sessiond: setup event notifier group for registering app
[lttng-tools.git] / src / bin / lttng-sessiond / ust-app.c
index 23f29c13ca414a0a02903952879701b73dda397b..b30089dbc2fd6029f590adf8c05752ddda2d3225 100644 (file)
@@ -1,23 +1,12 @@
 /*
- * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
- * Copyright (C) 2016 Jérémie Galarneau <jeremie.galarneau@efficios.com>
+ * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
+ * Copyright (C) 2016 Jérémie Galarneau <jeremie.galarneau@efficios.com>
  *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License, version 2 only,
- * as published by the Free Software Foundation.
+ * SPDX-License-Identifier: GPL-2.0-only
  *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License along
- * with this program; if not, write to the Free Software Foundation, Inc.,
- * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
 #define _LGPL_SOURCE
-#include <errno.h>
 #include <inttypes.h>
 #include <pthread.h>
 #include <stdio.h>
@@ -27,9 +16,9 @@
 #include <sys/types.h>
 #include <unistd.h>
 #include <urcu/compiler.h>
-#include <lttng/ust-error.h>
 #include <signal.h>
 
+#include <common/compat/errno.h>
 #include <common/common.h>
 #include <common/sessiond-comm/sessiond-comm.h>
 
 #include "health-sessiond.h"
 #include "ust-app.h"
 #include "ust-consumer.h"
-#include "ust-ctl.h"
+#include "lttng-ust-ctl.h"
+#include "lttng-ust-error.h"
 #include "utils.h"
 #include "session.h"
 #include "lttng-sessiond.h"
 #include "notification-thread-commands.h"
 #include "rotate.h"
 
+struct lttng_ht *ust_app_ht;
+struct lttng_ht *ust_app_ht_by_sock;
+struct lttng_ht *ust_app_ht_by_notify_sock;
+
 static
 int ust_app_flush_app_session(struct ust_app *app, struct ust_app_session *ua_sess);
 
@@ -250,7 +244,8 @@ static struct ust_registry_session *get_session_registry(
        case LTTNG_BUFFER_PER_UID:
        {
                struct buffer_reg_uid *reg_uid = buffer_reg_uid_find(
-                               ua_sess->tracing_id, ua_sess->bits_per_long, ua_sess->uid);
+                               ua_sess->tracing_id, ua_sess->bits_per_long,
+                               lttng_credentials_get_uid(&ua_sess->real_credentials));
                if (!reg_uid) {
                        goto error;
                }
@@ -437,6 +432,9 @@ void save_per_pid_lost_discarded_counters(struct ust_app_channel *ua_chan)
 
 end:
        rcu_read_unlock();
+       if (session) {
+               session_put(session);
+       }
 }
 
 /*
@@ -737,6 +735,10 @@ error:
  * nullified. The session lock MUST be held unless the application is
  * in the destroy path.
  *
+ * Do not hold the registry lock while communicating with the consumerd, because
+ * doing so causes inter-process deadlocks between consumerd and sessiond with
+ * the metadata request notification.
+ *
  * Return 0 on success else a negative value.
  */
 static int close_metadata(struct ust_registry_session *registry,
@@ -744,6 +746,8 @@ static int close_metadata(struct ust_registry_session *registry,
 {
        int ret;
        struct consumer_socket *socket;
+       uint64_t metadata_key;
+       bool registry_was_already_closed;
 
        assert(registry);
        assert(consumer);
@@ -751,8 +755,19 @@ static int close_metadata(struct ust_registry_session *registry,
        rcu_read_lock();
 
        pthread_mutex_lock(&registry->lock);
+       metadata_key = registry->metadata_key;
+       registry_was_already_closed = registry->metadata_closed;
+       if (metadata_key != 0) {
+               /*
+                * Metadata closed. Even on error this means that the consumer
+                * is not responding or not found so either way a second close
+                * should NOT be emit for this registry.
+                */
+               registry->metadata_closed = 1;
+       }
+       pthread_mutex_unlock(&registry->lock);
 
-       if (!registry->metadata_key || registry->metadata_closed) {
+       if (metadata_key == 0 || registry_was_already_closed) {
                ret = 0;
                goto end;
        }
@@ -762,23 +777,15 @@ static int close_metadata(struct ust_registry_session *registry,
                        consumer);
        if (!socket) {
                ret = -1;
-               goto error;
+               goto end;
        }
 
-       ret = consumer_close_metadata(socket, registry->metadata_key);
+       ret = consumer_close_metadata(socket, metadata_key);
        if (ret < 0) {
-               goto error;
+               goto end;
        }
 
-error:
-       /*
-        * Metadata closed. Even on error this means that the consumer is not
-        * responding or not found so either way a second close should NOT be emit
-        * for this registry.
-        */
-       registry->metadata_closed = 1;
 end:
-       pthread_mutex_unlock(&registry->lock);
        rcu_read_unlock();
        return ret;
 }
@@ -914,6 +921,28 @@ void delete_ust_app(struct ust_app *app)
        ht_cleanup_push(app->ust_sessions_objd);
        ht_cleanup_push(app->ust_objd);
 
+       /*
+        * This could be NULL if the event notifier setup failed (e.g the app
+        * was killed or the tracer does not support this feature).
+        */
+       if (app->event_notifier_group.object) {
+               enum lttng_error_code ret_code;
+               const int event_notifier_read_fd = lttng_pipe_get_readfd(
+                               app->event_notifier_group.event_pipe);
+
+               ret_code = notification_thread_command_remove_tracer_event_source(
+                               notification_thread_handle,
+                               event_notifier_read_fd);
+               if (ret_code != LTTNG_OK) {
+                       ERR("Failed to remove application tracer event source from notification thread");
+               }
+
+               ustctl_release_object(sock, app->event_notifier_group.object);
+               free(app->event_notifier_group.object);
+       }
+
+       lttng_pipe_destroy(app->event_notifier_group.event_pipe);
+
        /*
         * Wait until we have deleted the application from the sock hash table
         * before closing this socket, otherwise an application could re-use the
@@ -1012,7 +1041,7 @@ error_free:
  * Alloc new UST app channel.
  */
 static
-struct ust_app_channel *alloc_ust_app_channel(char *name,
+struct ust_app_channel *alloc_ust_app_channel(const char *name,
                struct ust_app_session *ua_sess,
                struct lttng_ust_channel_attr *attr)
 {
@@ -1096,7 +1125,7 @@ struct ust_app_event *alloc_ust_app_event(char *name,
        /* Init most of the default value by allocating and zeroing */
        ua_event = zmalloc(sizeof(struct ust_app_event));
        if (ua_event == NULL) {
-               PERROR("malloc");
+               PERROR("Failed to allocate ust_app_event structure");
                goto error;
        }
 
@@ -1136,7 +1165,7 @@ struct ust_app_ctx *alloc_ust_app_ctx(struct lttng_ust_context_attr *uctx)
        if (uctx) {
                memcpy(&ua_ctx->ctx, uctx, sizeof(ua_ctx->ctx));
                if (uctx->ctx == LTTNG_UST_CONTEXT_APP_CONTEXT) {
-                       char *provider_name = NULL, *ctx_name = NULL;
+                       char *provider_name = NULL, *ctx_name = NULL;
 
                        provider_name = strdup(uctx->u.app_ctx.provider_name);
                        ctx_name = strdup(uctx->u.app_ctx.ctx_name);
@@ -1187,7 +1216,7 @@ error:
  * Return allocated filter or NULL on error.
  */
 static struct lttng_ust_filter_bytecode *create_ust_bytecode_from_bytecode(
-               struct lttng_filter_bytecode *orig_f)
+               const struct lttng_filter_bytecode *orig_f)
 {
        struct lttng_ust_filter_bytecode *filter = NULL;
 
@@ -1257,7 +1286,7 @@ error:
  * Return an ust_app_event object or NULL on error.
  */
 static struct ust_app_event *find_ust_app_event(struct lttng_ht *ht,
-               char *name, struct lttng_filter_bytecode *filter,
+               const char *name, const struct lttng_filter_bytecode *filter,
                int loglevel_value,
                const struct lttng_event_exclusion *exclusion)
 {
@@ -1318,7 +1347,7 @@ int create_ust_channel_context(struct ust_app_channel *ua_chan,
                         * continue normally.
                         */
                        ret = 0;
-                       DBG3("UST app disable event failed. Application is dead.");
+                       DBG3("UST app add context failed. Application is dead.");
                }
                goto error;
        }
@@ -1336,33 +1365,28 @@ error:
 /*
  * Set the filter on the tracer.
  */
-static
-int set_ust_event_filter(struct ust_app_event *ua_event,
-               struct ust_app *app)
+static int set_ust_object_filter(struct ust_app *app,
+               const struct lttng_filter_bytecode *bytecode,
+               struct lttng_ust_object_data *ust_object)
 {
        int ret;
        struct lttng_ust_filter_bytecode *ust_bytecode = NULL;
 
        health_code_update();
 
-       if (!ua_event->filter) {
-               ret = 0;
-               goto error;
-       }
-
-       ust_bytecode = create_ust_bytecode_from_bytecode(ua_event->filter);
+       ust_bytecode = create_ust_bytecode_from_bytecode(bytecode);
        if (!ust_bytecode) {
                ret = -LTTNG_ERR_NOMEM;
                goto error;
        }
        pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_set_filter(app->sock, ust_bytecode,
-                       ua_event->obj);
+                       ust_object);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
-                       ERR("UST app event %s filter failed for app (pid: %d) "
-                                       "with ret %d", ua_event->attr.name, app->pid, ret);
+                       ERR("UST app set object filter failed for object %p of app (pid: %d) "
+                                       "with ret %d", ust_object, app->pid, ret);
                } else {
                        /*
                         * This is normal behavior, an application can die during the
@@ -1370,12 +1394,12 @@ int set_ust_event_filter(struct ust_app_event *ua_event,
                         * continue normally.
                         */
                        ret = 0;
-                       DBG3("UST app filter event failed. Application is dead.");
+                       DBG3("Failed to set UST app object filter. Application is dead.");
                }
                goto error;
        }
 
-       DBG2("UST filter set successfully for event %s", ua_event->name);
+       DBG2("UST filter successfully set for object %p", ust_object);
 
 error:
        health_code_update();
@@ -1385,7 +1409,7 @@ error:
 
 static
 struct lttng_ust_event_exclusion *create_ust_exclusion_from_exclusion(
-               struct lttng_event_exclusion *exclusion)
+               const struct lttng_event_exclusion *exclusion)
 {
        struct lttng_ust_event_exclusion *ust_exclusion = NULL;
        size_t exclusion_alloc_size = sizeof(struct lttng_ust_event_exclusion) +
@@ -1407,33 +1431,30 @@ end:
 /*
  * Set event exclusions on the tracer.
  */
-static
-int set_ust_event_exclusion(struct ust_app_event *ua_event,
-               struct ust_app *app)
+static int set_ust_object_exclusions(struct ust_app *app,
+               const struct lttng_event_exclusion *exclusions,
+               struct lttng_ust_object_data *ust_object)
 {
        int ret;
-       struct lttng_ust_event_exclusion *ust_exclusion = NULL;
+       struct lttng_ust_event_exclusion *ust_exclusions = NULL;
 
-       health_code_update();
+       assert(exclusions && exclusions->count > 0);
 
-       if (!ua_event->exclusion || !ua_event->exclusion->count) {
-               ret = 0;
-               goto error;
-       }
+       health_code_update();
 
