret = 0;
        struct lttng_payload msg_payload;
        struct notification_client_list_element *client_list_element, *tmp;
        const struct lttng_notification notification = {
                .trigger = (struct lttng_trigger *) trigger,
                .evaluation = (struct lttng_evaluation *) evaluation,
        };
        struct lttng_notification_channel_message msg_header;
        const struct lttng_credentials *trigger_creds = lttng_trigger_get_credentials(trigger);

        lttng_payload_init(&msg_payload);

        msg_header.type = (int8_t) LTTNG_NOTIFICATION_CHANNEL_MESSAGE_TYPE_NOTIFICATION;
        msg_header.size = 0;
        msg_header.fds = 0;

        ret = lttng_dynamic_buffer_append(&msg_payload.buffer, &msg_header, sizeof(msg_header));
        if (ret) {
                goto end;
        }

        ret = lttng_notification_serialize(&notification, &msg_payload);
        if (ret) {
                ERR("Failed to serialize notification");
                ret = -1;
                goto end;
        }

        /* Update payload size. */
        ((struct lttng_notification_channel_message *) msg_payload.buffer.data)->size =
                (uint32_t) (msg_payload.buffer.size - sizeof(msg_header));

        /* Update the payload number of fds. */
        {
                const struct lttng_payload_view pv =
                        lttng_payload_view_from_payload(&msg_payload, 0, -1);

                ((struct lttng_notification_channel_message *) msg_payload.buffer.data)->fds =
                        (uint32_t) lttng_payload_view_get_fd_handle_count(&pv);
        }

        pthread_mutex_lock(&client_list->lock);
        cds_list_for_each_entry_safe (client_list_element, tmp, &client_list->clients_list, node) {
                enum client_transmission_status transmission_status;
                struct notification_client *client = client_list_element->client;

                ret = 0;
                pthread_mutex_lock(&client->lock);
                if (!client->communication.active) {
                        /*
                         * Skip inactive client (protocol error or
                         * disconnecting).
                         */
                        DBG("Skipping client at it is marked as inactive");
                        goto skip_client;
                }

                if (lttng_trigger_is_hidden(trigger) && !client->is_sessiond) {
                        /*
                         * Notifications resulting from an hidden trigger are
                         * only sent to the session daemon.
                         */
                        DBG("Skipping client as the trigger is hidden and the client is not the session daemon");
                        goto skip_client;
                }

                if (source_object_creds) {
                        if (client->uid != lttng_credentials_get_uid(source_object_creds) &&
                            client->gid != lttng_credentials_get_gid(source_object_creds) &&
                            client->uid != 0) {
                                /*
                                 * Client is not allowed to monitor this
                                 * object.
                                 */
                                DBG("Skipping client at it does not have the object permission to receive notification for this trigger");
                                goto skip_client;
                        }
                }

                if (client->uid != lttng_credentials_get_uid(trigger_creds)) {
                        DBG("Skipping client at it does not have the permission to receive notification for this trigger");
                        goto skip_client;
                }

                DBG("Sending notification to client (fd = %i, %zu bytes)",
                    client->socket,
                    msg_payload.buffer.size);

                if (client_has_outbound_data_left(client)) {
                        /*
                         * Outgoing data is already buffered for this client;
                         * drop the notification and enqueue a "dropped
                         * notification" message if this is the first dropped
                         * notification since the socket spilled-over to the
                         * queue.
                         */
                        ret = client_notification_overflow(client);
                        if (ret) {
                                /* Fatal error. */
                                goto skip_client;
                        }
                }

                ret = lttng_payload_copy(&msg_payload, &client->communication.outbound.payload);
                if (ret) {
                        /* Fatal error. */
                        goto skip_client;
                }

                transmission_status = client_flush_outgoing_queue(client);
                pthread_mutex_unlock(&client->lock);
                ret = client_report(client, transmission_status, user_data);
                if (ret) {
                        /* Fatal error. */
                        goto end_unlock_list;
                }

                continue;

        skip_client:
                pthread_mutex_unlock(&client->lock);
                if (ret) {
                        /* Fatal error. */
                        goto end_unlock_list;
                }
        }
        ret = 0;

end_unlock_list:
        pthread_mutex_unlock(&client_list->lock);
end:
        lttng_payload_reset(&msg_payload);
        return ret;
}

static struct lttng_event_notifier_notification *
recv_one_event_notifier_notification(int notification_pipe_read_fd, enum lttng_domain_type domain)
{
        int ret;
        uint64_t token;
        struct lttng_event_notifier_notification *notification = nullptr;
        char *capture_buffer = nullptr;
        size_t capture_buffer_size;
        void *reception_buffer;
        size_t reception_size;

        struct lttng_ust_abi_event_notifier_notification ust_notification;
        struct lttng_kernel_abi_event_notifier_notification kernel_notification;

