ffmpeg: allow to set the thread message queue size.

doc/ffmpeg.texi

@@ -1158,6 +1158,12 @@ This option enables or disables accurate seeking in input files with the
 transcoding. Use @option{-noaccurate_seek} to disable it, which may be useful
 e.g. when copying some streams and transcoding the others.
 
+@item -thread_message_queue @var{size} (@emph{input})
+This option sets the maximum number of queued packets when reading from the
+file or device. With low latency / high rate live streams, packets may be
+discarded if they are not read in a timely manner; raising this value can
+avoid it.
+
 @item -override_ffserver (@emph{global})
 Overrides the input specifications from @command{ffserver}. Using this
 option you can map any input stream to @command{ffserver} and control

ffmpeg.c

@@ -3420,7 +3420,7 @@ static int init_input_threads(void)
             strcmp(f->ctx->iformat->name, "lavfi"))
             f->non_blocking = 1;
         ret = av_thread_message_queue_alloc(&f->in_thread_queue,
-                                            8, sizeof(AVPacket));
+                                            f->thread_queue_size, sizeof(AVPacket));
         if (ret < 0)
             return ret;
 

ffmpeg.h

@@ -111,6 +111,7 @@ typedef struct OptionsContext {
     int64_t input_ts_offset;
     int rate_emu;
     int accurate_seek;
+    int thread_queue_size;
 
     SpecifierOpt *ts_scale;
     int        nb_ts_scale;
@@ -350,6 +351,7 @@ typedef struct InputFile {
     pthread_t thread;           /* thread reading from this file */
     int non_blocking;           /* reading packets from the thread should not block */
     int joined;                 /* the thread has been joined */
+    int thread_queue_size;      /* maximum number of queued packets */
 #endif
 } InputFile;
 

ffmpeg_opt.c

@@ -951,6 +951,9 @@ static int open_input_file(OptionsContext *o, const char *filename)
     f->nb_streams = ic->nb_streams;
     f->rate_emu   = o->rate_emu;
     f->accurate_seek = o->accurate_seek;
+#if HAVE_PTHREADS
+    f->thread_queue_size = o->thread_queue_size > 0 ? o->thread_queue_size : 8;
+#endif
 
     /* check if all codec options have been used */
     unused_opts = strip_specifiers(o->g->codec_opts);
@@ -2957,6 +2960,9 @@ const OptionDef options[] = {
     { "disposition",    OPT_STRING | HAS_ARG | OPT_SPEC |
                         OPT_OUTPUT,                                  { .off = OFFSET(disposition) },
         "disposition", "" },
+    { "thread_queue_size", HAS_ARG | OPT_INT | OPT_OFFSET | OPT_EXPERT | OPT_INPUT,
+                                                                     { .off = OFFSET(thread_queue_size) },
+        "set the maximum number of queued packets from the demuxer" },
 
     /* video options */
     { "vframes",      OPT_VIDEO | HAS_ARG  | OPT_PERFILE | OPT_OUTPUT,           { .func_arg = opt_video_frames },