Fix: consumerd: double unlock on rotate channel error path
[lttng-tools.git] / src / common / consumer / consumer.c
index 0263489ddd8a908bacd2096ec14e68cfafbb1bd7..d45771e7eb19fb45123041736853143704db3aff 100644 (file)
@@ -1,22 +1,13 @@
 /*
- * Copyright (C) 2011 Julien Desfossez <julien.desfossez@polymtl.ca>
- *                      Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
- *               2012 - David Goulet <dgoulet@efficios.com>
+ * Copyright (C) 2011 Julien Desfossez <julien.desfossez@polymtl.ca>
+ * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ * Copyright (C) 2012 David Goulet <dgoulet@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.
  */
 
+#include "common/index/ctf-index.h"
 #define _LGPL_SOURCE
 #include <assert.h>
 #include <poll.h>
@@ -578,95 +569,6 @@ void consumer_stream_update_channel_attributes(
                        channel->tracefile_size;
 }
 
-struct lttng_consumer_stream *consumer_allocate_stream(uint64_t channel_key,
-               uint64_t stream_key,
-               const char *channel_name,
-               uint64_t relayd_id,
-               uint64_t session_id,
-               struct lttng_trace_chunk *trace_chunk,
-               int cpu,
-               int *alloc_ret,
-               enum consumer_channel_type type,
-               unsigned int monitor)
-{
-       int ret;
-       struct lttng_consumer_stream *stream;
-
-       stream = zmalloc(sizeof(*stream));
-       if (stream == NULL) {
-               PERROR("malloc struct lttng_consumer_stream");
-               ret = -ENOMEM;
-               goto end;
-       }
-
-       if (trace_chunk && !lttng_trace_chunk_get(trace_chunk)) {
-               ERR("Failed to acquire trace chunk reference during the creation of a stream");
-               ret = -1;
-               goto error;
-       }
-
-       rcu_read_lock();
-       stream->key = stream_key;
-       stream->trace_chunk = trace_chunk;
-       stream->out_fd = -1;
-       stream->out_fd_offset = 0;
-       stream->output_written = 0;
-       stream->net_seq_idx = relayd_id;
-       stream->session_id = session_id;
-       stream->monitor = monitor;
-       stream->endpoint_status = CONSUMER_ENDPOINT_ACTIVE;
-       stream->index_file = NULL;
-       stream->last_sequence_number = -1ULL;
-       stream->rotate_position = -1ULL;
-       pthread_mutex_init(&stream->lock, NULL);
-       pthread_mutex_init(&stream->metadata_timer_lock, NULL);
-
-       /* If channel is the metadata, flag this stream as metadata. */
-       if (type == CONSUMER_CHANNEL_TYPE_METADATA) {
-               stream->metadata_flag = 1;
-               /* Metadata is flat out. */
-               strncpy(stream->name, DEFAULT_METADATA_NAME, sizeof(stream->name));
-               /* Live rendez-vous point. */
-               pthread_cond_init(&stream->metadata_rdv, NULL);
-               pthread_mutex_init(&stream->metadata_rdv_lock, NULL);
-       } else {
-               /* Format stream name to <channel_name>_<cpu_number> */
-               ret = snprintf(stream->name, sizeof(stream->name), "%s_%d",
-                               channel_name, cpu);
-               if (ret < 0) {
-                       PERROR("snprintf stream name");
-                       goto error;
-               }
-       }
-
-       /* Key is always the wait_fd for streams. */
-       lttng_ht_node_init_u64(&stream->node, stream->key);
-
-       /* Init node per channel id key */
-       lttng_ht_node_init_u64(&stream->node_channel_id, channel_key);
-
-       /* Init session id node with the stream session id */
-       lttng_ht_node_init_u64(&stream->node_session_id, stream->session_id);
-
-       DBG3("Allocated stream %s (key %" PRIu64 ", chan_key %" PRIu64
-                       " relayd_id %" PRIu64 ", session_id %" PRIu64,
-                       stream->name, stream->key, channel_key,
-                       stream->net_seq_idx, stream->session_id);
-
-       rcu_read_unlock();
-       return stream;
-
-error:
-       rcu_read_unlock();
-       lttng_trace_chunk_put(stream->trace_chunk);
-       free(stream);
-end:
-       if (alloc_ret) {
-               *alloc_ret = ret;
-       }
-       return NULL;
-}
-
 /*
  * Add a stream to the global list protected by a mutex.
  */
@@ -1073,6 +975,7 @@ struct lttng_consumer_channel *consumer_allocate_channel(uint64_t key,
                uint64_t session_id_per_pid,
                unsigned int monitor,
                unsigned int live_timer_interval,
+               bool is_in_live_session,
                const char *root_shm_path,
                const char *shm_path)
 {
@@ -1104,6 +1007,7 @@ struct lttng_consumer_channel *consumer_allocate_channel(uint64_t key,
        channel->tracefile_count = tracefile_count;
        channel->monitor = monitor;
        channel->live_timer_interval = live_timer_interval;
+       channel->is_live = is_in_live_session;
        pthread_mutex_init(&channel->lock, NULL);
        pthread_mutex_init(&channel->timer_lock, NULL);
 