        /* Init lttng_event_notifier_notification */
        switch (domain) {
        case LTTNG_DOMAIN_UST:
                reception_buffer = (void *) &ust_notification;
                reception_size = sizeof(ust_notification);
                break;
        case LTTNG_DOMAIN_KERNEL:
                reception_buffer = (void *) &kernel_notification;
                reception_size = sizeof(kernel_notification);
                break;
        default:
                abort();
        }

        /*
         * The monitoring pipe only holds messages smaller than PIPE_BUF,
         * ensuring that read/write of tracer notifications are atomic.
         */
        ret = lttng_read(notification_pipe_read_fd, reception_buffer, reception_size);
        if (ret != reception_size) {
                PERROR("Failed to read from event source notification pipe: fd = %d, size to read = %zu, ret = %d",
                       notification_pipe_read_fd,
                       reception_size,
                       ret);
                ret = -1;
                goto end;
        }

        switch (domain) {
        case LTTNG_DOMAIN_UST:
                token = ust_notification.token;
                capture_buffer_size = ust_notification.capture_buf_size;
                break;
        case LTTNG_DOMAIN_KERNEL:
                token = kernel_notification.token;
                capture_buffer_size = kernel_notification.capture_buf_size;
                break;
        default:
                abort();
        }

        if (capture_buffer_size == 0) {
                capture_buffer = nullptr;
                goto skip_capture;
        }

        if (capture_buffer_size > MAX_CAPTURE_SIZE) {
                ERR("Event notifier has a capture payload size which exceeds the maximum allowed size: capture_payload_size = %zu bytes, max allowed size = %d bytes",
                    capture_buffer_size,
                    MAX_CAPTURE_SIZE);
                goto end;
        }

        capture_buffer = calloc<char>(capture_buffer_size);
        if (!capture_buffer) {
                ERR("Failed to allocate capture buffer");
                goto end;
        }

        /* Fetch additional payload (capture). */
        ret = lttng_read(notification_pipe_read_fd, capture_buffer, capture_buffer_size);
        if (ret != capture_buffer_size) {
                ERR("Failed to read from event source pipe (fd = %i)", notification_pipe_read_fd);
                goto end;
        }

skip_capture:
        notification = lttng_event_notifier_notification_create(
                token, domain, capture_buffer, capture_buffer_size);
        if (notification == nullptr) {
                goto end;
        }

        /*
         * Ownership transfered to the lttng_event_notifier_notification object.
         */
        capture_buffer = nullptr;

end:
        free(capture_buffer);
        return notification;
}

static int
dispatch_one_event_notifier_notification(struct notification_thread_state *state,
                                         struct lttng_event_notifier_notification *notification)
{
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;
        struct notification_trigger_tokens_ht_element *element;
        struct lttng_evaluation *evaluation = nullptr;
        enum action_executor_status executor_status;
        struct notification_client_list *client_list = nullptr;
        int ret;
        unsigned int capture_count = 0;

        /* Find triggers associated with this token. */
        lttng::urcu::read_lock_guard read_lock;
        cds_lfht_lookup(state->trigger_tokens_ht,
                        hash_key_u64(&notification->tracer_token, lttng_ht_seed),
                        match_trigger_token,
                        &notification->tracer_token,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (caa_unlikely(!node)) {
                /*
                 * This is not an error, slow consumption of the tracer
                 * notifications can lead to situations where a trigger is
                 * removed but we still get tracer notifications matching a
                 * trigger that no longer exists.
                 */
                ret = 0;
                goto end_unlock;
        }

        element = caa_container_of(node, struct notification_trigger_tokens_ht_element, node);

        if (lttng_condition_event_rule_matches_get_capture_descriptor_count(
                    lttng_trigger_get_const_condition(element->trigger), &capture_count) !=
            LTTNG_CONDITION_STATUS_OK) {
                ERR("Failed to get capture count");
                ret = -1;
                goto end;
        }

        if (!notification->capture_buffer && capture_count != 0) {
                ERR("Expected capture but capture buffer is null");
                ret = -1;
                goto end;
        }

