Fix: source.ctf.lttng-live: assertion on equal messages

The following assertion fails when consuming a per-pid trace
of short-lived applications.
 (╯°□°)╯︵ ┻━┻  muxing.c:849: common_muxing_compare_messages(): Assertion `left_msg != right_msg` failed.

The live source performs a muxing step during which it can see that a
trace has ended. When that happens, the trace is removed from the
array of traces and the iteration on that array resumes from the
beginning. This causes the message comparator to assert as two
identical messages can be compared.

Not reseting the trace index to 0 causes the iteration to continue
from the same position in the array. This is fine since the "fast"
variant of the glib pointer array removal function replaces the
removed pointer by the last one. This is both more efficient and
solved the problem of comparing a message to itself (the oldest
message) during a second pass on the traces array.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5045a0483b17f0bcb48ff7eb0d88f82bf19f68d4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2262
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>

src/plugins/ctf/lttng-live/lttng-live.c

@@ -1120,11 +1120,6 @@ enum lttng_live_iterator_status next_stream_iterator_for_session(
 
 	BT_ASSERT_DBG(session->traces);
 
-	/*
-	 * Use while loops here rather then for loops so we can restart the
-	 * iteration if an element is removed from the array during the
-	 * looping.
-	 */
 	while (trace_idx < session->traces->len) {
 		bool trace_is_ended = false;
 		struct lttng_live_stream_iterator *stream_iter;
@@ -1175,8 +1170,13 @@ enum lttng_live_iterator_status next_stream_iterator_for_session(
 			}
 			trace_idx++;
 		} else {
-			g_ptr_array_remove_index_fast(session->traces, trace_idx);
-			trace_idx = 0;
+			/*
+			 * trace_idx is not incremented since
+			 * g_ptr_array_remove_index_fast replaces the
+			 * element at trace_idx with the array's last element.
+			 */
+			g_ptr_array_remove_index_fast(session->traces,
+				trace_idx);
 		}
 	}
 	if (youngest_candidate_stream_iter) {