@@ -1472,7 +1376,7 @@ void lttng_consumer_sync_trace_file(struct lttng_consumer_stream *stream,
 struct lttng_consumer_local_data *lttng_consumer_create(
                enum lttng_consumer_type type,
                ssize_t (*buffer_ready)(struct lttng_consumer_stream *stream,
-                       struct lttng_consumer_local_data *ctx),
+                       struct lttng_consumer_local_data *ctx, bool locked_by_caller),
                int (*recv_channel)(struct lttng_consumer_channel *channel),
                int (*recv_stream)(struct lttng_consumer_stream *stream),
                int (*update_stream)(uint64_t stream_key, uint32_t state))
@@ -1679,19 +1583,18 @@ end:
  * Returns the number of bytes written
  */
 ssize_t lttng_consumer_on_read_subbuffer_mmap(
-               struct lttng_consumer_local_data *ctx,
-               struct lttng_consumer_stream *stream, unsigned long len,
-               unsigned long padding,
-               struct ctf_packet_index *index)
+               struct lttng_consumer_stream *stream,
+               const struct lttng_buffer_view *buffer,
+               unsigned long padding)
 {
-       unsigned long mmap_offset;
-       void *mmap_base;
        ssize_t ret = 0;
        off_t orig_offset = stream->out_fd_offset;
        /* Default is on the disk */
        int outfd = stream->out_fd;
        struct consumer_relayd_sock_pair *relayd = NULL;
        unsigned int relayd_hang_up = 0;
+       const size_t subbuf_content_size = buffer->size - padding;
+       size_t write_len;
 
        /* RCU lock for the relayd pointer */
        rcu_read_lock();
@@ -1707,39 +1610,9 @@ ssize_t lttng_consumer_on_read_subbuffer_mmap(
                }
        }
 
-       /* get the offset inside the fd to mmap */
-       switch (consumer_data.type) {
-       case LTTNG_CONSUMER_KERNEL:
-               mmap_base = stream->mmap_base;
-               ret = kernctl_get_mmap_read_offset(stream->wait_fd, &mmap_offset);
-               if (ret < 0) {
-                       PERROR("tracer ctl get_mmap_read_offset");
-                       goto end;
-               }
-               break;
-       case LTTNG_CONSUMER32_UST:
-       case LTTNG_CONSUMER64_UST:
-               mmap_base = lttng_ustctl_get_mmap_base(stream);
-               if (!mmap_base) {
-                       ERR("read mmap get mmap base for stream %s", stream->name);
-                       ret = -EPERM;
-                       goto end;
-               }
-               ret = lttng_ustctl_get_mmap_read_offset(stream, &mmap_offset);
-               if (ret != 0) {
-                       PERROR("tracer ctl get_mmap_read_offset");
-                       ret = -EINVAL;
-                       goto end;
-               }
-               break;
-       default:
-               ERR("Unknown consumer_data type");
-               assert(0);
-       }
-
        /* Handle stream on the relayd if the output is on the network */
        if (relayd) {
-               unsigned long netlen = len;
+               unsigned long netlen = subbuf_content_size;
 