-       ust_exclusion = create_ust_exclusion_from_exclusion(
-                       ua_event->exclusion);
-       if (!ust_exclusion) {
+       ust_exclusions = create_ust_exclusion_from_exclusion(
+                       exclusions);
+       if (!ust_exclusions) {
                ret = -LTTNG_ERR_NOMEM;
                goto error;
        }
        pthread_mutex_lock(&app->sock_lock);
-       ret = ustctl_set_exclusion(app->sock, ust_exclusion, ua_event->obj);
+       ret = ustctl_set_exclusion(app->sock, ust_exclusions, ust_object);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
-                       ERR("UST app event %s exclusions failed for app (pid: %d) "
-                                       "with ret %d", ua_event->attr.name, app->pid, ret);
+                       ERR("Failed to set UST app exclusions for object %p of app (pid: %d) "
+                                       "with ret %d", ust_object, app->pid, ret);
                } else {
                        /*
                         * This is normal behavior, an application can die during the
@@ -1441,37 +1462,36 @@ int set_ust_event_exclusion(struct ust_app_event *ua_event,
                         * continue normally.
                         */
                        ret = 0;
-                       DBG3("UST app event exclusion failed. Application is dead.");
+                       DBG3("Failed to set UST app object exclusions. Application is dead.");
                }
                goto error;
        }
 
-       DBG2("UST exclusion set successfully for event %s", ua_event->name);
+       DBG2("UST exclusions set successfully for object %p", ust_object);
 
 error:
        health_code_update();
-       free(ust_exclusion);
+       free(ust_exclusions);
        return ret;
 }
 
 /*
  * Disable the specified event on to UST tracer for the UST session.
  */
-static int disable_ust_event(struct ust_app *app,
-               struct ust_app_session *ua_sess, struct ust_app_event *ua_event)
+static int disable_ust_object(struct ust_app *app,
+               struct lttng_ust_object_data *object)
 {
        int ret;
 
        health_code_update();
 
        pthread_mutex_lock(&app->sock_lock);
-       ret = ustctl_disable(app->sock, ua_event->obj);
+       ret = ustctl_disable(app->sock, object);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
-                       ERR("UST app event %s disable failed for app (pid: %d) "
-                                       "and session handle %d with ret %d",
-                                       ua_event->attr.name, app->pid, ua_sess->handle, ret);
+                       ERR("Failed to disable UST app object %p app (pid: %d) with ret %d",
+                                       object, app->pid, ret);
                } else {
                        /*
                         * This is normal behavior, an application can die during the
@@ -1479,13 +1499,13 @@ static int disable_ust_event(struct ust_app *app,
                         * continue normally.
                         */
                        ret = 0;
-                       DBG3("UST app disable event failed. Application is dead.");
+                       DBG3("Failed to disable UST app object. Application is dead.");
                }
                goto error;
        }
 
-       DBG2("UST app event %s disabled successfully for app (pid: %d)",
-                       ua_event->attr.name, app->pid);
+       DBG2("UST app object %p disabled successfully for app (pid: %d)",
+                       object, app->pid);
 
 error:
        health_code_update();
@@ -1573,21 +1593,20 @@ error:
 /*
  * Enable the specified event on to UST tracer for the UST session.
  */
-static int enable_ust_event(struct ust_app *app,
-               struct ust_app_session *ua_sess, struct ust_app_event *ua_event)
+static int enable_ust_object(
+               struct ust_app *app, struct lttng_ust_object_data *ust_object)
 {
        int ret;
 
        health_code_update();
 
        pthread_mutex_lock(&app->sock_lock);
-       ret = ustctl_enable(app->sock, ua_event->obj);
+       ret = ustctl_enable(app->sock, ust_object);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
-                       ERR("UST app event %s enable failed for app (pid: %d) "
-                                       "and session handle %d with ret %d",
-                                       ua_event->attr.name, app->pid, ua_sess->handle, ret);
+                       ERR("UST app enable failed for object %p app (pid: %d) with ret %d",
+                                       ust_object, app->pid, ret);
                } else {
                        /*
                         * This is normal behavior, an application can die during the
@@ -1595,13 +1614,13 @@ static int enable_ust_event(struct ust_app *app,
                         * continue normally.
                         */
                        ret = 0;
-                       DBG3("UST app enable event failed. Application is dead.");
+                       DBG3("Failed to enable UST app object. Application is dead.");
                }
                goto error;
        }
 
-       DBG2("UST app event %s enabled successfully for app (pid: %d)",
-                       ua_event->attr.name, app->pid);
+       DBG2("UST app object %p enabled successfully for app (pid: %d)",
+                       ust_object, app->pid);
 
 error:
        health_code_update();
@@ -1680,6 +1699,7 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret != -EPIPE && ret != -LTTNG_UST_ERR_EXITING) {
+                       abort();
                        ERR("Error ustctl create event %s for app pid: %d with ret %d",
                                        ua_event->attr.name, app->pid, ret);
                } else {
@@ -1696,14 +1716,14 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
 
        ua_event->handle = ua_event->obj->handle;
 
-       DBG2("UST app event %s created successfully for pid:%d",
-                       ua_event->attr.name, app->pid);
+       DBG2("UST app event %s created successfully for pid:%d object: %p",
+                       ua_event->attr.name, app->pid, ua_event->obj);
 
        health_code_update();
 
        /* Set filter if one is present. */
        if (ua_event->filter) {
-               ret = set_ust_event_filter(ua_event, app);
+               ret = set_ust_object_filter(app, ua_event->filter, ua_event->obj);
                if (ret < 0) {
                        goto error;
                }
@@ -1711,7 +1731,7 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
 
        /* Set exclusions for the event */
        if (ua_event->exclusion) {
-               ret = set_ust_event_exclusion(ua_event, app);
+               ret = set_ust_object_exclusions(app, ua_event->exclusion, ua_event->obj);
                if (ret < 0) {
                        goto error;
                }
@@ -1723,7 +1743,7 @@ int create_ust_event(struct ust_app *app, struct ust_app_session *ua_sess,
                 * We now need to explicitly enable the event, since it
                 * is now disabled at creation.
                 */
-               ret = enable_ust_event(app, ua_sess, ua_event);
+               ret = enable_ust_object(app, ua_event->obj);
                if (ret < 0) {
                        /*
                         * If we hit an EPERM, something is wrong with our enable call. If
@@ -1792,11 +1812,6 @@ static void shadow_copy_event(struct ust_app_event *ua_event,
 static void shadow_copy_channel(struct ust_app_channel *ua_chan,
                struct ltt_ust_channel *uchan)
 {
-       struct lttng_ht_iter iter;
-       struct ltt_ust_event *uevent;
-       struct ltt_ust_context *uctx;
-       struct ust_app_event *ua_event;
-
        DBG2("UST app shadow copy of channel %s started", ua_chan->name);
 
        strncpy(ua_chan->name, uchan->name, sizeof(ua_chan->name));
@@ -1823,34 +1838,6 @@ static void shadow_copy_channel(struct ust_app_channel *ua_chan,
        ua_chan->enabled = uchan->enabled;
        ua_chan->tracing_channel_id = uchan->id;
 
-       cds_list_for_each_entry(uctx, &uchan->ctx_list, list) {
-               struct ust_app_ctx *ua_ctx = alloc_ust_app_ctx(&uctx->ctx);
-
-               if (ua_ctx == NULL) {
-                       continue;
-               }
-               lttng_ht_node_init_ulong(&ua_ctx->node,
-                               (unsigned long) ua_ctx->ctx.ctx);
-               lttng_ht_add_ulong(ua_chan->ctx, &ua_ctx->node);
-               cds_list_add_tail(&ua_ctx->list, &ua_chan->ctx_list);
-       }
-
-       /* Copy all events from ltt ust channel to ust app channel */
-       cds_lfht_for_each_entry(uchan->events->ht, &iter.iter, uevent, node.node) {
-               ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
-                               uevent->filter, uevent->attr.loglevel, uevent->exclusion);
-               if (ua_event == NULL) {
-                       DBG2("UST event %s not found on shadow copy channel",
-                                       uevent->attr.name);
-                       ua_event = alloc_ust_app_event(uevent->attr.name, &uevent->attr);
-                       if (ua_event == NULL) {
-                               continue;
-                       }
-                       shadow_copy_event(ua_event, uevent);
-                       add_unique_ust_app_event(ua_chan, ua_event);
-               }
-       }
-
        DBG3("UST app shadow copy of channel %s done", ua_chan->name);
 }
 
@@ -1860,29 +1847,22 @@ static void shadow_copy_channel(struct ust_app_channel *ua_chan,
 static void shadow_copy_session(struct ust_app_session *ua_sess,
                struct ltt_ust_session *usess, struct ust_app *app)
 {
-       struct lttng_ht_node_str *ua_chan_node;
-       struct lttng_ht_iter iter;
-       struct ltt_ust_channel *uchan;
-       struct ust_app_channel *ua_chan;
-       time_t rawtime;
        struct tm *timeinfo;
        char datetime[16];
        int ret;
        char tmp_shm_path[PATH_MAX];
 
-       /* Get date and time for unique app path */
-       time(&rawtime);
-       timeinfo = localtime(&rawtime);
+       timeinfo = localtime(&app->registration_time);
        strftime(datetime, sizeof(datetime), "%Y%m%d-%H%M%S", timeinfo);
 
        DBG2("Shadow copy of session handle %d", ua_sess->handle);
 
        ua_sess->tracing_id = usess->id;
        ua_sess->id = get_next_session_id();
-       ua_sess->uid = app->uid;
-       ua_sess->gid = app->gid;
-       ua_sess->euid = usess->uid;
-       ua_sess->egid = usess->gid;
+       LTTNG_OPTIONAL_SET(&ua_sess->real_credentials.uid, app->uid);
+       LTTNG_OPTIONAL_SET(&ua_sess->real_credentials.gid, app->gid);
+       LTTNG_OPTIONAL_SET(&ua_sess->effective_credentials.uid, usess->uid);
+       LTTNG_OPTIONAL_SET(&ua_sess->effective_credentials.gid, usess->gid);
        ua_sess->buffer_type = usess->buffer_type;
        ua_sess->bits_per_long = app->bits_per_long;
 
