Fix: rotation of a stopped session hangs indifinitely

The relay daemon control protocol expects a rotation position as "the
sequence number of the first packet _after_ the current trace chunk.

At the moment when the positions of the buffers are sampled, the
production position does not necessarily sit at a packet boundary. The
'active' flush operation above will push the production position to
the next packet boundary _if_ it is not already sitting at such a
boundary.

Assuming a current production position that is not on the bound of a
packet, the 'target' sequence number is
  (consumed_pos / subbuffer_size) + 1

Note the '+ 1' to ensure the current packet is part of the current
trace chunk.

However, if the production position is already at a packet boundary,
the '+ 1' is not necessary as the last packet of the current chunk is
already 'complete'.

In the case of a stopped session, performing the '+ 1'
indiscriminately results in a position that will not be reached until
another (not 'active') flush occurs.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>

diff --git a/src/common/consumer/consumer.c b/src/common/consumer/consumer.c
index b2f4c2686d2f7f81b5bc21c4249a681e92a5bad3..914eda8c5a1386f3650d6b47cc499e64986267a5 100644 (file)
--- a/src/common/consumer/consumer.c
+++ b/src/common/consumer/consumer.c
@@ -4073,6 +4073,10 @@ int lttng_consumer_rotate_channel(struct lttng_consumer_channel *channel,
                        stream->rotate_ready = true;
                }
 
+               /*
+                * Active flush; has no effect if the production position
+                * is at a packet boundary.
+                */
                ret = consumer_flush_buffer(stream, 1);
                if (ret < 0) {
                        ERR("Failed to flush stream %" PRIu64 " during channel rotation",
@@ -4081,10 +4085,34 @@ int lttng_consumer_rotate_channel(struct lttng_consumer_channel *channel,
                }
 
                if (!is_local_trace) {
+                       /*
+                        * The relay daemon control protocol expects a rotation
+                        * position as "the sequence number of the first packet
+                        * _after_ the current trace chunk.
+                        *
+                        * At the moment when the positions of the buffers are
+                        * sampled, the production position does not necessarily
+                        * sit at a packet boundary. The 'active' flush
+                        * operation above will push the production position to
+                        * the next packet boundary _if_ it is not already
+                        * sitting at such a boundary.
+                        *
+                        * Assuming a current production position that is not
+                        * on the bound of a packet, the 'target' sequence
+                        * number is
+                        *   (consumed_pos / subbuffer_size) + 1
+                        * Note the '+ 1' to ensure the current packet is
+                        * part of the current trace chunk.
+                        *
+                        * However, if the production position is already at
+                        * a packet boundary, the '+ 1' is not necessary as the
+                        * last packet of the current chunk is already
+                        * 'complete'.
+                        */
                        const struct relayd_stream_rotation_position position = {
                                .stream_id = stream->relayd_stream_id,
-                               .rotate_at_seq_num = (stream->rotate_position /
-                                               stream->max_sb_size) + 1,
+                               .rotate_at_seq_num = (stream->rotate_position / stream->max_sb_size) +
+                                       !!(stream->rotate_position % stream->max_sb_size),
                        };
 
                        ret = lttng_dynamic_array_add_element(