                /*
                 * Lock the control socket for the complete duration of the function
@@ -1777,10 +1650,10 @@ ssize_t lttng_consumer_on_read_subbuffer_mmap(
                                goto write_error;
                        }
                }
-       } else {
-               /* No streaming, we have to set the len with the full padding */
-               len += padding;
 
+               write_len = subbuf_content_size;
+       } else {
+               /* No streaming; we have to write the full padding. */
                if (stream->metadata_flag && stream->reset_metadata_flag) {
                        ret = utils_truncate_stream_file(stream->out_fd, 0);
                        if (ret < 0) {
@@ -1794,7 +1667,7 @@ ssize_t lttng_consumer_on_read_subbuffer_mmap(
                 * Check if we need to change the tracefile before writing the packet.
                 */
                if (stream->chan->tracefile_size > 0 &&
-                               (stream->tracefile_size_current + len) >
+                               (stream->tracefile_size_current + buffer->size) >
                                stream->chan->tracefile_size) {
                        ret = consumer_stream_rotate_output_files(stream);
                        if (ret) {
@@ -1803,19 +1676,17 @@ ssize_t lttng_consumer_on_read_subbuffer_mmap(
                        outfd = stream->out_fd;
                        orig_offset = 0;
                }
-               stream->tracefile_size_current += len;
-               if (index) {
-                       index->offset = htobe64(stream->out_fd_offset);
-               }
+               stream->tracefile_size_current += buffer->size;
+               write_len = buffer->size;
        }
 
        /*
         * This call guarantee that len or less is returned. It's impossible to
         * receive a ret value that is bigger than len.
         */
-       ret = lttng_write(outfd, mmap_base + mmap_offset, len);
-       DBG("Consumer mmap write() ret %zd (len %lu)", ret, len);
-       if (ret < 0 || ((size_t) ret != len)) {
+       ret = lttng_write(outfd, buffer->data, write_len);
+       DBG("Consumer mmap write() ret %zd (len %zu)", ret, write_len);
+       if (ret < 0 || ((size_t) ret != write_len)) {
                /*
                 * Report error to caller if nothing was written else at least send the
                 * amount written.
@@ -1836,7 +1707,8 @@ ssize_t lttng_consumer_on_read_subbuffer_mmap(
                        DBG("Consumer mmap write detected relayd hang up");
                } else {
                        /* Unhandled error, print it and stop function right now. */
-                       PERROR("Error in write mmap (ret %zd != len %lu)", ret, len);
+                       PERROR("Error in write mmap (ret %zd != write_len %zu)", ret,
+                                       write_len);
                }
                goto write_error;
        }
@@ -1845,9 +1717,9 @@ ssize_t lttng_consumer_on_read_subbuffer_mmap(
        /* This call is useless on a socket so better save a syscall. */
        if (!relayd) {
                /* This won't block, but will start writeout asynchronously */
-               lttng_sync_file_range(outfd, stream->out_fd_offset, len,
+               lttng_sync_file_range(outfd, stream->out_fd_offset, write_len,
                                SYNC_FILE_RANGE_WRITE);
-               stream->out_fd_offset += len;
+               stream->out_fd_offset += write_len;
                lttng_consumer_sync_trace_file(stream, orig_offset);
        }
 
@@ -1881,8 +1753,7 @@ end:
 ssize_t lttng_consumer_on_read_subbuffer_splice(
                struct lttng_consumer_local_data *ctx,
                struct lttng_consumer_stream *stream, unsigned long len,
-               unsigned long padding,
-               struct ctf_packet_index *index)
+               unsigned long padding)
 {
        ssize_t ret = 0, written = 0, ret_splice = 0;
        loff_t offset = 0;
@@ -1986,7 +1857,6 @@ ssize_t lttng_consumer_on_read_subbuffer_splice(
                        orig_offset = 0;
                }
                stream->tracefile_size_current += len;
-               index->offset = htobe64(stream->out_fd_offset);
        }
 
        while (len > 0) {
@@ -2553,7 +2423,7 @@ restart:
                                do {
                                        health_code_update();
 
-                                       len = ctx->on_buffer_ready(stream, ctx);
+                                       len = ctx->on_buffer_ready(stream, ctx, false);
                                        /*
                                         * We don't check the return value here since if we get
                                         * a negative len, it means an error occurred thus we
@@ -2580,7 +2450,7 @@ restart:
                                        do {
                                                health_code_update();
 
