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 cb3a2747b131c614572378e40ac0f597e1c2fd54..58b8080b56dff398d82e7262257e770682f1afcc 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 e92e7109715e2bc96002ac643381d11fe0333a49..e315c4681c94def1d2d36305637f6218a7ff3a20 100644 (file)
--- a/src/common/ust-consumer/ust-consumer.c
+++ b/src/common/ust-consumer/ust-consumer.c
@@ -1390,8 +1390,15 @@ 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;
                }