From: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Date: Fri, 9 Aug 2019 15:11:28 +0000 (-0400)
Subject: Fix: rotation of a stopped session hangs indifinitely
X-Git-Tag: v2.11.0-rc3~79
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=63b2fe4881ad4b1c8f6e5f8f3e69d9aafda0ad97;p=lttng-tools.git

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 b2f4c2686..914eda8c5 100644
--- 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(