@@ -1903,7 +1883,9 @@ static void shadow_copy_session(struct ust_app_session *ua_sess,
                break;
        case LTTNG_BUFFER_PER_UID:
                ret = snprintf(ua_sess->path, sizeof(ua_sess->path),
-                               DEFAULT_UST_TRACE_UID_PATH, ua_sess->uid, app->bits_per_long);
+                               DEFAULT_UST_TRACE_UID_PATH,
+                               lttng_credentials_get_uid(&ua_sess->real_credentials),
+                               app->bits_per_long);
                break;
        default:
                assert(0);
@@ -1925,12 +1907,12 @@ static void shadow_copy_session(struct ust_app_session *ua_sess,
                switch (ua_sess->buffer_type) {
                case LTTNG_BUFFER_PER_PID:
                        ret = snprintf(tmp_shm_path, sizeof(tmp_shm_path),
-                                       DEFAULT_UST_TRACE_PID_PATH "/%s-%d-%s",
+                                       "/" DEFAULT_UST_TRACE_PID_PATH "/%s-%d-%s",
                                        app->name, app->pid, datetime);
                        break;
                case LTTNG_BUFFER_PER_UID:
                        ret = snprintf(tmp_shm_path, sizeof(tmp_shm_path),
-                                       DEFAULT_UST_TRACE_UID_PATH,
+                                       "/" DEFAULT_UST_TRACE_UID_PATH,
                                        app->uid, app->bits_per_long);
                        break;
                default:
@@ -1946,37 +1928,6 @@ static void shadow_copy_session(struct ust_app_session *ua_sess,
                        sizeof(ua_sess->shm_path) - strlen(ua_sess->shm_path) - 1);
                ua_sess->shm_path[sizeof(ua_sess->shm_path) - 1] = '\0';
        }
-
-       /* Iterate over all channels in global domain. */
-       cds_lfht_for_each_entry(usess->domain_global.channels->ht, &iter.iter,
-                       uchan, node.node) {
-               struct lttng_ht_iter uiter;
-
-               lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &uiter);
-               ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
-               if (ua_chan_node != NULL) {
-                       /* Session exist. Contiuing. */
-                       continue;
-               }
-
-               DBG2("Channel %s not found on shadow session copy, creating it",
-                               uchan->name);
-               ua_chan = alloc_ust_app_channel(uchan->name, ua_sess,
-                               &uchan->attr);
-               if (ua_chan == NULL) {
-                       /* malloc failed FIXME: Might want to do handle ENOMEM .. */
-                       continue;
-               }
-               shadow_copy_channel(ua_chan, uchan);
-               /*
-                * The concept of metadata channel does not exist on the tracing
-                * registry side of the session daemon so this can only be a per CPU
-                * channel and not metadata.
-                */
-               ua_chan->attr.type = LTTNG_UST_CHAN_PER_CPU;
-
-               lttng_ht_add_unique_str(ua_sess->channels, &ua_chan->node);
-       }
        return;
 
 error:
@@ -1987,7 +1938,7 @@ error:
  * Lookup sesison wrapper.
  */
 static
-void __lookup_session_by_app(struct ltt_ust_session *usess,
+void __lookup_session_by_app(const struct ltt_ust_session *usess,
                        struct ust_app *app, struct lttng_ht_iter *iter)
 {
        /* Get right UST app session from app */
@@ -1999,7 +1950,7 @@ void __lookup_session_by_app(struct ltt_ust_session *usess,
  * id.
  */
 static struct ust_app_session *lookup_session_by_app(
-               struct ltt_ust_session *usess, struct ust_app *app)
+               const struct ltt_ust_session *usess, struct ust_app *app)
 {
        struct lttng_ht_iter iter;
        struct lttng_ht_node_u64 *node;
@@ -2054,10 +2005,12 @@ static int setup_buffer_reg_pid(struct ust_app_session *ua_sess,
                        app->bits_per_long, app->uint8_t_alignment,
                        app->uint16_t_alignment, app->uint32_t_alignment,
                        app->uint64_t_alignment, app->long_alignment,
-                       app->byte_order, app->version.major,
-                       app->version.minor, reg_pid->root_shm_path,
-                       reg_pid->shm_path,
-                       ua_sess->euid, ua_sess->egid);
+                       app->byte_order, app->version.major, app->version.minor,
+                       reg_pid->root_shm_path, reg_pid->shm_path,
+                       lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                       lttng_credentials_get_gid(&ua_sess->effective_credentials),
+                       ua_sess->tracing_id,
+                       app->uid);
        if (ret < 0) {
                /*
                 * reg_pid->registry->reg.ust is NULL upon error, so we need to
@@ -2124,7 +2077,8 @@ static int setup_buffer_reg_uid(struct ltt_ust_session *usess,
                        app->uint64_t_alignment, app->long_alignment,
                        app->byte_order, app->version.major,
                        app->version.minor, reg_uid->root_shm_path,
-                       reg_uid->shm_path, usess->uid, usess->gid);
+                       reg_uid->shm_path, usess->uid, usess->gid,
+                       ua_sess->tracing_id, app->uid);
        if (ret < 0) {
                /*
                 * reg_uid->registry->reg.ust is NULL upon error, so we need to
@@ -2355,7 +2309,7 @@ end:
  */
 static
 int create_ust_app_channel_context(struct ust_app_channel *ua_chan,
-               struct lttng_ust_context_attr *uctx,
+               struct lttng_ust_context_attr *uctx,
                struct ust_app *app)
 {
        int ret = 0;
@@ -2400,7 +2354,7 @@ int enable_ust_app_event(struct ust_app_session *ua_sess,
 {
        int ret;
 
-       ret = enable_ust_event(app, ua_sess, ua_event);
+       ret = enable_ust_object(app, ua_event->obj);
        if (ret < 0) {
                goto error;
        }
@@ -2419,7 +2373,7 @@ static int disable_ust_app_event(struct ust_app_session *ua_sess,
 {
        int ret;
 
-       ret = disable_ust_event(app, ua_sess, ua_event);
+       ret = disable_ust_object(app, ua_event->obj);
        if (ret < 0) {
                goto error;
        }
@@ -2526,7 +2480,7 @@ static int do_consumer_create_channel(struct ltt_ust_session *usess,
         * stream we have to expect.
         */
        ret = ust_consumer_ask_channel(ua_sess, ua_chan, usess->consumer, socket,
-                       registry, trace_archive_id);
+                       registry, usess->current_trace_chunk);
        if (ret < 0) {
                goto error_ask;
        }
@@ -2865,7 +2819,7 @@ static int create_channel_per_uid(struct ust_app *app,
        int ret;
        struct buffer_reg_uid *reg_uid;
        struct buffer_reg_channel *reg_chan;
-       struct ltt_session *session;
+       struct ltt_session *session = NULL;
        enum lttng_error_code notification_ret;
        struct ust_registry_channel *chan_reg;
 
@@ -2909,7 +2863,7 @@ static int create_channel_per_uid(struct ust_app *app,
         */
        ret = do_consumer_create_channel(usess, ua_sess, ua_chan,
                        app->bits_per_long, reg_uid->registry->reg.ust,
-                       session->current_archive_id);
+                       session->most_recent_chunk_id.value);
        if (ret < 0) {
                ERR("Error creating UST channel \"%s\" on the consumer daemon",
                                ua_chan->name);
@@ -2946,10 +2900,10 @@ static int create_channel_per_uid(struct ust_app *app,
 
        notification_ret = notification_thread_command_add_channel(
                        notification_thread_handle, session->name,
-                       ua_sess->euid, ua_sess->egid,
+                       lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                       lttng_credentials_get_gid(&ua_sess->effective_credentials),
                        ua_chan->name,
-                       ua_chan->key,
-                       LTTNG_DOMAIN_UST,
+                       ua_chan->key, LTTNG_DOMAIN_UST,
                        ua_chan->attr.subbuf_size * ua_chan->attr.num_subbuf);
        if (notification_ret != LTTNG_OK) {
                ret = - (int) notification_ret;
@@ -2968,6 +2922,9 @@ send_channel:
        }
 
 error:
+       if (session) {
+               session_put(session);
+       }
        return ret;
 }
 
@@ -2986,7 +2943,7 @@ static int create_channel_per_pid(struct ust_app *app,
        int ret;
        struct ust_registry_session *registry;
        enum lttng_error_code cmd_ret;
-       struct ltt_session *session;
+       struct ltt_session *session = NULL;
        uint64_t chan_reg_key;
        struct ust_registry_channel *chan_reg;
 
@@ -3020,7 +2977,7 @@ static int create_channel_per_pid(struct ust_app *app,
        /* Create and get channel on the consumer side. */
        ret = do_consumer_create_channel(usess, ua_sess, ua_chan,
                        app->bits_per_long, registry,
-                       session->current_archive_id);
+                       session->most_recent_chunk_id.value);
        if (ret < 0) {
                ERR("Error creating UST channel \"%s\" on the consumer daemon",
                        ua_chan->name);
@@ -3044,10 +3001,10 @@ static int create_channel_per_pid(struct ust_app *app,
 
        cmd_ret = notification_thread_command_add_channel(
                        notification_thread_handle, session->name,
-                       ua_sess->euid, ua_sess->egid,
+                       lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                       lttng_credentials_get_gid(&ua_sess->effective_credentials),
                        ua_chan->name,
-                       ua_chan->key,
-                       LTTNG_DOMAIN_UST,
+                       ua_chan->key, LTTNG_DOMAIN_UST,
                        ua_chan->attr.subbuf_size * ua_chan->attr.num_subbuf);
        if (cmd_ret != LTTNG_OK) {
                ret = - (int) cmd_ret;
@@ -3061,12 +3018,15 @@ error_remove_from_registry:
        }
 error:
        rcu_read_unlock();
+       if (session) {
+               session_put(session);
+       }
        return ret;
 }
 
 /*
  * From an already allocated ust app channel, create the channel buffers if
- * need and send it to the application. This MUST be called with a RCU read
+ * needed and send them to the application. This MUST be called with a RCU read
  * side lock acquired.
  *
  * Called with UST app session lock held.
@@ -3074,7 +3034,7 @@ error:
  * Return 0 on success or else a negative value. Returns -ENOTCONN if
  * the application exited concurrently.
  */
-static int do_create_channel(struct ust_app *app,
+static int ust_app_channel_send(struct ust_app *app,
                struct ltt_ust_session *usess, struct ust_app_session *ua_sess,
                struct ust_app_channel *ua_chan)
 {
@@ -3082,6 +3042,7 @@ static int do_create_channel(struct ust_app *app,
 
        assert(app);
        assert(usess);
+       assert(usess->active);
        assert(ua_sess);
        assert(ua_chan);
 
@@ -3126,16 +3087,14 @@ error:
 }
 
 /*
- * Create UST app channel and create it on the tracer. Set ua_chanp of the
- * newly created channel if not NULL.
+ * Create UST app channel and return it through ua_chanp if not NULL.
  *
  * Called with UST app session lock and RCU read-side lock held.
  *
- * Return 0 on success or else a negative value. Returns -ENOTCONN if
- * the application exited concurrently.
+ * Return 0 on success or else a negative value.
  */
-static int create_ust_app_channel(struct ust_app_session *ua_sess,
-               struct ltt_ust_channel *uchan, struct ust_app *app,
+static int ust_app_channel_allocate(struct ust_app_session *ua_sess,
+               struct ltt_ust_channel *uchan,
                enum lttng_ust_chan_type type, struct ltt_ust_session *usess,
                struct ust_app_channel **ua_chanp)
 {
@@ -3156,21 +3115,13 @@ static int create_ust_app_channel(struct ust_app_session *ua_sess,
        if (ua_chan == NULL) {
                /* Only malloc can fail here */
                ret = -ENOMEM;
-               goto error_alloc;
+               goto error;
        }
        shadow_copy_channel(ua_chan, uchan);
 
        /* Set channel type. */
        ua_chan->attr.type = type;
 
-       ret = do_create_channel(app, usess, ua_sess, ua_chan);
-       if (ret < 0) {
-               goto error;
-       }
-
-       DBG2("UST app create channel %s for PID %d completed", ua_chan->name,
-                       app->pid);
-
        /* Only add the channel if successful on the tracer side. */
        lttng_ht_add_unique_str(ua_sess->channels, &ua_chan->node);
 end:
@@ -3182,8 +3133,6 @@ end:
        return 0;
 
 error:
-       delete_ust_app_channel(ua_chan->is_sent ? app->sock : -1, ua_chan, app);
-error_alloc:
        return ret;
 }
 
@@ -3200,18 +3149,9 @@ int create_ust_app_event(struct ust_app_session *ua_sess,
        int ret = 0;
        struct ust_app_event *ua_event;
 
