Fix: consumer-stream: live viewers observe timestamps going backwards

[lttng-tools.git] / src / common / consumer / consumer-stream.cpp

diff --git a/src/common/consumer/consumer-stream.cpp b/src/common/consumer/consumer-stream.cpp
index e466c01551be7d73755f05483c89987dfd6d87b2..f9807cb0173f4bf6f4a7c0e9f838afb4631c16ec 100644 (file)
--- a/src/common/consumer/consumer-stream.cpp
+++ b/src/common/consumer/consumer-stream.cpp
@@ -419,6 +419,7 @@ static int consumer_stream_sync_metadata_index(
                const struct stream_subbuffer *subbuffer,
                struct lttng_consumer_local_data *ctx)
 {
+       bool missed_metadata_flush;
        int ret;
 
        /* Block until all the metadata is sent. */
@@ -431,18 +432,34 @@ static int consumer_stream_sync_metadata_index(
 
        pthread_mutex_lock(&stream->metadata_timer_lock);
        stream->waiting_on_metadata = false;
-       if (stream->missed_metadata_flush) {
+       missed_metadata_flush = stream->missed_metadata_flush;
+       if (missed_metadata_flush) {
                stream->missed_metadata_flush = false;
-               pthread_mutex_unlock(&stream->metadata_timer_lock);
-               (void) stream->read_subbuffer_ops.send_live_beacon(stream);
-       } else {
-               pthread_mutex_unlock(&stream->metadata_timer_lock);
        }
+       pthread_mutex_unlock(&stream->metadata_timer_lock);
        if (ret < 0) {
                goto end;
        }
 
        ret = consumer_stream_send_index(stream, subbuffer, ctx);
+       /*
+        * Send the live inactivity beacon to handle the situation where
+        * the live timer is prevented from sampling this stream
+        * because the stream lock was being held while this stream is
+        * waiting on metadata. This ensures live viewer progress in the
+        * unlikely scenario where a live timer would be prevented from
+        * locking a stream lock repeatedly due to a steady flow of
+        * incoming metadata, for a stream which is mostly inactive.
+        *
+        * It is important to send the inactivity beacon packet to
+        * relayd _after_ sending the index associated with the data
+        * that was just sent, otherwise this can cause live viewers to
+        * observe timestamps going backwards between an inactivity
+        * beacon and a following trace packet.
+        */
+       if (missed_metadata_flush) {
+               (void) stream->read_subbuffer_ops.send_live_beacon(stream);
+       }
 end:
        return ret;
 }