-                                               len = ctx->on_buffer_ready(stream, ctx);
+                                               len = ctx->on_buffer_ready(stream, ctx, false);
                                                /*
                                                 * We don't check the return value here since if we get
                                                 * a negative len, it means an error occurred thus we
@@ -2796,7 +2666,7 @@ void *consumer_thread_data_poll(void *data)
                        if (pollfd[i].revents & POLLPRI) {
                                DBG("Urgent read on fd %d", pollfd[i].fd);
                                high_prio = 1;
-                               len = ctx->on_buffer_ready(local_stream[i], ctx);
+                               len = ctx->on_buffer_ready(local_stream[i], ctx, false);
                                /* it's ok to have an unavailable sub-buffer */
                                if (len < 0 && len != -EAGAIN && len != -ENODATA) {
                                        /* Clean the stream and free it. */
@@ -2827,7 +2697,7 @@ void *consumer_thread_data_poll(void *data)
                                        local_stream[i]->hangup_flush_done ||
                                        local_stream[i]->has_data) {
                                DBG("Normal read on fd %d", pollfd[i].fd);
-                               len = ctx->on_buffer_ready(local_stream[i], ctx);
+                               len = ctx->on_buffer_ready(local_stream[i], ctx, false);
                                /* it's ok to have an unavailable sub-buffer */
                                if (len < 0 && len != -EAGAIN && len != -ENODATA) {
                                        /* Clean the stream and free it. */
@@ -3440,40 +3310,146 @@ error_testpoint:
        return NULL;
 }
 
-ssize_t lttng_consumer_read_subbuffer(struct lttng_consumer_stream *stream,
+static int post_consume(struct lttng_consumer_stream *stream,
+               const struct stream_subbuffer *subbuffer,
                struct lttng_consumer_local_data *ctx)
 {
-       ssize_t ret;
+       size_t i;
+       int ret = 0;
+       const size_t count = lttng_dynamic_array_get_count(
+                       &stream->read_subbuffer_ops.post_consume_cbs);
 
-       pthread_mutex_lock(&stream->chan->lock);
-       pthread_mutex_lock(&stream->lock);
-       if (stream->metadata_flag) {
-               pthread_mutex_lock(&stream->metadata_rdv_lock);
+       for (i = 0; i < count; i++) {
+               const post_consume_cb op = *(post_consume_cb *) lttng_dynamic_array_get_element(
+                               &stream->read_subbuffer_ops.post_consume_cbs,
+                               i);
+
+               ret = op(stream, subbuffer, ctx);
+               if (ret) {
+                       goto end;
+               }
        }
+end:
+       return ret;
+}
 
-       switch (consumer_data.type) {
-       case LTTNG_CONSUMER_KERNEL:
-               ret = lttng_kconsumer_read_subbuffer(stream, ctx);
-               break;
-       case LTTNG_CONSUMER32_UST:
-       case LTTNG_CONSUMER64_UST:
-               ret = lttng_ustconsumer_read_subbuffer(stream, ctx);
-               break;
-       default:
-               ERR("Unknown consumer_data type");
-               assert(0);
-               ret = -ENOSYS;
-               break;
+ssize_t lttng_consumer_read_subbuffer(struct lttng_consumer_stream *stream,
+               struct lttng_consumer_local_data *ctx,
+               bool locked_by_caller)
+{
+       ssize_t ret, written_bytes = 0;
+       int rotation_ret;
+       struct stream_subbuffer subbuffer = {};
+
+       if (!locked_by_caller) {
+               stream->read_subbuffer_ops.lock(stream);
        }
 
