lavf: Add a flag to enable/disable per-packet flushing

This is enabled by default and can be disabled with
"-fflags -flush_packets".

Inspired by a patch from Nicolas George <nicolas.george@normalesup.org>.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/avformat.h

@@ -894,6 +894,7 @@ typedef struct AVFormatContext {
 #define AVFMT_FLAG_NOBUFFER     0x0040 ///< Do not buffer frames when possible
 #define AVFMT_FLAG_CUSTOM_IO    0x0080 ///< The caller has supplied a custom AVIOContext, don't avio_close() it.
 #define AVFMT_FLAG_DISCARD_CORRUPT  0x0100 ///< Discard frames marked corrupted
+#define AVFMT_FLAG_FLUSH_PACKETS    0x0200 ///< Flush the AVIOContext every packet.
 
     /**
      * decoding: size of data to probe; encoding: unused.

libavformat/mux.c

@@ -404,6 +404,7 @@ static int compute_pkt_fields2(AVFormatContext *s, AVStream *st, AVPacket *pkt)
 
 static int write_packet(AVFormatContext *s, AVPacket *pkt)
 {
+    int ret;
     if (!(s->oformat->flags & (AVFMT_TS_NEGATIVE | AVFMT_NOTIMESTAMPS))) {
         AVRational time_base = s->streams[pkt->stream_index]->time_base;
         int64_t offset = 0;
@@ -420,7 +421,12 @@ static int write_packet(AVFormatContext *s, AVPacket *pkt)
         if (pkt->pts != AV_NOPTS_VALUE)
             pkt->pts += offset;
     }
-    return s->oformat->write_packet(s, pkt);
+    ret = s->oformat->write_packet(s, pkt);
+
+    if (s->pb && ret >= 0 && s->flags & AVFMT_FLAG_FLUSH_PACKETS)
+        avio_flush(s->pb);
+
+    return ret;
 }
 
 int av_write_frame(AVFormatContext *s, AVPacket *pkt)

libavformat/options_table.h

@@ -33,7 +33,8 @@
 static const AVOption avformat_options[] = {
 {"probesize", "set probing size", OFFSET(probesize), AV_OPT_TYPE_INT, {.i64 = 5000000 }, 32, INT_MAX, D},
 {"packetsize", "set packet size", OFFSET(packet_size), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX, E},
-{"fflags", NULL, OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "fflags"},
+{"fflags", NULL, OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = AVFMT_FLAG_FLUSH_PACKETS }, INT_MIN, INT_MAX, D|E, "fflags"},
+{"flush_packets", "reduce the latency by flushing out packets immediately", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_FLUSH_PACKETS }, INT_MIN, INT_MAX, D, "fflags"},
 {"ignidx", "ignore index", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_IGNIDX }, INT_MIN, INT_MAX, D, "fflags"},
 {"genpts", "generate pts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_GENPTS }, INT_MIN, INT_MAX, D, "fflags"},
 {"nofillin", "do not fill in missing values that can be exactly calculated", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOFILLIN }, INT_MIN, INT_MAX, D, "fflags"},

libavformat/version.h

@@ -30,7 +30,7 @@
 #include "libavutil/avutil.h"
 
 #define LIBAVFORMAT_VERSION_MAJOR 55
-#define LIBAVFORMAT_VERSION_MINOR  3
+#define LIBAVFORMAT_VERSION_MINOR  4
 #define LIBAVFORMAT_VERSION_MICRO  0
 
 #define LIBAVFORMAT_VERSION_INT AV_VERSION_INT(LIBAVFORMAT_VERSION_MAJOR, \