fftools/sync_queue: make sure audio duration matches sample count

For audio AVFrames, nb_samples is typically more trustworthy than
duration. Since sync queues look at durations, make sure they match the
sample count.

The last audio frame in the fate-shortest test is now gone. This is more
correct, since it outlasts the last video frame.

fftools/sync_queue.c

@@ -116,6 +116,11 @@ static int64_t frame_ts(const SyncQueue *sq, SyncQueueFrame frame)
            frame.f->pts + frame.f->duration;
 }
 
+static int frame_samples(const SyncQueue *sq, SyncQueueFrame frame)
+{
+    return (sq->type == SYNC_QUEUE_PACKETS) ? 0 : frame.f->nb_samples;
+}
+
 static int frame_null(const SyncQueue *sq, SyncQueueFrame frame)
 {
     return (sq->type == SYNC_QUEUE_PACKETS) ? (frame.p == NULL) : (frame.f == NULL);
@@ -293,7 +298,7 @@ int sq_send(SyncQueue *sq, unsigned int stream_idx, SyncQueueFrame frame)
     SyncQueueStream *st;
     SyncQueueFrame dst;
     int64_t ts;
-    int ret;
+    int ret, nb_samples;
 
     av_assert0(stream_idx < sq->nb_streams);
     st = &sq->streams[stream_idx];
@@ -313,6 +318,14 @@ int sq_send(SyncQueue *sq, unsigned int stream_idx, SyncQueueFrame frame)
 
     frame_move(sq, dst, frame);
 
+    nb_samples = frame_samples(sq, dst);
+    // make sure frame duration is consistent with sample count
+    if (nb_samples) {
+        av_assert0(dst.f->sample_rate > 0);
+        dst.f->duration = av_rescale_q(nb_samples, (AVRational){ 1, dst.f->sample_rate },
+                                       dst.f->time_base);
+    }
+
     ts = frame_ts(sq, dst);
 
     ret = av_fifo_write(st->fifo, &dst, 1);

tests/ref/fate/shortest

@@ -115,4 +115,3 @@
 0,         48,         48,        1,    11212, 0xc61a3f0a, S=1,        8
 1,      85760,      85760,     1536,      418, 0xae06ca91
 0,         49,         49,        1,     1423, 0x45fba9e4, F=0x0, S=1,        8
-1,      87296,      87296,     1536,      418, 0x7bdcc3c7