[lttng-dev] [PATCH babeltrace] Fix: lttng-live discarded event count after inactivity

[Julien Desfossez]

When a stream is inactive, the consumer produces fake indexes which are
beacons to let the viewer know a stream has not produced any data up to
a certain timestamp. These beacon are actually real packet indexes with
all the fields set to 0 except for the timestamp_end. Currently we keep
these beacons just like we keep real indexes. The problem is that when
we switch packet, we compare the events_discarded field in the index we
just received with the same field in the previous index. In the case
where a stream has been inactive, we have received inactivity beacons,
and set the discarded_event field to 0, so the difference with the next
real index might be wrong.

In fact, since the inactivity beacons are only used to push the
timestamp end of a stream, we don't need to keep them and we actually
need to keep most of the data from the real previous index. So we now
copy the entire prev_index into the cur_index when we receive an
inactivity beacon. We could refactor the code to avoid performing the
pointer swap of cur and prev indexes, but this implies a redesign of
much of the packet switching code which would affect other code paths.

Signed-off-by: Julien Desfossez <jdesfossez at efficios.com>
---
 formats/lttng-live/lttng-live-comm.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/formats/lttng-live/lttng-live-comm.c b/formats/lttng-live/lttng-live-comm.c
index 77bf34a..3ffa257 100644
--- a/formats/lttng-live/lttng-live-comm.c
+++ b/formats/lttng-live/lttng-live-comm.c
@@ -1275,6 +1275,7 @@ retry:
 		pos->offset = 0;
 	}
 
+	/* Beacon packet index */
 	if (cur_index->content_size == 0) {
 		if (file_stream->parent.stream_class) {
 			file_stream->parent.cycles_timestamp =
@@ -1282,8 +1283,23 @@ retry:
 			file_stream->parent.real_timestamp = ctf_get_real_timestamp(
 					&file_stream->parent,
 					cur_index->ts_cycles.timestamp_end);
+
+			/*
+			 * Duplicate the data from the previous index, because
+			 * the one we just received is only a beacon with no
+			 * relevant information except the timestamp_end. We
+			 * don't need to keep this timestamp_end because we already
+			 * updated the file_stream timestamps, so we only need
+			 * to keep the last real index data as prev_index. That
+			 * way, we keep the original prev timestamps and
+			 * discarded events counter. This is the same behaviour
+			 * as if we were reading a local trace, we would not
+			 * have fake indexes between real indexes.
+			 */
+			memcpy(cur_index, prev_index, sizeof(struct packet_index));
 		}
 	} else {
+	/* Real packet index */
 		if (file_stream->parent.stream_class) {
 			/* Convert the timestamps and append to the real_index. */
 			cur_index->ts_real.timestamp_begin = ctf_get_real_timestamp(
-- 
2.7.4