-       if (stream->metadata_flag) {
-               pthread_cond_broadcast(&stream->metadata_rdv);
-               pthread_mutex_unlock(&stream->metadata_rdv_lock);
+       if (stream->read_subbuffer_ops.on_wake_up) {
+               ret = stream->read_subbuffer_ops.on_wake_up(stream);
+               if (ret) {
+                       goto end;
+               }
+       }
+
+       /*
+        * If the stream was flagged to be ready for rotation before we extract
+        * the next packet, rotate it now.
+        */
+       if (stream->rotate_ready) {
+               DBG("Rotate stream before consuming data");
+               ret = lttng_consumer_rotate_stream(ctx, stream);
+               if (ret < 0) {
+                       ERR("Stream rotation error before consuming data");
+                       goto end;
+               }
+       }
+
+       ret = stream->read_subbuffer_ops.get_next_subbuffer(stream, &subbuffer);
+       if (ret) {
+               if (ret == -ENODATA) {
+                       /* Not an error. */
+                       ret = 0;
+                       goto sleep_stream;
+               }
+               goto end;
+       }
+
+       ret = stream->read_subbuffer_ops.pre_consume_subbuffer(
+                       stream, &subbuffer);
+       if (ret) {
+               goto error_put_subbuf;
+       }
+
+       written_bytes = stream->read_subbuffer_ops.consume_subbuffer(
+                       ctx, stream, &subbuffer);
+       /*
+        * Should write subbuf_size amount of data when network streaming or
+        * the full padded size when we are not streaming.
+        */
+       if ((written_bytes != subbuffer.info.data.subbuf_size &&
+                           stream->net_seq_idx != (uint64_t) -1ULL) ||
+                       (written_bytes != subbuffer.info.data.padded_subbuf_size &&
+                                       stream->net_seq_idx ==
+                                                       (uint64_t) -1ULL)) {
+               /*
+                * Display the error but continue processing to try to
+                * release the subbuffer. This is a DBG statement
+                * since this can happen without being a critical
+                * error.
+                */
+               DBG("Failed to write to tracefile (written_bytes: %zd != padded subbuffer size: %lu, subbuffer size: %lu)",
+                               written_bytes, subbuffer.info.data.padded_subbuf_size,
+                               subbuffer.info.data.subbuf_size);
+       }
+
+       ret = stream->read_subbuffer_ops.put_next_subbuffer(stream, &subbuffer);
+       if (ret) {
+               goto end;
+       }
+
+       ret = post_consume(stream, &subbuffer, ctx);
+       if (ret) {
+               goto end;
+       }
+
+       /*
+        * After extracting the packet, we check if the stream is now ready to
+        * be rotated and perform the action immediately.
+        *
+        * Don't overwrite `ret` as callers expect the number of bytes
+        * consumed to be returned on success.
+        */
+       rotation_ret = lttng_consumer_stream_is_rotate_ready(stream);
+       if (rotation_ret == 1) {
+               rotation_ret = lttng_consumer_rotate_stream(ctx, stream);
+               if (rotation_ret < 0) {
+                       ret = rotation_ret;
+                       ERR("Stream rotation error after consuming data");
+                       goto end;
+               }
+
+       } else if (rotation_ret < 0) {
+               ret = rotation_ret;
+               ERR("Failed to check if stream was ready to rotate after consuming data");
+               goto end;
+       }
+
+sleep_stream:
+       if (stream->read_subbuffer_ops.on_sleep) {
+               stream->read_subbuffer_ops.on_sleep(stream, ctx);
+       }
+
+       ret = written_bytes;
+end:
+       if (!locked_by_caller) {
+               stream->read_subbuffer_ops.unlock(stream);
        }
-       pthread_mutex_unlock(&stream->lock);
-       pthread_mutex_unlock(&stream->chan->lock);
 
        return ret;
+error_put_subbuf:
+       (void) stream->read_subbuffer_ops.put_next_subbuffer(stream, &subbuffer);
+       goto end;
 }
 
 int lttng_consumer_on_recv_stream(struct lttng_consumer_stream *stream)
@@ -3947,40 +3923,6 @@ unsigned long consumer_get_consume_start_pos(unsigned long consumed_pos,
        return start_pos;
 }
 