        evaluation = lttng_evaluation_event_rule_matches_create(
                lttng::utils::container_of(lttng_trigger_get_const_condition(element->trigger),
                                           &lttng_condition_event_rule_matches::parent),
                notification->capture_buffer,
                notification->capture_buf_size,
                false);

        if (evaluation == nullptr) {
                ERR("Failed to create event rule matches evaluation while creating and enqueuing action executor job");
                ret = -1;
                goto end_unlock;
        }

        client_list = get_client_list_from_condition(
                state, lttng_trigger_get_const_condition(element->trigger));
        executor_status = action_executor_enqueue_trigger(
                state->executor, element->trigger, evaluation, nullptr, client_list);
        switch (executor_status) {
        case ACTION_EXECUTOR_STATUS_OK:
                ret = 0;
                break;
        case ACTION_EXECUTOR_STATUS_OVERFLOW:
        {
                struct notification_client_list_element *client_list_element, *tmp;

                /*
                 * Not a fatal error; this is expected and simply means the
                 * executor has too much work queued already.
                 */
                ret = 0;

                /* No clients subscribed to notifications for this trigger. */
                if (!client_list) {
                        break;
                }

                /* Warn clients that a notification (or more) was dropped. */
                pthread_mutex_lock(&client_list->lock);
                cds_list_for_each_entry_safe (
                        client_list_element, tmp, &client_list->clients_list, node) {
                        enum client_transmission_status transmission_status;
                        struct notification_client *client = client_list_element->client;

                        pthread_mutex_lock(&client->lock);
                        ret = client_notification_overflow(client);
                        if (ret) {
                                /* Fatal error. */
                                goto next_client;
                        }

                        transmission_status = client_flush_outgoing_queue(client);
                        ret = client_handle_transmission_status(client, transmission_status, state);
                        if (ret) {
                                /* Fatal error. */
                                goto next_client;
                        }
                next_client:
                        pthread_mutex_unlock(&client->lock);
                        if (ret) {
                                break;
                        }
                }

                pthread_mutex_unlock(&client_list->lock);
                break;
        }
        case ACTION_EXECUTOR_STATUS_INVALID:
        case ACTION_EXECUTOR_STATUS_ERROR:
                /* Fatal error, shut down everything. */
                ERR("Fatal error encoutered while enqueuing action to the action executor");
                ret = -1;
                goto end_unlock;
        default:
                /* Unhandled error. */
                abort();
        }

end_unlock:
        notification_client_list_put(client_list);
end:
        return ret;
}

static int handle_one_event_notifier_notification(struct notification_thread_state *state,
                                                  int pipe,
                                                  enum lttng_domain_type domain)
{
        int ret = 0;
        struct lttng_event_notifier_notification *notification = nullptr;

        notification = recv_one_event_notifier_notification(pipe, domain);
        if (notification == nullptr) {
                /* Reception failed, don't consider it fatal. */
                ERR("Error receiving an event notifier notification from tracer: fd = %i, domain = %s",
                    pipe,
                    lttng_domain_type_str(domain));
                goto end;
        }

        ret = dispatch_one_event_notifier_notification(state, notification);
        if (ret) {
                ERR("Error dispatching an event notifier notification from tracer: fd = %i, domain = %s",
                    pipe,
                    lttng_domain_type_str(domain));
                goto end;
        }

end:
        lttng_event_notifier_notification_destroy(notification);
        return ret;
}

int handle_notification_thread_event_notification(struct notification_thread_state *state,
                                                  int pipe,
                                                  enum lttng_domain_type domain)
{
        return handle_one_event_notifier_notification(state, pipe, domain);
}

int handle_notification_thread_channel_sample(struct notification_thread_state *state,
                                              int pipe,
                                              enum lttng_domain_type domain)
{
        int ret = 0;
        struct lttcomm_consumer_channel_monitor_msg sample_msg;
        struct channel_info *channel_info = nullptr;
        struct cds_lfht_node *node;
        struct cds_lfht_iter iter;
        struct lttng_channel_trigger_list *channel_trigger_list;
        struct lttng_session_trigger_list *session_trigger_list;
        struct lttng_trigger_list_element *trigger_list_element;
        bool previous_sample_available = false;
        struct channel_state_sample channel_previous_sample, channel_new_sample;
        struct session_state_sample session_new_sample;
        struct lttng_credentials channel_creds = {};
        struct lttng_credentials session_creds = {};
        struct session_info *session;
        lttng::urcu::read_lock_guard read_lock;

