lavf/mux: add the flush_packets option.

Note: a lot of muxers already do the flushing explicitly.

doc/ffmpeg-formats.texi

@@ -151,6 +151,11 @@ been without shifting.
 
 Also note that this affects only leading negative timestamps, and not
 non-monotonic negative timestamps.
+
+@item flush_packets @var{integer} (@emph{output})
+Flush the underlying I/O stream after each packet. Default 1 enables it, and
+has the effect of reducing the latency; 0 disables it and may slightly
+increase performance in some cases.
 @end table
 
 @c man end FORMAT OPTIONS

libavformat/avformat.h

@@ -1213,6 +1213,13 @@ typedef struct AVFormatContext {
      */
     int seek2any;
 
+    /**
+     * Flush the I/O context after each packet.
+     * - encoding: Set by user via AVOptions (NO direct access)
+     * - decoding: unused
+     */
+    int flush_packets;
+
     /*****************************************************************
      * All fields below this line are not part of the public API. They
      * may not be used outside of libavformat and can be changed and

libavformat/mux.c

@@ -494,6 +494,8 @@ static inline int split_write_packet(AVFormatContext *s, AVPacket *pkt)
 
     did_split = av_packet_split_side_data(pkt);
     ret = s->oformat->write_packet(s, pkt);
+    if (s->flush_packets && s->pb && s->pb->error >= 0)
+        avio_flush(s->pb);
     if (did_split)
         av_packet_merge_side_data(pkt);
     return ret;

libavformat/options_table.h

@@ -75,6 +75,7 @@ static const AVOption options[]={
 {"avoid_negative_ts", "shift timestamps to make them positive. 1 enables, 0 disables, default of -1 enables when required by target format.", OFFSET(avoid_negative_ts), AV_OPT_TYPE_INT, {.i64 = -1}, -1, 1, E},
 {"skip_initial_bytes", "skip initial bytes", OFFSET(skip_initial_bytes), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, D},
 {"correct_ts_overflow", "correct single timestamp overflows", OFFSET(correct_ts_overflow), AV_OPT_TYPE_INT, {.i64 = 1}, 0, 1, D},
+{"flush_packets", "enable flushing of the I/O context after each packet", OFFSET(flush_packets), AV_OPT_TYPE_INT, {.i64 = 1}, 0, 1, E},
 {NULL},
 };