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 cc0d235841a853e662ca48d7a01d7e5e6a8871b1..de4aab55a0f9cde742df9f9dad9ac4c0031f72e2 100644 (file)
--- a/src/bin/lttng-sessiond/ust-app.c
+++ b/src/bin/lttng-sessiond/ust-app.c
@@ -464,6 +464,20 @@ 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 6e7e1679476c742cfd2b71c3454ecf2c348d5fed..ad6abffa97d35e09b5e7d15c455672621e66961b 100644 (file)
--- a/src/common/ust-consumer/ust-consumer.c
+++ b/src/common/ust-consumer/ust-consumer.c
@@ -1133,8 +1133,15 @@ 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;
                }