-static
-int consumer_flush_buffer(struct lttng_consumer_stream *stream, int producer_active)
-{
-       int ret = 0;
-
-       switch (consumer_data.type) {
-       case LTTNG_CONSUMER_KERNEL:
-               if (producer_active) {
-                       ret = kernctl_buffer_flush(stream->wait_fd);
-                       if (ret < 0) {
-                               ERR("Failed to flush kernel stream");
-                               goto end;
-                       }
-               } else {
-                       ret = kernctl_buffer_flush_empty(stream->wait_fd);
-                       if (ret < 0) {
-                               ERR("Failed to flush kernel stream");
-                               goto end;
-                       }
-               }
-               break;
-       case LTTNG_CONSUMER32_UST:
-       case LTTNG_CONSUMER64_UST:
-               lttng_ustconsumer_flush_buffer(stream, producer_active);
-               break;
-       default:
-               ERR("Unknown consumer_data type");
-               abort();
-       }
-
-end:
-       return ret;
-}
-
 /*
  * Sample the rotate position for all the streams of a channel. If a stream
  * is already at the rotate position (produced == consumed), we flag it as
@@ -4004,11 +3946,15 @@ int lttng_consumer_rotate_channel(struct lttng_consumer_channel *channel,
        const bool is_local_trace = relayd_id == -1ULL;
        struct consumer_relayd_sock_pair *relayd = NULL;
        bool rotating_to_new_chunk = true;
+       /* Array of `struct lttng_consumer_stream *` */
+       struct lttng_dynamic_pointer_array streams_packet_to_open;
+       size_t stream_idx;
 
        DBG("Consumer sample rotate position for channel %" PRIu64, key);
 
        lttng_dynamic_array_init(&stream_rotation_positions,
                        sizeof(struct relayd_stream_rotation_position), NULL);
+       lttng_dynamic_pointer_array_init(&streams_packet_to_open, NULL);
 
        rcu_read_lock();
 
@@ -4050,7 +3996,8 @@ int lttng_consumer_rotate_channel(struct lttng_consumer_channel *channel,
                         * ensures we have at least one packet in each stream per trace
                         * chunk, even if no data was produced.
                         */
-                       ret = consumer_flush_buffer(stream, stream->metadata_flag ? 1 : 0);
+                       ret = consumer_stream_flush_buffer(
+                                       stream, stream->metadata_flag ? 1 : 0);
                        if (ret < 0) {
                                ERR("Failed to flush stream %" PRIu64 " during channel rotation",
                                                stream->key);
@@ -4138,36 +4085,135 @@ int lttng_consumer_rotate_channel(struct lttng_consumer_channel *channel,
                        }
                        stream_count++;
                }
+
+               stream->opened_packet_in_current_trace_chunk = false;
+
+               if (rotating_to_new_chunk && !stream->metadata_flag) {
+                       /*
+                        * Attempt to flush an empty packet as close to the
+                        * rotation point as possible. In the event where a
+                        * stream remains inactive after the rotation point,
+                        * this ensures that the new trace chunk has a
+                        * beginning timestamp set at the begining of the
+                        * trace chunk instead of only creating an empty
+                        * packet when the trace chunk is stopped.
+                        *
+                        * This indicates to the viewers that the stream
+                        * was being recorded, but more importantly it
+                        * allows viewers to determine a useable trace
+                        * intersection.
+                        *
+                        * This presents a problem in the case where the
+                        * ring-buffer is completely full.
+                        *
+                        * Consider the following scenario:
+                        *   - The consumption of data is slow (slow network,
+                        *     for instance),
+                        *   - The ring buffer is full,
+                        *   - A rotation is initiated,
+                        *     - The flush below does nothing (no space left to
+                        *       open a new packet),
+                        *   - The other streams rotate very soon, and new
+                        *     data is produced in the new chunk,
+                        *   - This stream completes its rotation long after the
+                        *     rotation was initiated
+                        *   - The session is stopped before any event can be
+                        *     produced in this stream's buffers.
+                        *
+                        * The resulting trace chunk will have a single packet
+                        * temporaly at the end of the trace chunk for this
+                        * stream making the stream intersection more narrow
+                        * than it should be.
+                        *
+                        * To work-around this, an empty flush is performed
+                        * after the first consumption of a packet during a
+                        * rotation if open_packet fails. The idea is that
+                        * consuming a packet frees enough space to switch
+                        * packets in this scenario and allows the tracer to
+                        * "stamp" the beginning of the new trace chunk at the
+                        * earliest possible point.
+                        *
+                        * The packet open is performed after the channel
+                        * rotation to ensure that no attempt to open a packet
+                        * is performed in a stream that has no active trace
+                        * chunk.
+                        */
+                       ret = lttng_dynamic_pointer_array_add_pointer(
+                                       &streams_packet_to_open, stream);
+                       if (ret) {
+                               PERROR("Failed to add a stream pointer to array of streams in which to open a packet");
+                               ret = -1;
+                               goto end_unlock_stream;
+                       }
+               }
+
                pthread_mutex_unlock(&stream->lock);
        }
        stream = NULL;
-       pthread_mutex_unlock(&channel->lock);
 
