Fix: flush empty packets on snapshot channel

Snapshot operation on a non-stopped stream should use a "final" flush to
ensure empty packets are flushed, so we gather timestamps at the moment
where the snapshot is taken. This is important for streams that have a
low amount of activity.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/lib/ringbuffer/ring_buffer_frontend.c b/lib/ringbuffer/ring_buffer_frontend.c
index 42cf317a01d56482ae07401b8d1d8123b332a1d5..708bc6d39af56e50629b34f9e3c66c849d5f9c0c 100644 (file)
--- a/lib/ringbuffer/ring_buffer_frontend.c
+++ b/lib/ringbuffer/ring_buffer_frontend.c
@@ -900,6 +900,15 @@ int lib_ring_buffer_snapshot(struct lib_ring_buffer *buf,
        unsigned long consumed_cur, write_offset;
        int finalized;
 
+       /*
+        * First, ensure we perform a "final" flush onto the stream.  This will
+        * ensure we create a packet of padding if we encounter an empty
+        * packet. This ensures the time-stamps right before the snapshot is
+        * used as end of packet timestamp.
+        */
+       if (!buf->quiescent)
+               _lib_ring_buffer_switch_remote(buf, SWITCH_FLUSH);
+
 retry:
        finalized = ACCESS_ONCE(buf->finalized);
        /*
@@ -1289,7 +1298,8 @@ void lib_ring_buffer_print_errors(struct channel *chan,
 /*
  * lib_ring_buffer_switch_old_start: Populate old subbuffer header.
  *
- * Only executed when the buffer is finalized, in SWITCH_FLUSH.
+ * Only executed by SWITCH_FLUSH, which can be issued while tracing is active
+ * or at buffer finalization (destroy).
  */
 static
 void lib_ring_buffer_switch_old_start(struct lib_ring_buffer *buf,
@@ -1480,12 +1490,14 @@ int lib_ring_buffer_try_switch_slow(enum switch_mode mode,
                unsigned long sb_index, commit_count;
 
                /*
-                * We are performing a SWITCH_FLUSH. At this stage, there are no
-                * concurrent writes into the buffer.
+                * We are performing a SWITCH_FLUSH. There may be concurrent
+                * writes into the buffer if e.g. invoked while performing a
+                * snapshot on an active trace.
                 *
-                * The client does not save any header information.  Don't
-                * switch empty subbuffer on finalize, because it is invalid to
-                * deliver a completely empty subbuffer.
+                * If the client does not save any header information (sub-buffer
+                * header size == 0), don't switch empty subbuffer on finalize,
+                * because it is invalid to deliver a completely empty
+                * subbuffer.
                 */
                if (!config->cb.subbuffer_header_size())
                        return -1;