        /*
         * The monitoring pipe only holds messages smaller than PIPE_BUF,
         * ensuring that read/write of sampling messages are atomic.
         */
        ret = lttng_read(pipe, &sample_msg, sizeof(sample_msg));
        if (ret != sizeof(sample_msg)) {
                ERR("Failed to read from monitoring pipe (fd = %i)", pipe);
                ret = -1;
                goto end;
        }

        ret = 0;
        channel_new_sample.key.key = sample_msg.key;
        channel_new_sample.key.domain = domain;
        channel_new_sample.highest_usage = sample_msg.highest;
        channel_new_sample.lowest_usage = sample_msg.lowest;

        session = get_session_info_by_id(state, sample_msg.session_id);
        if (!session) {
                DBG("Received a sample for an unknown session from consumerd: session id = %" PRIu64,
                    sample_msg.session_id);
                goto end_unlock;
        }

        session_new_sample = session->last_state_sample;
        session_new_sample.consumed_data_size += sample_msg.consumed_since_last_sample;
        session_creds = {
                .uid = LTTNG_OPTIONAL_INIT_VALUE(session->uid),
                .gid = LTTNG_OPTIONAL_INIT_VALUE(session->gid),
        };

        session_trigger_list = get_session_trigger_list(state, session->name);
        LTTNG_ASSERT(session_trigger_list);
        cds_list_for_each_entry (trigger_list_element, &session_trigger_list->list, node) {
                const struct lttng_condition *condition;
                struct lttng_trigger *trigger;
                struct notification_client_list *client_list = nullptr;
                struct lttng_evaluation *evaluation = nullptr;
                enum action_executor_status executor_status;

                ret = 0;
                trigger = trigger_list_element->trigger;
                condition = lttng_trigger_get_const_condition(trigger);
                LTTNG_ASSERT(condition);

                ret = evaluate_session_condition(
                        condition, session, &session_new_sample, &evaluation);
                if (caa_unlikely(ret)) {
                        break;
                }

                if (caa_likely(!evaluation)) {
                        continue;
                }

                /*
                 * Ownership of `evaluation` transferred to the action executor
                 * no matter the result. The callee acquires a reference to the
                 * client list: we can release our own.
                 */
                client_list = get_client_list_from_condition(state, condition);
                executor_status = action_executor_enqueue_trigger(
                        state->executor, trigger, evaluation, &session_creds, client_list);
                notification_client_list_put(client_list);
                evaluation = nullptr;
                switch (executor_status) {
                case ACTION_EXECUTOR_STATUS_OK:
                        break;
                case ACTION_EXECUTOR_STATUS_ERROR:
                case ACTION_EXECUTOR_STATUS_INVALID:
                        /*
                         * TODO Add trigger identification (name/id) when
                         * it is added to the API.
                         */
                        ERR("Fatal error occurred while enqueuing action associated with buffer-condition trigger");
                        ret = -1;
                        goto end_unlock;
                case ACTION_EXECUTOR_STATUS_OVERFLOW:
                        /*
                         * TODO Add trigger identification (name/id) when
                         * it is added to the API.
                         *
                         * Not a fatal error.
                         */
                        WARN("No space left when enqueuing action associated with buffer-condition trigger");
                        ret = 0;
                        goto end_unlock;
                default:
                        abort();
                }
        }

        /* Retrieve the channel's informations */
        cds_lfht_lookup(state->channels_ht,
                        hash_channel_key(&channel_new_sample.key),
                        match_channel_info,
                        &channel_new_sample.key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (caa_unlikely(!node)) {
                /*
                 * Not an error since the consumer can push a sample to the pipe
                 * and the rest of the session daemon could notify us of the
                 * channel's destruction before we get a chance to process that
                 * sample.
                 */
                DBG("Received a sample for an unknown channel from consumerd, key = %" PRIu64
                    " in %s domain",
                    channel_new_sample.key.key,
                    lttng_domain_type_str(domain));
                goto end_unlock;
        }

        channel_info = caa_container_of(node, struct channel_info, channels_ht_node);
        DBG("Handling channel sample for channel %s (key = %" PRIu64
            ") in session %s (highest usage = %" PRIu64 ", lowest usage = %" PRIu64
            ", consumed since last sample = %" PRIu64 ")",
            channel_info->name,
            channel_new_sample.key.key,
            channel_info->session_info->name,
            channel_new_sample.highest_usage,
            channel_new_sample.lowest_usage,
            sample_msg.consumed_since_last_sample);