-       if (is_local_trace) {
-               ret = 0;
-               goto end;
-       }
+       if (!is_local_trace) {
+               relayd = consumer_find_relayd(relayd_id);
+               if (!relayd) {
+                       ERR("Failed to find relayd %" PRIu64, relayd_id);
+                       ret = -1;
+                       goto end_unlock_channel;
+               }
 
-       relayd = consumer_find_relayd(relayd_id);
-       if (!relayd) {
-               ERR("Failed to find relayd %" PRIu64, relayd_id);
-               ret = -1;
-               goto end;
+               pthread_mutex_lock(&relayd->ctrl_sock_mutex);
+               ret = relayd_rotate_streams(&relayd->control_sock, stream_count,
+                               rotating_to_new_chunk ? &next_chunk_id : NULL,
+                               (const struct relayd_stream_rotation_position *)
+                                               stream_rotation_positions.buffer
+                                                               .data);
+               pthread_mutex_unlock(&relayd->ctrl_sock_mutex);
+               if (ret < 0) {
+                       ERR("Relayd rotate stream failed. Cleaning up relayd %" PRIu64,
+                                       relayd->net_seq_idx);
+                       lttng_consumer_cleanup_relayd(relayd);
+                       goto end_unlock_channel;
+               }
        }
 
-       pthread_mutex_lock(&relayd->ctrl_sock_mutex);
-       ret = relayd_rotate_streams(&relayd->control_sock, stream_count,
-                       rotating_to_new_chunk ? &next_chunk_id : NULL,
-                       (const struct relayd_stream_rotation_position *)
-                                       stream_rotation_positions.buffer.data);
-       pthread_mutex_unlock(&relayd->ctrl_sock_mutex);
-       if (ret < 0) {
-               ERR("Relayd rotate stream failed. Cleaning up relayd %" PRIu64,
-                               relayd->net_seq_idx);
-               lttng_consumer_cleanup_relayd(relayd);
-               goto end;
+       for (stream_idx = 0;
+                       stream_idx < lttng_dynamic_pointer_array_get_count(
+                               &streams_packet_to_open);
+                       stream_idx++) {
+               enum consumer_stream_open_packet_status status;
+
+               stream = lttng_dynamic_pointer_array_get_pointer(
+                               &streams_packet_to_open, stream_idx);
+
+               pthread_mutex_lock(&stream->lock);
+               status = consumer_stream_open_packet(stream);
+               pthread_mutex_unlock(&stream->lock);
+               switch (status) {
+               case CONSUMER_STREAM_OPEN_PACKET_STATUS_OPENED:
+                       DBG("Opened a packet after a rotation: stream id = %" PRIu64
+                           ", channel name = %s, session id = %" PRIu64,
+                                       stream->key, stream->chan->name,
+                                       stream->chan->session_id);
+                       break;
+               case CONSUMER_STREAM_OPEN_PACKET_STATUS_NO_SPACE:
+                       /*
+                        * Can't open a packet as there is no space left
+                        * in the buffer. A new packet will be opened
+                        * once one has been consumed.
+                        */
+                       DBG("No space left to open a packet after a rotation: stream id = %" PRIu64
+                           ", channel name = %s, session id = %" PRIu64,
+                                       stream->key, stream->chan->name,
+                                       stream->chan->session_id);
+                       break;
+               case CONSUMER_STREAM_OPEN_PACKET_STATUS_ERROR:
+                       /* Logged by callee. */
+                       ret = -1;
+                       goto end_unlock_channel;
+               default:
+                       abort();
+               }
        }
 
+       pthread_mutex_unlock(&channel->lock);
        ret = 0;
        goto end;
 
