fftools/ffmpeg: make specifying thread_queue_size turn on threaded input

Threaded input can increase smoothness of e.g. x11grab significantly. Before
this patch, in order to activate threaded input the user had to specify a
"dummy" additional input, with this change it is no longer required.

Signed-off-by: Marton Balint <cus@passwd.hu>

doc/ffmpeg.texi

@@ -1689,8 +1689,9 @@ not start from timestamp 0, such as transport streams.
 @item -thread_queue_size @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.
+discarded if they are not read in a timely manner; setting this value can
+force ffmpeg to use a separate input thread and read packets as soon as they
+arrive. By default ffmpeg only do this if multiple inputs are specified.
 
 @item -sdp_file @var{file} (@emph{global})
 Print sdp information for an output stream to @var{file}.

fftools/ffmpeg.c

@@ -4051,7 +4051,9 @@ static int init_input_thread(int i)
     int ret;
     InputFile *f = input_files[i];
 
-    if (nb_input_files == 1)
+    if (f->thread_queue_size < 0)
+        f->thread_queue_size = (nb_input_files > 1 ? 8 : 0);
+    if (!f->thread_queue_size)
         return 0;
 
     if (f->ctx->pb ? !f->ctx->pb->seekable :
@@ -4105,7 +4107,7 @@ static int get_input_packet(InputFile *f, AVPacket *pkt)
     }
 
 #if HAVE_THREADS
-    if (nb_input_files > 1)
+    if (f->thread_queue_size)
         return get_input_packet_mt(f, pkt);
 #endif
     return av_read_frame(f->ctx, pkt);

fftools/ffmpeg_opt.c

@@ -228,6 +228,7 @@ static void init_options(OptionsContext *o)
     o->limit_filesize = UINT64_MAX;
     o->chapters_input_file = INT_MAX;
     o->accurate_seek  = 1;
+    o->thread_queue_size = -1;
 }
 
 static int show_hwaccels(void *optctx, const char *opt, const char *arg)
@@ -1270,7 +1271,7 @@ static int open_input_file(OptionsContext *o, const char *filename)
     f->duration = 0;
     f->time_base = (AVRational){ 1, 1 };
 #if HAVE_THREADS
-    f->thread_queue_size = o->thread_queue_size > 0 ? o->thread_queue_size : 8;
+    f->thread_queue_size = o->thread_queue_size;
 #endif
 
     /* check if all codec options have been used */