-       /* Get event node */
-       ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
-                       uevent->filter, uevent->attr.loglevel, uevent->exclusion);
-       if (ua_event != NULL) {
-               ret = -EEXIST;
-               goto end;
-       }
-
-       /* Does not exist so create one */
        ua_event = alloc_ust_app_event(uevent->attr.name, &uevent->attr);
        if (ua_event == NULL) {
-               /* Only malloc can failed so something is really wrong */
+               /* Only failure mode of alloc_ust_app_event(). */
                ret = -ENOMEM;
                goto end;
        }
@@ -3220,8 +3160,19 @@ int create_ust_app_event(struct ust_app_session *ua_sess,
        /* Create it on the tracer side */
        ret = create_ust_event(app, ua_sess, ua_chan, ua_event);
        if (ret < 0) {
-               /* Not found previously means that it does not exist on the tracer */
-               assert(ret != -LTTNG_UST_ERR_EXIST);
+               /*
+                * Not found previously means that it does not exist on the
+                * tracer. If the application reports that the event existed,
+                * it means there is a bug in the sessiond or lttng-ust
+                * (or corruption, etc.)
+                */
+               if (ret == -LTTNG_UST_ERR_EXIST) {
+                       ERR("Tracer for application reported that an event being created already existed: "
+                                       "event_name = \"%s\", pid = %d, ppid = %d, uid = %d, gid = %d",
+                                       uevent->attr.name,
+                                       app->pid, app->ppid, app->uid,
+                                       app->gid);
+               }
                goto error;
        }
 
@@ -3251,7 +3202,7 @@ static int create_ust_app_metadata(struct ust_app_session *ua_sess,
        struct ust_app_channel *metadata;
        struct consumer_socket *socket;
        struct ust_registry_session *registry;
-       struct ltt_session *session;
+       struct ltt_session *session = NULL;
 
        assert(ua_sess);
        assert(app);
@@ -3314,7 +3265,7 @@ static int create_ust_app_metadata(struct ust_app_session *ua_sess,
         * consumer.
         */
        ret = ust_consumer_ask_channel(ua_sess, metadata, consumer, socket,
-                       registry, session->current_archive_id);
+                       registry, session->current_trace_chunk);
        if (ret < 0) {
                /* Nullify the metadata key so we don't try to close it later on. */
                registry->metadata_key = 0;
@@ -3342,6 +3293,9 @@ error_consumer:
        delete_ust_app_channel(-1, metadata, app);
 error:
        pthread_mutex_unlock(&registry->lock);
+       if (session) {
+               session_put(session);
+       }
        return ret;
 }
 
@@ -3380,6 +3334,7 @@ error:
 struct ust_app *ust_app_create(struct ust_register_msg *msg, int sock)
 {
        struct ust_app *lta = NULL;
+       struct lttng_pipe *event_notifier_event_source_pipe = NULL;
 
        assert(msg);
        assert(sock >= 0);
@@ -3396,12 +3351,21 @@ struct ust_app *ust_app_create(struct ust_register_msg *msg, int sock)
                goto error;
        }
 
+       event_notifier_event_source_pipe = lttng_pipe_open(FD_CLOEXEC);
+       if (!event_notifier_event_source_pipe) {
+               PERROR("Failed to open application event source pipe: '%s' (ppid = %d)",
+                               msg->name, msg->ppid);
+               goto error;
+       }
+
        lta = zmalloc(sizeof(struct ust_app));
        if (lta == NULL) {
                PERROR("malloc");
-               goto error;
+               goto error_free_pipe;
        }
 
+       lta->event_notifier_group.event_pipe = event_notifier_event_source_pipe;
+
        lta->ppid = msg->ppid;
        lta->uid = msg->uid;
        lta->gid = msg->gid;
@@ -3439,8 +3403,12 @@ struct ust_app *ust_app_create(struct ust_register_msg *msg, int sock)
        lttng_ht_node_init_ulong(&lta->sock_n, (unsigned long) lta->sock);
 
        CDS_INIT_LIST_HEAD(&lta->teardown_head);
-error:
        return lta;
+
+error_free_pipe:
+       lttng_pipe_destroy(event_notifier_event_source_pipe);
+error:
+       return NULL;
 }
 
 /*
@@ -3451,6 +3419,8 @@ void ust_app_add(struct ust_app *app)
        assert(app);
        assert(app->notify_sock >= 0);
 
+       app->registration_time = time(NULL);
+
        rcu_read_lock();
 
        /*
@@ -3504,6 +3474,61 @@ int ust_app_version(struct ust_app *app)
        return ret;
 }
 
+/*
+ * Setup the base event notifier group.
+ *
+ * Return 0 on success else a negative value either an errno code or a
+ * LTTng-UST error code.
+ */
+int ust_app_setup_event_notifier_group(struct ust_app *app)
+{
+       int ret;
+       int event_pipe_write_fd;
+       struct lttng_ust_object_data *event_notifier_group = NULL;
+       enum lttng_error_code lttng_ret;
+
+       assert(app);
+
+       /* Get the write side of the pipe. */
+       event_pipe_write_fd = lttng_pipe_get_writefd(
+                       app->event_notifier_group.event_pipe);
+
+       pthread_mutex_lock(&app->sock_lock);
+       ret = ustctl_create_event_notifier_group(app->sock,
+                       event_pipe_write_fd, &event_notifier_group);
+       pthread_mutex_unlock(&app->sock_lock);
+       if (ret < 0) {
+               if (ret != -LTTNG_UST_ERR_EXITING && ret != -EPIPE) {
+                       ERR("Failed to create application event notifier group: ret = %d, app socket fd = %d, event_pipe_write_fd = %d",
+                                       ret, app->sock, event_pipe_write_fd);
+               } else {
+                       DBG("Failed to create application event notifier group (application is dead): app socket fd = %d",
+                                       app->sock);
+               }
+
+               goto error;
+       }
+
+       lttng_ret = notification_thread_command_add_tracer_event_source(
+                       notification_thread_handle,
+                       lttng_pipe_get_readfd(app->event_notifier_group.event_pipe),
+                       LTTNG_DOMAIN_UST);
+       if (lttng_ret != LTTNG_OK) {
+               ERR("Failed to add tracer event source to notification thread");
+               ret = - 1;
+               goto error;
+       }
+
+       /* Assign handle only when the complete setup is valid. */
+       app->event_notifier_group.object = event_notifier_group;
+       return ret;
+
+error:
+       ustctl_release_object(app->sock, app->event_notifier_group.object);
+       free(app->event_notifier_group.object);
+       return ret;
+}
+
 /*
  * Unregister app by removing it from the global traceable app list and freeing
  * the data struct.
@@ -3909,6 +3934,24 @@ void ust_app_clean_list(void)
 
        rcu_read_lock();
 
+       /* Cleanup notify socket hash table */
+       if (ust_app_ht_by_notify_sock) {
+               cds_lfht_for_each_entry(ust_app_ht_by_notify_sock->ht, &iter.iter, app,
+                               notify_sock_n.node) {
+                       struct cds_lfht_node *node;
+                       struct ust_app *app;
+
+                       node = cds_lfht_iter_get_node(&iter.iter);
+                       if (!node) {
+                               continue;
+                       }
+
+                       app = container_of(node, struct ust_app,
+                                       notify_sock_n.node);
+                       ust_app_notify_sock_unregister(app->notify_sock);
+               }
+       }
+
        if (ust_app_ht) {
                cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        ret = lttng_ht_del(ust_app_ht, &iter);
@@ -3926,14 +3969,6 @@ void ust_app_clean_list(void)
                }
        }
 
-       /* Cleanup notify socket hash table */
-       if (ust_app_ht_by_notify_sock) {
-               cds_lfht_for_each_entry(ust_app_ht_by_notify_sock->ht, &iter.iter, app,
-                               notify_sock_n.node) {
-                       ret = lttng_ht_del(ust_app_ht_by_notify_sock, &iter);
-                       assert(!ret);
-               }
-       }
        rcu_read_unlock();
 
        /* Destroy is done only when the ht is empty */
@@ -3981,12 +4016,7 @@ int ust_app_disable_channel_glb(struct ltt_ust_session *usess,
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;
 
-       if (usess == NULL || uchan == NULL) {
-               ERR("Disabling UST global channel with NULL values");
-               ret = -1;
-               goto error;
-       }
-
+       assert(usess->active);
        DBG2("UST app disabling channel %s from global domain for session id %" PRIu64,
                        uchan->name, usess->id);
 
@@ -4026,8 +4056,6 @@ int ust_app_disable_channel_glb(struct ltt_ust_session *usess,
        }
 
        rcu_read_unlock();
-
-error:
        return ret;
 }
 
@@ -4042,12 +4070,7 @@ int ust_app_enable_channel_glb(struct ltt_ust_session *usess,
        struct ust_app *app;
        struct ust_app_session *ua_sess;
 
-       if (usess == NULL || uchan == NULL) {
-               ERR("Adding UST global channel to NULL values");
-               ret = -1;
-               goto error;
-       }
-
+       assert(usess->active);
        DBG2("UST app enabling channel %s to global domain for session id %" PRIu64,
                        uchan->name, usess->id);
 
@@ -4076,8 +4099,6 @@ int ust_app_enable_channel_glb(struct ltt_ust_session *usess,
        }
 
        rcu_read_unlock();
-
-error:
        return ret;
 }
 
@@ -4095,6 +4116,7 @@ int ust_app_disable_event_glb(struct ltt_ust_session *usess,
        struct ust_app_channel *ua_chan;
        struct ust_app_event *ua_event;
 
+       assert(usess->active);
        DBG("UST app disabling event %s for all apps in channel "
                        "%s for session id %" PRIu64,
                        uevent->attr.name, uchan->name, usess->id);
@@ -4143,107 +4165,82 @@ int ust_app_disable_event_glb(struct ltt_ust_session *usess,
        }
 
        rcu_read_unlock();
-
        return ret;
 }
 
-/*
- * For a specific UST session, create the channel for all registered apps.
- */
-int ust_app_create_channel_glb(struct ltt_ust_session *usess,
-               struct ltt_ust_channel *uchan)
+/* The ua_sess lock must be held by the caller.  */
+static
+int ust_app_channel_create(struct ltt_ust_session *usess,
+               struct ust_app_session *ua_sess,
+               struct ltt_ust_channel *uchan, struct ust_app *app,
+               struct ust_app_channel **_ua_chan)
 {
-       int ret = 0, created;
-       struct lttng_ht_iter iter;
-       struct ust_app *app;
-       struct ust_app_session *ua_sess = NULL;
-
-       /* Very wrong code flow */
-       assert(usess);
-       assert(uchan);
-
-       DBG2("UST app adding channel %s to UST domain for session id %" PRIu64,
-                       uchan->name, usess->id);
+       int ret = 0;
+       struct ust_app_channel *ua_chan = NULL;
 
-       rcu_read_lock();
+       assert(ua_sess);
+       ASSERT_LOCKED(ua_sess->lock);
 
-       /* For every registered applications */
-       cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
-               if (!app->compatible) {
-                       /*
-                        * TODO: In time, we should notice the caller of this error by
-                        * telling him that this is a version error.
-                        */
-                       continue;
-               }
-               if (!trace_ust_pid_tracker_lookup(usess, app->pid)) {
-                       /* Skip. */
-                       continue;
-               }
+       if (!strncmp(uchan->name, DEFAULT_METADATA_NAME,
+                    sizeof(uchan->name))) {
+               copy_channel_attr_to_ustctl(&ua_sess->metadata_attr,
+                       &uchan->attr);
+               ret = 0;
+       } else {
+               struct ltt_ust_context *uctx = NULL;
 