@@ -4178,6 +4224,7 @@ end_unlock_channel:
 end:
        rcu_read_unlock();
        lttng_dynamic_array_reset(&stream_rotation_positions);
+       lttng_dynamic_pointer_array_reset(&streams_packet_to_open);
        return ret;
 }
 
@@ -4203,7 +4250,7 @@ int consumer_clear_buffer(struct lttng_consumer_stream *stream)
        case LTTNG_CONSUMER_KERNEL:
                ret = kernctl_buffer_clear(stream->wait_fd);
                if (ret < 0) {
-                       ERR("Failed to flush kernel stream");
+                       ERR("Failed to clear kernel stream (ret = %d)", ret);
                        goto end;
                }
                break;
@@ -4236,7 +4283,7 @@ int consumer_clear_stream(struct lttng_consumer_stream *stream)
 {
        int ret;
 
-       ret = consumer_flush_buffer(stream, 1);
+       ret = consumer_stream_flush_buffer(stream, 1);
        if (ret < 0) {
                ERR("Failed to flush stream %" PRIu64 " during channel clear",
                                stream->key);
@@ -4282,11 +4329,6 @@ error_unlock:
        pthread_mutex_unlock(&stream->lock);
        pthread_mutex_unlock(&channel->lock);
        rcu_read_unlock();
-       if (ret) {
-               goto error;
-       }
-       ret = LTTCOMM_CONSUMERD_SUCCESS;
-error:
        return ret;
 }
 
@@ -4998,3 +5040,70 @@ int lttng_consumer_clear_channel(struct lttng_consumer_channel *channel)
 end:
        return ret;
 }
+
+enum lttcomm_return_code lttng_consumer_open_channel_packets(
+               struct lttng_consumer_channel *channel)
+{
+       struct lttng_consumer_stream *stream;
+       enum lttcomm_return_code ret = LTTCOMM_CONSUMERD_SUCCESS;
+
+       if (channel->metadata_stream) {
+               ERR("Open channel packets command attempted on a metadata channel");
+               ret = LTTCOMM_CONSUMERD_INVALID_PARAMETERS;
+               goto end;
+       }
+
+       rcu_read_lock();
+       cds_list_for_each_entry(stream, &channel->streams.head, send_node) {
+               enum consumer_stream_open_packet_status status;
+
+               pthread_mutex_lock(&stream->lock);
+               if (cds_lfht_is_node_deleted(&stream->node.node)) {
+                       goto next;
+               }
+
+               status = consumer_stream_open_packet(stream);
+               switch (status) {
+               case CONSUMER_STREAM_OPEN_PACKET_STATUS_OPENED:
+                       DBG("Opened a packet in \"open channel packets\" command: stream id = %" PRIu64
+                                       ", channel name = %s, session id = %" PRIu64,
+                                       stream->key, stream->chan->name,
+                                       stream->chan->session_id);
+                       stream->opened_packet_in_current_trace_chunk = true;
+                       break;
+               case CONSUMER_STREAM_OPEN_PACKET_STATUS_NO_SPACE:
+                       DBG("No space left to open a packet in \"open channel packets\" command: stream id = %" PRIu64
+                                       ", channel name = %s, session id = %" PRIu64,
+                                       stream->key, stream->chan->name,
+                                       stream->chan->session_id);
+                       break;
+               case CONSUMER_STREAM_OPEN_PACKET_STATUS_ERROR:
+                       /*
+                        * Only unexpected internal errors can lead to this
+                        * failing. Report an unknown error.
+                        */
+                       ERR("Failed to flush empty buffer in \"open channel packets\" command: stream id = %" PRIu64
+                                       ", channel id = %" PRIu64
+                                       ", channel name = %s"
+                                       ", session id = %" PRIu64,
+                                       stream->key, channel->key,
+                                       channel->name, channel->session_id);
+                       ret = LTTCOMM_CONSUMERD_UNKNOWN_ERROR;
+                       goto error_unlock;
+               default:
+                       abort();
+               }
+
+       next:
+               pthread_mutex_unlock(&stream->lock);
+       }
+
+end_rcu_unlock:
+       rcu_read_unlock();
+end:
+       return ret;
+
+error_unlock:
+       pthread_mutex_unlock(&stream->lock);
+       goto end_rcu_unlock;
+}
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