fftools/ffmpeg: rework -shortest implementation

The -shortest option (which finishes the output file at the time the
shortest stream ends) is currently implemented by faking the -t option
when an output stream ends. This approach is fragile, since it depends
on the frames/packets being processed in a specific order. E.g. there
are currently some situations in which the output file length will
depend unpredictably on unrelated factors like encoder delay. More
importantly, the present work aiming at splitting various ffmpeg
components into different threads will make this approach completely
unworkable, since the frames/packets will arrive in effectively random
order.

This commit introduces a "sync queue", which is essentially a collection
of FIFOs, one per stream. Frames/packets are submitted to these FIFOs
and are then released for further processing (encoding or muxing) when
it is ensured that the frame in question will not cause its stream to
get ahead of the other streams (the logic is similar to libavformat's
interleaving queue).

These sync queues are then used for encoding and/or muxing when the
-shortest option is specified.

A new option – -shortest_buf_duration – controls the maximum number of
queued packets, to avoid runaway memory usage.

This commit changes the results of the following tests:
- copy-shortest[12]: the last audio frame is now gone. This is
  correct, since it actually outlasts the last video frame.
- shortest-sub: the video packets following the last subtitle packet are
  now gone. This is also correct.

1 files changed, 100 insertions, 0 deletions

diff --git a/fftools/sync_queue.h b/fftools/sync_queue.h
new file mode 100644
index 0000000000..e08780b7bf
--- /dev/null
+++ b/fftools/sync_queue.h
@@ -0,0 +1,100 @@
+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef FFTOOLS_SYNC_QUEUE_H
+#define FFTOOLS_SYNC_QUEUE_H
+
+#include <stdint.h>
+
+#include "libavcodec/packet.h"
+
+#include "libavutil/frame.h"
+
+enum SyncQueueType {
+    SYNC_QUEUE_PACKETS,
+    SYNC_QUEUE_FRAMES,
+};
+
+typedef union SyncQueueFrame {
+    AVFrame  *f;
+    AVPacket *p;
+} SyncQueueFrame;
+
+#define SQFRAME(frame) ((SyncQueueFrame){ .f = (frame) })
+#define SQPKT(pkt)     ((SyncQueueFrame){ .p = (pkt) })
+
+typedef struct SyncQueue SyncQueue;
+
+/**
+ * Allocate a sync queue of the given type.
+ *
+ * @param buf_size_us maximum duration that will be buffered in microseconds
+ */
+SyncQueue *sq_alloc(enum SyncQueueType type, int64_t buf_size_us);
+void       sq_free(SyncQueue **sq);
+
+/**
+ * Add a new stream to the sync queue.
+ *
+ * @return
+ * - a non-negative stream index on success
+ * - a negative error code on error
+ */
+int sq_add_stream(SyncQueue *sq);
+
+/**
+ * Set the timebase for the stream with index stream_idx. Should be called
+ * before sending any frames for this stream.
+ */
+void sq_set_tb(SyncQueue *sq, unsigned int stream_idx, AVRational tb);
+
+/**
+ * Submit a frame for the stream with index stream_idx.
+ *
+ * On success, the sync queue takes ownership of the frame and will reset the
+ * contents of the supplied frame. On failure, the frame remains owned by the
+ * caller.
+ *
+ * Sending a frame with NULL contents marks the stream as finished.
+ *
+ * @return
+ * - 0 on success
+ * - AVERROR_EOF when no more frames should be submitted for this stream
+ * - another a negative error code on failure
+ */
+int sq_send(SyncQueue *sq, unsigned int stream_idx, SyncQueueFrame frame);
+
+/**
+ * Read a frame from the queue.
+ *
+ * @param stream_idx index of the stream to read a frame for. May be -1, then
+ *                   try to read a frame from any stream that is ready for
+ *                   output.
+ * @param frame output frame will be written here on success. The frame is owned
+ *              by the caller.
+ *
+ * @return
+ * - a non-negative index of the stream to which the returned frame belongs
+ * - AVERROR(EAGAIN) when more frames need to be submitted to the queue
+ * - AVERROR_EOF when no more frames will be available for this stream (for any
+ *               stream if stream_idx is -1)
+ * - another negative error code on failure
+ */
+int sq_receive(SyncQueue *sq, int stream_idx, SyncQueueFrame frame);
+
+#endif // FFTOOLS_SYNC_QUEUE_H