                /*
-                * Create session on the tracer side and add it to app session HT. Note
-                * that if session exist, it will simply return a pointer to the ust
-                * app session.
+                * Create channel onto application and synchronize its
+                * configuration.
                 */
-               ret = find_or_create_ust_app_session(usess, app, &ua_sess, &created);
+               ret = ust_app_channel_allocate(ua_sess, uchan,
+                       LTTNG_UST_CHAN_PER_CPU, usess,
+                       &ua_chan);
                if (ret < 0) {
-                       switch (ret) {
-                       case -ENOTCONN:
-                               /*
-                                * The application's socket is not valid. Either a bad socket
-                                * or a timeout on it. We can't inform the caller that for a
-                                * specific app, the session failed so lets continue here.
-                                */
-                               ret = 0;        /* Not an error. */
-                               continue;
-                       case -ENOMEM:
-                       default:
-                               goto error_rcu_unlock;
-                       }
+                       goto error;
                }
-               assert(ua_sess);
-
-               pthread_mutex_lock(&ua_sess->lock);
 
-               if (ua_sess->deleted) {
-                       pthread_mutex_unlock(&ua_sess->lock);
-                       continue;
+               ret = ust_app_channel_send(app, usess,
+                       ua_sess, ua_chan);
+               if (ret) {
+                       goto error;
                }
 
-               if (!strncmp(uchan->name, DEFAULT_METADATA_NAME,
-                                       sizeof(uchan->name))) {
-                       copy_channel_attr_to_ustctl(&ua_sess->metadata_attr, &uchan->attr);
-                       ret = 0;
-               } else {
-                       /* Create channel onto application. We don't need the chan ref. */
-                       ret = create_ust_app_channel(ua_sess, uchan, app,
-                                       LTTNG_UST_CHAN_PER_CPU, usess, NULL);
-               }
-               pthread_mutex_unlock(&ua_sess->lock);
-               if (ret < 0) {
-                       /* Cleanup the created session if it's the case. */
-                       if (created) {
-                               destroy_app_session(app, ua_sess);
-                       }
-                       switch (ret) {
-                       case -ENOTCONN:
-                               /*
-                                * The application's socket is not valid. Either a bad socket
-                                * or a timeout on it. We can't inform the caller that for a
-                                * specific app, the session failed so lets continue here.
-                                */
-                               ret = 0;        /* Not an error. */
-                               continue;
-                       case -ENOMEM:
-                       default:
-                               goto error_rcu_unlock;
+               /* Add contexts. */
+               cds_list_for_each_entry(uctx, &uchan->ctx_list, list) {
+                       ret = create_ust_app_channel_context(ua_chan,
+                               &uctx->ctx, app);
+                       if (ret) {
+                               goto error;
                        }
                }
        }
 
-error_rcu_unlock:
-       rcu_read_unlock();
+error:
+       if (ret < 0) {
+               switch (ret) {
+               case -ENOTCONN:
+                       /*
+                        * The application's socket is not valid. Either a bad socket
+                        * or a timeout on it. We can't inform the caller that for a
+                        * specific app, the session failed so lets continue here.
+                        */
+                       ret = 0;        /* Not an error. */
+                       break;
+               case -ENOMEM:
+               default:
+                       break;
+               }
+       }
+
+       if (ret == 0 && _ua_chan) {
+               /*
+                * Only return the application's channel on success. Note
+                * that the channel can still be part of the application's
+                * channel hashtable on error.
+                */
+               *_ua_chan = ua_chan;
+       }
        return ret;
 }
 
@@ -4261,6 +4258,7 @@ int ust_app_enable_event_glb(struct ltt_ust_session *usess,
        struct ust_app_channel *ua_chan;
        struct ust_app_event *ua_event;
 
+       assert(usess->active);
        DBG("UST app enabling event %s for all apps for session id %" PRIu64,
                        uevent->attr.name, usess->id);
 
@@ -4346,6 +4344,7 @@ int ust_app_create_event_glb(struct ltt_ust_session *usess,
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;
 
+       assert(usess->active);
        DBG("UST app creating event %s for all apps for session id %" PRIu64,
                        uevent->attr.name, usess->id);
 
@@ -4395,7 +4394,6 @@ int ust_app_create_event_glb(struct ltt_ust_session *usess,
        }
 
        rcu_read_unlock();
-
        return ret;
 }
 
@@ -4432,61 +4430,20 @@ int ust_app_start_trace(struct ltt_ust_session *usess, struct ust_app *app)
                goto end;
        }
 
-       /* Upon restart, we skip the setup, already done */
-       if (ua_sess->started) {
-               goto skip_setup;
-       }
-
-       /* Create directories if consumer is LOCAL and has a path defined. */
-       if (usess->consumer->type == CONSUMER_DST_LOCAL &&
-                       usess->consumer->dst.session_root_path[0] != '\0') {
-               char *tmp_path;
-
-               tmp_path = zmalloc(LTTNG_PATH_MAX);
-               if (!tmp_path) {
-                       ERR("Alloc tmp_path");
-                       goto error_unlock;
-               }
-               ret = snprintf(tmp_path, LTTNG_PATH_MAX, "%s%s%s",
-                               usess->consumer->dst.session_root_path,
-                               usess->consumer->chunk_path,
-                               usess->consumer->subdir);
-               if (ret >= LTTNG_PATH_MAX) {
-                       ERR("Local destination path exceeds the maximal allowed length of %i bytes (needs %i bytes) with path = \"%s%s%s\"",
-                                       LTTNG_PATH_MAX, ret,
-                                       usess->consumer->dst.session_root_path,
-                                       usess->consumer->chunk_path,
-                                       usess->consumer->subdir);
-                       free(tmp_path);
-                       goto error_unlock;
-               }
-
-               DBG("Creating directory path for local tracing: \"%s\"",
-                               tmp_path);
-               ret = run_as_mkdir_recursive(tmp_path, S_IRWXU | S_IRWXG,
-                               ua_sess->euid, ua_sess->egid);
-               free(tmp_path);
-               if (ret < 0) {
-                       if (errno != EEXIST) {
-                               ERR("Trace directory creation error");
-                               goto error_unlock;
-                       }
-               }
+       if (ua_sess->enabled) {
+               pthread_mutex_unlock(&ua_sess->lock);
+               goto end;
        }
 
-       /*
-        * Create the metadata for the application. This returns gracefully if a
-        * metadata was already set for the session.
-        */
-       ret = create_ust_app_metadata(ua_sess, app, usess->consumer);
-       if (ret < 0) {
-               goto error_unlock;
+       /* Upon restart, we skip the setup, already done */
+       if (ua_sess->started) {
+               goto skip_setup;
        }
 
        health_code_update();
 
 skip_setup:
-       /* This start the UST tracing */
+       /* This starts the UST tracing */
        pthread_mutex_lock(&app->sock_lock);
        ret = ustctl_start_session(app->sock, ua_sess->handle);
        pthread_mutex_unlock(&app->sock_lock);
@@ -4509,6 +4466,7 @@ skip_setup:
 
        /* Indicate that the session has been started once */
        ua_sess->started = 1;
+       ua_sess->enabled = 1;
 
        pthread_mutex_unlock(&ua_sess->lock);
 
@@ -4598,6 +4556,7 @@ int ust_app_stop_trace(struct ltt_ust_session *usess, struct ust_app *app)
        }
 
        health_code_update();
+       ua_sess->enabled = 0;
 
        /* Quiescent wait after stopping trace */
        pthread_mutex_lock(&app->sock_lock);
@@ -4968,12 +4927,17 @@ end:
  */
 int ust_app_start_trace_all(struct ltt_ust_session *usess)
 {
-       int ret = 0;
        struct lttng_ht_iter iter;
        struct ust_app *app;
 
        DBG("Starting all UST traces");
 
+       /*
+        * Even though the start trace might fail, flag this session active so
+        * other application coming in are started by default.
+        */
+       usess->active = 1;
+
        rcu_read_lock();
 
        /*
@@ -4985,11 +4949,7 @@ int ust_app_start_trace_all(struct ltt_ust_session *usess)
        (void) ust_app_clear_quiescent_session(usess);
 
        cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
-               ret = ust_app_start_trace(usess, app);
-               if (ret < 0) {
-                       /* Continue to next apps even on error */
-                       continue;
-               }
+               ust_app_global_update(usess, app);
        }
 
        rcu_read_unlock();
@@ -5009,6 +4969,12 @@ int ust_app_stop_trace_all(struct ltt_ust_session *usess)
 
        DBG("Stopping all UST traces");
 
+       /*
+        * Even though the stop trace might fail, flag this session inactive so
+        * other application coming in are not started by default.
+        */
+       usess->active = 0;
+
        rcu_read_lock();
 
        cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
@@ -5052,92 +5018,163 @@ int ust_app_destroy_trace_all(struct ltt_ust_session *usess)
        return 0;
 }
 
+/* The ua_sess lock must be held by the caller. */
 static
-void ust_app_global_create(struct ltt_ust_session *usess, struct ust_app *app)
+int find_or_create_ust_app_channel(
+               struct ltt_ust_session *usess,
+               struct ust_app_session *ua_sess,
+               struct ust_app *app,
+               struct ltt_ust_channel *uchan,
+               struct ust_app_channel **ua_chan)
 {
        int ret = 0;
-       struct lttng_ht_iter iter, uiter;
+       struct lttng_ht_iter iter;
+       struct lttng_ht_node_str *ua_chan_node;
+
+       lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &iter);
+       ua_chan_node = lttng_ht_iter_get_node_str(&iter);
+       if (ua_chan_node) {
+               *ua_chan = caa_container_of(ua_chan_node,
+                       struct ust_app_channel, node);
+               goto end;
+       }
+
+       ret = ust_app_channel_create(usess, ua_sess, uchan, app, ua_chan);
+       if (ret) {
+               goto end;
+       }
+end:
+       return ret;
+}
+
+static
+int ust_app_channel_synchronize_event(struct ust_app_channel *ua_chan,
+               struct ltt_ust_event *uevent, struct ust_app_session *ua_sess,
+               struct ust_app *app)
+{
+       int ret = 0;
+       struct ust_app_event *ua_event = NULL;
+
+       ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
+               uevent->filter, uevent->attr.loglevel, uevent->exclusion);
+       if (!ua_event) {
+               ret = create_ust_app_event(ua_sess, ua_chan, uevent, app);
+               if (ret < 0) {
+                       goto end;
+               }
+       } else {
+               if (ua_event->enabled != uevent->enabled) {
+                       ret = uevent->enabled ?
+                               enable_ust_app_event(ua_sess, ua_event, app) :
+                               disable_ust_app_event(ua_sess, ua_event, app);
+               }
+       }
+
+end:
+       return ret;
+}
+
+/*
+ * The caller must ensure that the application is compatible and is tracked
+ * by the process attribute trackers.
+ */
+static
+void ust_app_synchronize(struct ltt_ust_session *usess,
+               struct ust_app *app)
+{
+       int ret = 0;
+       struct cds_lfht_iter uchan_iter;
+       struct ltt_ust_channel *uchan;
        struct ust_app_session *ua_sess = NULL;
-       struct ust_app_channel *ua_chan;
-       struct ust_app_event *ua_event;
-       struct ust_app_ctx *ua_ctx;
-       int is_created = 0;
 
