Fix: don't fail on push metadata if no channel

The comments in the code explains it well but in a nutshell, this is an
acceptable race between the creation of the metadata on the consumer
side and the push metadata from the session daemon for that channel.

This race is resolved by either having the consumer requesting metadata
or the session is stopped which will in both situation push the metadata
to the consumer.

Without that fix, the session daemon flags the registry's metadata to be
"closed" which usually indicates that the consumer is not responding
leading to the consumer thread exiting in the session daemon.

Acked-by: Julien Desfossez <julien.desfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>

diff --git a/src/bin/lttng-sessiond/ust-app.c b/src/bin/lttng-sessiond/ust-app.c
index 41a321f0a3a8dc5a10d1c7d15eca79bca621241c..72a3223c1fa53baa0bf8f413e9e9e99d7e6d335d 100644 (file)
--- a/src/bin/lttng-sessiond/ust-app.c
+++ b/src/bin/lttng-sessiond/ust-app.c
@@ -481,6 +481,20 @@ push_data:
        ret = consumer_push_metadata(socket, registry->metadata_key,
                        metadata_str, len, offset);
        if (ret < 0) {
+               /*
+                * There is an acceptable race here between the registry metadata key
+                * assignment and the creation on the consumer. The session daemon can
+                * concurrently push metadata for this registry while being created on
+                * the consumer since the metadata key of the registry is assigned
+                * *before* it is setup to avoid the consumer to ask for metadata that
+                * could possibly be not found in the session daemon.
+                *
+                * The metadata will get pushed either by the session being stopped or
+                * the consumer requesting metadata if that race is triggered.
+                */
+               if (ret == -LTTCOMM_CONSUMERD_CHANNEL_FAIL) {
+                       ret = 0;
+               }
                ret_val = ret;
                goto error_push;
        }

diff --git a/src/common/ust-consumer/ust-consumer.c b/src/common/ust-consumer/ust-consumer.c
index 83aa5986f7f663e122a92f58dd7895836d807149..a077faafba12de1533e5015cc58f71c4ca5d530d 100644 (file)
--- a/src/common/ust-consumer/ust-consumer.c
+++ b/src/common/ust-consumer/ust-consumer.c
@@ -1451,8 +1451,15 @@ int lttng_ustconsumer_recv_cmd(struct lttng_consumer_local_data *ctx,
 
                channel = consumer_find_channel(key);
                if (!channel) {
-                       ERR("UST consumer push metadata %" PRIu64 " not found", key);
-                       ret_code = LTTNG_ERR_UST_CHAN_NOT_FOUND;
+                       /*
+                        * This is possible if the metadata creation on the consumer side
+                        * is in flight vis-a-vis a concurrent push metadata from the
+                        * session daemon.  Simply return that the channel failed and the
+                        * session daemon will handle that message correctly considering
+                        * that this race is acceptable thus the DBG() statement here.
+                        */
+                       DBG("UST consumer push metadata %" PRIu64 " not found", key);
+                       ret_code = LTTCOMM_CONSUMERD_CHANNEL_FAIL;
                        goto end_msg_sessiond;
                }