        /* Retrieve the channel's last sample, if it exists, and update it. */
        cds_lfht_lookup(state->channel_state_ht,
                        hash_channel_key(&channel_new_sample.key),
                        match_channel_state_sample,
                        &channel_new_sample.key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        if (caa_likely(node)) {
                struct channel_state_sample *stored_sample;

                /* Update the sample stored. */
                stored_sample =
                        caa_container_of(node, struct channel_state_sample, channel_state_ht_node);

                memcpy(&channel_previous_sample, stored_sample, sizeof(channel_previous_sample));
                stored_sample->highest_usage = channel_new_sample.highest_usage;
                stored_sample->lowest_usage = channel_new_sample.lowest_usage;
                previous_sample_available = true;
        } else {
                /*
                 * This is the channel's first sample, allocate space for and
                 * store the new sample.
                 */
                struct channel_state_sample *stored_sample;

                stored_sample = zmalloc<channel_state_sample>();
                if (!stored_sample) {
                        ret = -1;
                        goto end_unlock;
                }

                memcpy(stored_sample, &channel_new_sample, sizeof(*stored_sample));
                cds_lfht_node_init(&stored_sample->channel_state_ht_node);
                cds_lfht_add(state->channel_state_ht,
                             hash_channel_key(&stored_sample->key),
                             &stored_sample->channel_state_ht_node);
        }

        /* Find triggers associated with this channel. */
        cds_lfht_lookup(state->channel_triggers_ht,
                        hash_channel_key(&channel_new_sample.key),
                        match_channel_trigger_list,
                        &channel_new_sample.key,
                        &iter);
        node = cds_lfht_iter_get_node(&iter);
        LTTNG_ASSERT(node);

        channel_creds = (typeof(channel_creds)){
                .uid = LTTNG_OPTIONAL_INIT_VALUE(channel_info->session_info->uid),
                .gid = LTTNG_OPTIONAL_INIT_VALUE(channel_info->session_info->gid),
        };

        channel_trigger_list =
                caa_container_of(node, struct lttng_channel_trigger_list, channel_triggers_ht_node);
        cds_list_for_each_entry (trigger_list_element, &channel_trigger_list->list, node) {
                const struct lttng_condition *condition;
                struct lttng_trigger *trigger;
                struct notification_client_list *client_list = nullptr;
                struct lttng_evaluation *evaluation = nullptr;
                enum action_executor_status executor_status;

                ret = 0;
                trigger = trigger_list_element->trigger;
                condition = lttng_trigger_get_const_condition(trigger);
                LTTNG_ASSERT(condition);

                ret = evaluate_buffer_condition(
                        condition,
                        &evaluation,
                        state,
                        previous_sample_available ? &channel_previous_sample : nullptr,
                        &channel_new_sample,
                        channel_info);
                if (caa_unlikely(ret)) {
                        break;
                }

                if (caa_likely(!evaluation)) {
                        continue;
                }

                /*
                 * Ownership of `evaluation` transferred to the action executor
                 * no matter the result. The callee acquires a reference to the
                 * client list: we can release our own.
                 */
                client_list = get_client_list_from_condition(state, condition);
                executor_status = action_executor_enqueue_trigger(
                        state->executor, trigger, evaluation, &channel_creds, client_list);
                notification_client_list_put(client_list);
                evaluation = nullptr;
                switch (executor_status) {
                case ACTION_EXECUTOR_STATUS_OK:
                        break;
                case ACTION_EXECUTOR_STATUS_ERROR:
                case ACTION_EXECUTOR_STATUS_INVALID:
                        /*
                         * TODO Add trigger identification (name/id) when
                         * it is added to the API.
                         */
                        ERR("Fatal error occurred while enqueuing action associated with buffer-condition trigger");
                        ret = -1;
                        goto end_unlock;
                case ACTION_EXECUTOR_STATUS_OVERFLOW:
                        /*
                         * TODO Add trigger identification (name/id) when
                         * it is added to the API.
                         *
                         * Not a fatal error.
                         */
                        WARN("No space left when enqueuing action associated with buffer-condition trigger");
                        ret = 0;
                        goto end_unlock;
                default:
                        abort();
                }
        }
end_unlock:
        if (session) {
                session->last_state_sample = session_new_sample;
        }
        session_info_put(session);
end:
        return ret;
}