-       ret = find_or_create_ust_app_session(usess, app, &ua_sess, &is_created);
+       /*
+        * The application's configuration should only be synchronized for
+        * active sessions.
+        */
+       assert(usess->active);
+
+       ret = find_or_create_ust_app_session(usess, app, &ua_sess, NULL);
        if (ret < 0) {
                /* Tracer is probably gone or ENOMEM. */
                goto error;
        }
-       if (!is_created) {
-               /* App session already created. */
-               goto end;
-       }
        assert(ua_sess);
 
        pthread_mutex_lock(&ua_sess->lock);
-
        if (ua_sess->deleted) {
                pthread_mutex_unlock(&ua_sess->lock);
                goto end;
        }
 
-       /*
-        * We can iterate safely here over all UST app session since the create ust
-        * app session above made a shadow copy of the UST global domain from the
-        * ltt ust session.
-        */
-       cds_lfht_for_each_entry(ua_sess->channels->ht, &iter.iter, ua_chan,
-                       node.node) {
-               ret = do_create_channel(app, usess, ua_sess, ua_chan);
-               if (ret < 0 && ret != -ENOTCONN) {
-                       /*
-                        * Stop everything. On error, the application
-                        * failed, no more file descriptor are available
-                        * or ENOMEM so stopping here is the only thing
-                        * we can do for now. The only exception is
-                        * -ENOTCONN, which indicates that the application
-                        * has exit.
-                        */
-                       goto error_unlock;
-               }
+       rcu_read_lock();
+
+       cds_lfht_for_each_entry(usess->domain_global.channels->ht, &uchan_iter,
+                       uchan, node.node) {
+               struct ust_app_channel *ua_chan;
+               struct cds_lfht_iter uevent_iter;
+               struct ltt_ust_event *uevent;
 
                /*
-                * Add context using the list so they are enabled in the same order the
-                * user added them.
+                * Search for a matching ust_app_channel. If none is found,
+                * create it. Creating the channel will cause the ua_chan
+                * structure to be allocated, the channel buffers to be
+                * allocated (if necessary) and sent to the application, and
+                * all enabled contexts will be added to the channel.
                 */
-               cds_list_for_each_entry(ua_ctx, &ua_chan->ctx_list, list) {
-                       ret = create_ust_channel_context(ua_chan, ua_ctx, app);
-                       if (ret < 0) {
-                               goto error_unlock;
-                       }
+               ret = find_or_create_ust_app_channel(usess, ua_sess,
+                       app, uchan, &ua_chan);
+               if (ret) {
+                       /* Tracer is probably gone or ENOMEM. */
+                       goto error_unlock;
                }
 
+               if (!ua_chan) {
+                       /* ua_chan will be NULL for the metadata channel */
+                       continue;
+               }
 
-               /* For each events */
-               cds_lfht_for_each_entry(ua_chan->events->ht, &uiter.iter, ua_event,
+               cds_lfht_for_each_entry(uchan->events->ht, &uevent_iter, uevent,
                                node.node) {
-                       ret = create_ust_event(app, ua_sess, ua_chan, ua_event);
-                       if (ret < 0) {
+                       ret = ust_app_channel_synchronize_event(ua_chan,
+                               uevent, ua_sess, app);
+                       if (ret) {
                                goto error_unlock;
                        }
                }
-       }
-
-       pthread_mutex_unlock(&ua_sess->lock);
 
-       if (usess->active) {
-               ret = ust_app_start_trace(usess, app);
-               if (ret < 0) {
-                       goto error;
+               if (ua_chan->enabled != uchan->enabled) {
+                       ret = uchan->enabled ?
+                               enable_ust_app_channel(ua_sess, uchan, app) :
+                               disable_ust_app_channel(ua_sess, ua_chan, app);
+                       if (ret) {
+                               goto error_unlock;
+                       }
                }
+       }
 
-               DBG2("UST trace started for app pid %d", app->pid);
+       /*
+        * Create the metadata for the application. This returns gracefully if a
+        * metadata was already set for the session.
+        *
+        * The metadata channel must be created after the data channels as the
+        * consumer daemon assumes this ordering. When interacting with a relay
+        * daemon, the consumer will use this assumption to send the
+        * "STREAMS_SENT" message to the relay daemon.
+        */
+       ret = create_ust_app_metadata(ua_sess, app, usess->consumer);
+       if (ret < 0) {
+               goto error_unlock;
        }
+
+       rcu_read_unlock();
+
 end:
+       pthread_mutex_unlock(&ua_sess->lock);
        /* Everything went well at this point. */
        return;
 
 error_unlock:
+       rcu_read_unlock();
        pthread_mutex_unlock(&ua_sess->lock);
 error:
        if (ua_sess) {
@@ -5167,6 +5204,7 @@ void ust_app_global_destroy(struct ltt_ust_session *usess, struct ust_app *app)
 void ust_app_global_update(struct ltt_ust_session *usess, struct ust_app *app)
 {
        assert(usess);
+       assert(usess->active);
 
        DBG2("UST app global update for app sock %d for session id %" PRIu64,
                        app->sock, usess->id);
@@ -5174,9 +5212,20 @@ void ust_app_global_update(struct ltt_ust_session *usess, struct ust_app *app)
        if (!app->compatible) {
                return;
        }
-
-       if (trace_ust_pid_tracker_lookup(usess, app->pid)) {
-               ust_app_global_create(usess, app);
+       if (trace_ust_id_tracker_lookup(LTTNG_PROCESS_ATTR_VIRTUAL_PROCESS_ID,
+                           usess, app->pid) &&
+                       trace_ust_id_tracker_lookup(
+                                       LTTNG_PROCESS_ATTR_VIRTUAL_USER_ID,
+                                       usess, app->uid) &&
+                       trace_ust_id_tracker_lookup(
+                                       LTTNG_PROCESS_ATTR_VIRTUAL_GROUP_ID,
+                                       usess, app->gid)) {
+               /*
+                * Synchronize the application's internal tracing configuration
+                * and start tracing.
+                */
+               ust_app_synchronize(usess, app);
+               ust_app_start_trace(usess, app);
        } else {
                ust_app_global_destroy(usess, app);
        }
@@ -5210,8 +5259,9 @@ int ust_app_add_ctx_channel_glb(struct ltt_ust_session *usess,
        struct ust_app_session *ua_sess;
        struct ust_app *app;
 
-       rcu_read_lock();
+       assert(usess->active);
 
+       rcu_read_lock();
        cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                if (!app->compatible) {
                        /*
@@ -5252,79 +5302,6 @@ int ust_app_add_ctx_channel_glb(struct ltt_ust_session *usess,
        return ret;
 }
 
-/*
- * Enable event for a channel from a UST session for a specific PID.
- */
-int ust_app_enable_event_pid(struct ltt_ust_session *usess,
-               struct ltt_ust_channel *uchan, struct ltt_ust_event *uevent, pid_t pid)
-{
-       int ret = 0;
-       struct lttng_ht_iter iter;
-       struct lttng_ht_node_str *ua_chan_node;
-       struct ust_app *app;
-       struct ust_app_session *ua_sess;
-       struct ust_app_channel *ua_chan;
-       struct ust_app_event *ua_event;
-
-       DBG("UST app enabling event %s for PID %d", uevent->attr.name, pid);
-
-       rcu_read_lock();
-
-       app = ust_app_find_by_pid(pid);
-       if (app == NULL) {
-               ERR("UST app enable event per PID %d not found", pid);
-               ret = -1;
-               goto end;
-       }
-
-       if (!app->compatible) {
-               ret = 0;
-               goto end;
-       }
-
-       ua_sess = lookup_session_by_app(usess, app);
-       if (!ua_sess) {
-               /* The application has problem or is probably dead. */
-               ret = 0;
-               goto end;
-       }
-
-       pthread_mutex_lock(&ua_sess->lock);
-
-       if (ua_sess->deleted) {
-               ret = 0;
-               goto end_unlock;
-       }
-
-       /* Lookup channel in the ust app session */
-       lttng_ht_lookup(ua_sess->channels, (void *)uchan->name, &iter);
-       ua_chan_node = lttng_ht_iter_get_node_str(&iter);
-       /* If the channel is not found, there is a code flow error */
-       assert(ua_chan_node);
-
-       ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
-
-       ua_event = find_ust_app_event(ua_chan->events, uevent->attr.name,
-                       uevent->filter, uevent->attr.loglevel, uevent->exclusion);
-       if (ua_event == NULL) {
-               ret = create_ust_app_event(ua_sess, ua_chan, uevent, app);
-               if (ret < 0) {
-                       goto end_unlock;
-               }
-       } else {
-               ret = enable_ust_app_event(ua_sess, ua_event, app);
-               if (ret < 0) {
-                       goto end_unlock;
-               }
-       }
-
-end_unlock:
-       pthread_mutex_unlock(&ua_sess->lock);
-end:
-       rcu_read_unlock();
-       return ret;
-}
-
 /*
  * Receive registration and populate the given msg structure.
  *
@@ -5952,30 +5929,24 @@ void ust_app_destroy(struct ust_app *app)
  * Take a snapshot for a given UST session. The snapshot is sent to the given
  * output.
  *
- * Return 0 on success or else a negative value.
+ * Returns LTTNG_OK on success or a LTTNG_ERR error code.
  */
-int ust_app_snapshot_record(struct ltt_ust_session *usess,
-               struct snapshot_output *output, int wait,
+enum lttng_error_code ust_app_snapshot_record(
+               const struct ltt_ust_session *usess,
+               const struct consumer_output *output, int wait,
                uint64_t nb_packets_per_stream)
 {
        int ret = 0;
+       enum lttng_error_code status = LTTNG_OK;
        struct lttng_ht_iter iter;
        struct ust_app *app;
-       char pathname[PATH_MAX];
-       struct ltt_session *session;
-       uint64_t trace_archive_id;
+       char *trace_path = NULL;
 
        assert(usess);
        assert(output);
 
        rcu_read_lock();
 
-       session = session_find_by_id(usess->id);
-       assert(session);
-       assert(pthread_mutex_trylock(&session->lock));
-       assert(session_trylock_list());
-       trace_archive_id = session->current_archive_id;
-
        switch (usess->buffer_type) {
        case LTTNG_BUFFER_PER_UID:
        {
@@ -5984,6 +5955,8 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
                cds_list_for_each_entry(reg, &usess->buffer_reg_uid_list, lnode) {
                        struct buffer_reg_channel *reg_chan;
                        struct consumer_socket *socket;
+                       char pathname[PATH_MAX];
+                       size_t consumer_path_offset = 0;
 
                        if (!reg->registry->reg.ust->metadata_key) {
                                /* Skip since no metadata is present */
@@ -5994,7 +5967,7 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
                        socket = consumer_find_socket_by_bitness(reg->bits_per_long,
                                        usess->consumer);
                        if (!socket) {
-                               ret = -EINVAL;
+                               status = LTTNG_ERR_INVALID;
                                goto error;
                        }
 
@@ -6004,27 +5977,34 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
                                        reg->uid, reg->bits_per_long);
                        if (ret < 0) {
                                PERROR("snprintf snapshot path");
+                               status = LTTNG_ERR_INVALID;
+                               goto error;
+                       }
+                       /* Free path allowed on previous iteration. */
+                       free(trace_path);
+                       trace_path = setup_channel_trace_path(usess->consumer, pathname,
+                                               &consumer_path_offset);
+                       if (!trace_path) {
+                               status = LTTNG_ERR_INVALID;
                                goto error;
                        }
-
                        /* Add the UST default trace dir to path. */
                        cds_lfht_for_each_entry(reg->registry->channels->ht, &iter.iter,
                                        reg_chan, node.node) {
-                               ret = consumer_snapshot_channel(socket,
+                               status = consumer_snapshot_channel(socket,
                                                reg_chan->consumer_key,
                                                output, 0, usess->uid,
-                                               usess->gid, pathname, wait,
-                                               nb_packets_per_stream,
-                                               trace_archive_id);
-                               if (ret < 0) {
+                                               usess->gid, &trace_path[consumer_path_offset], wait,
+                                               nb_packets_per_stream);
+                               if (status != LTTNG_OK) {
                                        goto error;
                                }
                        }
-                       ret = consumer_snapshot_channel(socket,
+                       status = consumer_snapshot_channel(socket,
                                        reg->registry->reg.ust->metadata_key, output, 1,
-                                       usess->uid, usess->gid, pathname, wait, 0,
-                                       trace_archive_id);
-                       if (ret < 0) {
+                                       usess->uid, usess->gid, &trace_path[consumer_path_offset],
+                                       wait, 0);
+                       if (status != LTTNG_OK) {
                                goto error;
                        }
                }
@@ -6038,6 +6018,8 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
                        struct ust_app_channel *ua_chan;
                        struct ust_app_session *ua_sess;
                        struct ust_registry_session *registry;
+                       char pathname[PATH_MAX];
+                       size_t consumer_path_offset = 0;
 
                        ua_sess = lookup_session_by_app(usess, app);
                        if (!ua_sess) {
@@ -6047,9 +6029,9 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
 
                        /* Get the right consumer socket for the application. */
                        socket = consumer_find_socket_by_bitness(app->bits_per_long,
-                                       output->consumer);
+                                       output);
                        if (!socket) {
-                               ret = -EINVAL;
+                               status = LTTNG_ERR_INVALID;
                                goto error;
                        }
 
@@ -6058,22 +6040,32 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
                        ret = snprintf(pathname, sizeof(pathname), DEFAULT_UST_TRACE_DIR "/%s",
                                        ua_sess->path);
                        if (ret < 0) {
+                               status = LTTNG_ERR_INVALID;
                                PERROR("snprintf snapshot path");
                                goto error;
                        }
-
+                       /* Free path allowed on previous iteration. */
+                       free(trace_path);
+                       trace_path = setup_channel_trace_path(usess->consumer, pathname,
+                                       &consumer_path_offset);
+                       if (!trace_path) {
+                               status = LTTNG_ERR_INVALID;
+                               goto error;
+                       }
                        cds_lfht_for_each_entry(ua_sess->channels->ht, &chan_iter.iter,
                                        ua_chan, node.node) {
-                               ret = consumer_snapshot_channel(socket,
-                                               ua_chan->key, output,
-                                               0, ua_sess->euid, ua_sess->egid,
-                                               pathname, wait,
-                                               nb_packets_per_stream,
-                                               trace_archive_id);
-                               if (ret < 0) {
-                                       if (ret == -LTTNG_ERR_CHAN_NOT_FOUND) {
-                                               continue;
-                                       }
+                               status = consumer_snapshot_channel(socket,
+                                               ua_chan->key, output, 0,
+                                               lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                                               lttng_credentials_get_gid(&ua_sess->effective_credentials),
+                                               &trace_path[consumer_path_offset], wait,
+                                               nb_packets_per_stream);
+                               switch (status) {
+                               case LTTNG_OK:
+                                       break;
+                               case LTTNG_ERR_CHAN_NOT_FOUND:
+                                       continue;
+                               default:
                                        goto error;
                                }
                        }
@@ -6083,15 +6075,17 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
                                DBG("Application session is being torn down. Skip application.");
                                continue;
                        }
-                       ret = consumer_snapshot_channel(socket,
-                                       registry->metadata_key, output,
-                                       1, ua_sess->euid, ua_sess->egid,
-                                       pathname, wait, 0,
-                                       trace_archive_id);
-                       if (ret < 0) {
-                               if (ret == -LTTNG_ERR_CHAN_NOT_FOUND) {
-                                       continue;
-                               }
+                       status = consumer_snapshot_channel(socket,
+                                       registry->metadata_key, output, 1,
+                                       lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                                       lttng_credentials_get_gid(&ua_sess->effective_credentials),
+                                       &trace_path[consumer_path_offset], wait, 0);
+                       switch (status) {
+                       case LTTNG_OK:
+                               break;
+                       case LTTNG_ERR_CHAN_NOT_FOUND:
+                               continue;
+                       default:
                                goto error;
                        }
                }
@@ -6103,15 +6097,16 @@ int ust_app_snapshot_record(struct ltt_ust_session *usess,
        }
 
 error:
+       free(trace_path);
        rcu_read_unlock();
-       return ret;
+       return status;
 }
 
 /*
  * Return the size taken by one more packet per stream.
  */
-uint64_t ust_app_get_size_one_more_packet_per_stream(struct ltt_ust_session *usess,
-               uint64_t cur_nr_packets)
+uint64_t ust_app_get_size_one_more_packet_per_stream(
+               const struct ltt_ust_session *usess, uint64_t cur_nr_packets)
 {
        uint64_t tot_size = 0;
        struct ust_app *app;
@@ -6350,7 +6345,6 @@ enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
        struct lttng_ht_iter iter;
        struct ust_app *app;
        struct ltt_ust_session *usess = session->ust_session;
-       char pathname[LTTNG_PATH_MAX];
 
        assert(usess);
 
@@ -6365,6 +6359,11 @@ enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
                        struct buffer_reg_channel *reg_chan;
                        struct consumer_socket *socket;
 
+                       if (!reg->registry->reg.ust->metadata_key) {
+                               /* Skip since no metadata is present */
+                               continue;
+                       }
+
                        /* Get consumer socket to use to push the metadata.*/
                        socket = consumer_find_socket_by_bitness(reg->bits_per_long,
                                        usess->consumer);
@@ -6373,24 +6372,14 @@ enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
                                goto error;
                        }
 
-                       ret = snprintf(pathname, sizeof(pathname),
-                                       DEFAULT_UST_TRACE_DIR "/" DEFAULT_UST_TRACE_UID_PATH,
-                                       reg->uid, reg->bits_per_long);
-                       if (ret < 0 || ret == sizeof(pathname)) {
-                               PERROR("Failed to format rotation path");
-                               cmd_ret = LTTNG_ERR_INVALID;
-                               goto error;
-                       }
-
                        /* Rotate the data channels. */
                        cds_lfht_for_each_entry(reg->registry->channels->ht, &iter.iter,
                                        reg_chan, node.node) {
                                ret = consumer_rotate_channel(socket,
                                                reg_chan->consumer_key,
                                                usess->uid, usess->gid,
-                                               usess->consumer, pathname,
-                                               /* is_metadata_channel */ false,
-                                               session->current_archive_id);
+                                               usess->consumer,
+                                               /* is_metadata_channel */ false);
                                if (ret < 0) {
                                        cmd_ret = LTTNG_ERR_ROTATION_FAIL_CONSUMER;
                                        goto error;
@@ -6402,9 +6391,8 @@ enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
                        ret = consumer_rotate_channel(socket,
                                        reg->registry->reg.ust->metadata_key,
                                        usess->uid, usess->gid,
-                                       usess->consumer, pathname,
-                                       /* is_metadata_channel */ true,
-                                       session->current_archive_id);
+                                       usess->consumer,
+                                       /* is_metadata_channel */ true);
                        if (ret < 0) {
                                cmd_ret = LTTNG_ERR_ROTATION_FAIL_CONSUMER;
                                goto error;
@@ -6426,14 +6414,6 @@ enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
                                /* Session not associated with this app. */
                                continue;
                        }
-                       ret = snprintf(pathname, sizeof(pathname),
-                                       DEFAULT_UST_TRACE_DIR "/%s",
-                                       ua_sess->path);
-                       if (ret < 0 || ret == sizeof(pathname)) {
-                               PERROR("Failed to format rotation path");
-                               cmd_ret = LTTNG_ERR_INVALID;
-                               goto error;
-                       }
 
                        /* Get the right consumer socket for the application. */
                        socket = consumer_find_socket_by_bitness(app->bits_per_long,
@@ -6449,15 +6429,15 @@ enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
                                continue;
                        }
 
-
                        /* Rotate the data channels. */
                        cds_lfht_for_each_entry(ua_sess->channels->ht, &chan_iter.iter,
                                        ua_chan, node.node) {
-                               ret = consumer_rotate_channel(socket, ua_chan->key,
-                                               ua_sess->euid, ua_sess->egid,
-                                               ua_sess->consumer, pathname,
-                                               /* is_metadata_channel */ false,
-                                               session->current_archive_id);
+                               ret = consumer_rotate_channel(socket,
+                                               ua_chan->key,
+                                               lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                                               lttng_credentials_get_gid(&ua_sess->effective_credentials),
+                                               ua_sess->consumer,
+                                               /* is_metadata_channel */ false);
                                if (ret < 0) {
                                        /* Per-PID buffer and application going away. */
                                        if (ret == -LTTNG_ERR_CHAN_NOT_FOUND)
@@ -6469,11 +6449,12 @@ enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
 
                        /* Rotate the metadata channel. */
                        (void) push_metadata(registry, usess->consumer);
-                       ret = consumer_rotate_channel(socket, registry->metadata_key,
-                                       ua_sess->euid, ua_sess->egid,
-                                       ua_sess->consumer, pathname,
-                                       /* is_metadata_channel */ true,
-                                       session->current_archive_id);
+                       ret = consumer_rotate_channel(socket,
+                                       registry->metadata_key,
+                                       lttng_credentials_get_uid(&ua_sess->effective_credentials),
+                                       lttng_credentials_get_gid(&ua_sess->effective_credentials),
+                                       ua_sess->consumer,
+                                       /* is_metadata_channel */ true);
                        if (ret < 0) {
                                /* Per-PID buffer and application going away. */
                                if (ret == -LTTNG_ERR_CHAN_NOT_FOUND)
@@ -6495,3 +6476,381 @@ error:
        rcu_read_unlock();
        return cmd_ret;
 }
+
+enum lttng_error_code ust_app_create_channel_subdirectories(
+               const struct ltt_ust_session *usess)
+{
+       enum lttng_error_code ret = LTTNG_OK;
+       struct lttng_ht_iter iter;
+       enum lttng_trace_chunk_status chunk_status;
+       char *pathname_index;
+       int fmt_ret;
+
+       assert(usess->current_trace_chunk);
+       rcu_read_lock();
+
+       switch (usess->buffer_type) {
+       case LTTNG_BUFFER_PER_UID:
+       {
+               struct buffer_reg_uid *reg;
+
+               cds_list_for_each_entry(reg, &usess->buffer_reg_uid_list, lnode) {
+                       fmt_ret = asprintf(&pathname_index,
+                                      DEFAULT_UST_TRACE_DIR "/" DEFAULT_UST_TRACE_UID_PATH "/" DEFAULT_INDEX_DIR,
+                                      reg->uid, reg->bits_per_long);
+                       if (fmt_ret < 0) {
+                               ERR("Failed to format channel index directory");
+                               ret = LTTNG_ERR_CREATE_DIR_FAIL;
+                               goto error;
+                       }
+
+                       /*
+                        * Create the index subdirectory which will take care
+                        * of implicitly creating the channel's path.
+                        */
+                       chunk_status = lttng_trace_chunk_create_subdirectory(
+                                       usess->current_trace_chunk,
+                                       pathname_index);
+                       free(pathname_index);
+                       if (chunk_status != LTTNG_TRACE_CHUNK_STATUS_OK) {
+                               ret = LTTNG_ERR_CREATE_DIR_FAIL;
+                               goto error;
+                       }
+               }
+               break;
+       }
+       case LTTNG_BUFFER_PER_PID:
+       {
+               struct ust_app *app;
+
+               /*
+                * Create the toplevel ust/ directory in case no apps are running.
+                */
+               chunk_status = lttng_trace_chunk_create_subdirectory(
+                               usess->current_trace_chunk,
+                               DEFAULT_UST_TRACE_DIR);
+               if (chunk_status != LTTNG_TRACE_CHUNK_STATUS_OK) {
+                       ret = LTTNG_ERR_CREATE_DIR_FAIL;
+                       goto error;
+               }
+
+               cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app,
+                               pid_n.node) {
+                       struct ust_app_session *ua_sess;
+                       struct ust_registry_session *registry;
+
+                       ua_sess = lookup_session_by_app(usess, app);
+                       if (!ua_sess) {
+                               /* Session not associated with this app. */
+                               continue;
+                       }
+
+                       registry = get_session_registry(ua_sess);
+                       if (!registry) {
+                               DBG("Application session is being torn down. Skip application.");
+                               continue;
+                       }
+
+                       fmt_ret = asprintf(&pathname_index,
+                                       DEFAULT_UST_TRACE_DIR "/%s/" DEFAULT_INDEX_DIR,
+                                       ua_sess->path);
+                       if (fmt_ret < 0) {
+                               ERR("Failed to format channel index directory");
+                               ret = LTTNG_ERR_CREATE_DIR_FAIL;
+                               goto error;
+                       }
+                       /*
+                        * Create the index subdirectory which will take care
+                        * of implicitly creating the channel's path.
+                        */
+                       chunk_status = lttng_trace_chunk_create_subdirectory(
+                                       usess->current_trace_chunk,
+                                       pathname_index);
+                       free(pathname_index);
+                       if (chunk_status != LTTNG_TRACE_CHUNK_STATUS_OK) {
+                               ret = LTTNG_ERR_CREATE_DIR_FAIL;
+                               goto error;
+                       }
+               }
+               break;
+       }
+       default:
+               abort();
+       }
+
+       ret = LTTNG_OK;
+error:
+       rcu_read_unlock();
+       return ret;
+}
+
+/*
+ * Clear all the channels of a session.
+ *
+ * Return LTTNG_OK on success or else an LTTng error code.
+ */
+enum lttng_error_code ust_app_clear_session(struct ltt_session *session)
+{
+       int ret;
+       enum lttng_error_code cmd_ret = LTTNG_OK;
+       struct lttng_ht_iter iter;
+       struct ust_app *app;
+       struct ltt_ust_session *usess = session->ust_session;
+
+       assert(usess);
+
+       rcu_read_lock();
+
+       if (usess->active) {
+               ERR("Expecting inactive session %s (%" PRIu64 ")", session->name, session->id);
+               cmd_ret = LTTNG_ERR_FATAL;
+               goto end;
+       }
+
+       switch (usess->buffer_type) {
+       case LTTNG_BUFFER_PER_UID:
+       {
+               struct buffer_reg_uid *reg;
+
+               cds_list_for_each_entry(reg, &usess->buffer_reg_uid_list, lnode) {
+                       struct buffer_reg_channel *reg_chan;
+                       struct consumer_socket *socket;
+
+                       /* Get consumer socket to use to push the metadata.*/
+                       socket = consumer_find_socket_by_bitness(reg->bits_per_long,
+                                       usess->consumer);
+                       if (!socket) {
+                               cmd_ret = LTTNG_ERR_INVALID;
+                               goto error_socket;
+                       }
+
+                       /* Clear the data channels. */
+                       cds_lfht_for_each_entry(reg->registry->channels->ht, &iter.iter,
+                                       reg_chan, node.node) {
+                               ret = consumer_clear_channel(socket,
+                                               reg_chan->consumer_key);
+                               if (ret < 0) {
+                                       goto error;
+                               }
+                       }
+
+                       (void) push_metadata(reg->registry->reg.ust, usess->consumer);
+
+                       /*
+                        * Clear the metadata channel.
+                        * Metadata channel is not cleared per se but we still need to
+                        * perform a rotation operation on it behind the scene.
+                        */
+                       ret = consumer_clear_channel(socket,
+                                       reg->registry->reg.ust->metadata_key);
+                       if (ret < 0) {
+                               goto error;
+                       }
+               }
+               break;
+       }
+       case LTTNG_BUFFER_PER_PID:
+       {
+               cds_lfht_for_each_entry(ust_app_ht->ht, &iter.iter, app, pid_n.node) {
+                       struct consumer_socket *socket;
+                       struct lttng_ht_iter chan_iter;
+                       struct ust_app_channel *ua_chan;
+                       struct ust_app_session *ua_sess;
+                       struct ust_registry_session *registry;
+
+                       ua_sess = lookup_session_by_app(usess, app);
+                       if (!ua_sess) {
+                               /* Session not associated with this app. */
+                               continue;
+                       }
+
+                       /* Get the right consumer socket for the application. */
+                       socket = consumer_find_socket_by_bitness(app->bits_per_long,
+                                       usess->consumer);
+                       if (!socket) {
+                               cmd_ret = LTTNG_ERR_INVALID;
+                               goto error_socket;
+                       }
+
+                       registry = get_session_registry(ua_sess);
+                       if (!registry) {
+                               DBG("Application session is being torn down. Skip application.");
+                               continue;
+                       }
+
+                       /* Clear the data channels. */
+                       cds_lfht_for_each_entry(ua_sess->channels->ht, &chan_iter.iter,
+                                       ua_chan, node.node) {
+                               ret = consumer_clear_channel(socket, ua_chan->key);
+                               if (ret < 0) {
+                                       /* Per-PID buffer and application going away. */
+                                       if (ret == -LTTNG_ERR_CHAN_NOT_FOUND) {
+                                               continue;
+                                       }
+                                       goto error;
+                               }
+                       }
+
+                       (void) push_metadata(registry, usess->consumer);
+
+                       /*
+                        * Clear the metadata channel.
+                        * Metadata channel is not cleared per se but we still need to
+                        * perform rotation operation on it behind the scene.
+                        */
+                       ret = consumer_clear_channel(socket, registry->metadata_key);
+                       if (ret < 0) {
+                               /* Per-PID buffer and application going away. */
+                               if (ret == -LTTNG_ERR_CHAN_NOT_FOUND) {
+                                       continue;
+                               }
+                               goto error;
+                       }
+               }
+               break;
+       }
+       default:
+               assert(0);
+               break;
+       }
+
+       cmd_ret = LTTNG_OK;
+       goto end;
+
+error:
+       switch (-ret) {
+       case LTTCOMM_CONSUMERD_RELAYD_CLEAR_DISALLOWED:
+               cmd_ret = LTTNG_ERR_CLEAR_RELAY_DISALLOWED;
+               break;
+       default:
+               cmd_ret = LTTNG_ERR_CLEAR_FAIL_CONSUMER;
+       }
+
+error_socket:
+end:
+       rcu_read_unlock();
+       return cmd_ret;
+}
+
+/*
+ * This function skips the metadata channel as the begin/end timestamps of a
+ * metadata packet are useless.
+ *
+ * Moreover, opening a packet after a "clear" will cause problems for live
+ * sessions as it will introduce padding that was not part of the first trace
+ * chunk. The relay daemon expects the content of the metadata stream of
+ * successive metadata trace chunks to be strict supersets of one another.
+ *
+ * For example, flushing a packet at the beginning of the metadata stream of
+ * a trace chunk resulting from a "clear" session command will cause the
+ * size of the metadata stream of the new trace chunk to not match the size of
+ * the metadata stream of the original chunk. This will confuse the relay
+ * daemon as the same "offset" in a metadata stream will no longer point
+ * to the same content.
+ */
+enum lttng_error_code ust_app_open_packets(struct ltt_session *session)
+{
+       enum lttng_error_code ret = LTTNG_OK;
+       struct lttng_ht_iter iter;
+       struct ltt_ust_session *usess = session->ust_session;
+
+       assert(usess);
+
+       rcu_read_lock();
+
+       switch (usess->buffer_type) {
+       case LTTNG_BUFFER_PER_UID:
+       {
+               struct buffer_reg_uid *reg;
+
+               cds_list_for_each_entry (
+                               reg, &usess->buffer_reg_uid_list, lnode) {
+                       struct buffer_reg_channel *reg_chan;
+                       struct consumer_socket *socket;
+
+                       socket = consumer_find_socket_by_bitness(
+                                       reg->bits_per_long, usess->consumer);
+                       if (!socket) {
+                               ret = LTTNG_ERR_FATAL;
+                               goto error;
+                       }
+
+                       cds_lfht_for_each_entry(reg->registry->channels->ht,
+                                       &iter.iter, reg_chan, node.node) {
+                               const int open_ret =
+                                               consumer_open_channel_packets(
+                                                       socket,
+                                                       reg_chan->consumer_key);
+
+                               if (open_ret < 0) {
+                                       ret = LTTNG_ERR_UNK;
+                                       goto error;
+                               }
+                       }
+               }
+               break;
+       }
+       case LTTNG_BUFFER_PER_PID:
+       {
+               struct ust_app *app;
+
+               cds_lfht_for_each_entry (
+                               ust_app_ht->ht, &iter.iter, app, pid_n.node) {
+                       struct consumer_socket *socket;
+                       struct lttng_ht_iter chan_iter;
+                       struct ust_app_channel *ua_chan;
+                       struct ust_app_session *ua_sess;
+                       struct ust_registry_session *registry;
+
+                       ua_sess = lookup_session_by_app(usess, app);
+                       if (!ua_sess) {
+                               /* Session not associated with this app. */
+                               continue;
+                       }
+
+                       /* Get the right consumer socket for the application. */
+                       socket = consumer_find_socket_by_bitness(
+                                       app->bits_per_long, usess->consumer);
+                       if (!socket) {
+                               ret = LTTNG_ERR_FATAL;
+                               goto error;
+                       }
+
+                       registry = get_session_registry(ua_sess);
+                       if (!registry) {
+                               DBG("Application session is being torn down. Skip application.");
+                               continue;
+                       }
+
+                       cds_lfht_for_each_entry(ua_sess->channels->ht,
+                                       &chan_iter.iter, ua_chan, node.node) {
+                               const int open_ret =
+                                               consumer_open_channel_packets(
+                                                       socket,
+                                                       ua_chan->key);
+
+                               if (open_ret < 0) {
+                                       /*
+                                        * Per-PID buffer and application going
+                                        * away.
+                                        */
+                                       if (open_ret == -LTTNG_ERR_CHAN_NOT_FOUND) {
+                                               continue;
+                                       }
+
+                                       ret = LTTNG_ERR_UNK;
+                                       goto error;
+                               }
+                       }
+               }
+               break;
+       }
+       default:
+               abort();
+               break;
+       }
+
+error:
+       rcu_read_unlock();
+       return ret;
+}
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