ffmpeg: set extra_hw_frames to account for frames held in queues

Since e0da916b8f the ffmpeg utility has
held multiple frames output by the decoder in internal queues without
telling the decoder that it is going to do so.  When the decoder has a
fixed-size pool of frames (common in some hardware APIs where the output
frames must be stored as an array texture) this could lead to the pool
being exhausted and the decoder getting stuck.  Fix this by telling the
decoder to allocate additional frames according to the queue size.

fftools/ffmpeg_dec.c

@@ -1207,6 +1207,19 @@ static int dec_open(DecoderPriv *dp, AVDictionary **dec_opts,
         return ret;
     }
 
+    if (dp->dec_ctx->hw_device_ctx) {
+        // Update decoder extra_hw_frames option to account for the
+        // frames held in queues inside the ffmpeg utility.  This is
+        // called after avcodec_open2() because the user-set value of
+        // extra_hw_frames becomes valid in there, and we need to add
+        // this on top of it.
+        int extra_frames = DEFAULT_FRAME_THREAD_QUEUE_SIZE;
+        if (dp->dec_ctx->extra_hw_frames >= 0)
+            dp->dec_ctx->extra_hw_frames += extra_frames;
+        else
+            dp->dec_ctx->extra_hw_frames = extra_frames;
+    }
+
     ret = check_avoptions(*dec_opts);
     if (ret < 0)
         return ret;

fftools/ffmpeg_sched.c

@@ -365,7 +365,21 @@ static int queue_alloc(ThreadQueue **ptq, unsigned nb_streams, unsigned queue_si
     ThreadQueue *tq;
     ObjPool *op;
 
-    queue_size = queue_size > 0 ? queue_size : 8;
+    if (queue_size <= 0) {
+        if (type == QUEUE_FRAMES)
+            queue_size = DEFAULT_FRAME_THREAD_QUEUE_SIZE;
+        else
+            queue_size = DEFAULT_PACKET_THREAD_QUEUE_SIZE;
+    }
+
+    if (type == QUEUE_FRAMES) {
+        // This queue length is used in the decoder code to ensure that
+        // there are enough entries in fixed-size frame pools to account
+        // for frames held in queues inside the ffmpeg utility.  If this
+        // can ever dynamically change then the corresponding decode
+        // code needs to be updated as well.
+        av_assert0(queue_size == DEFAULT_FRAME_THREAD_QUEUE_SIZE);
+    }
 
     op = (type == QUEUE_PACKETS) ? objpool_alloc_packets() :
                                    objpool_alloc_frames();

fftools/ffmpeg_sched.h

@@ -233,6 +233,18 @@ int sch_add_filtergraph(Scheduler *sch, unsigned nb_inputs, unsigned nb_outputs,
  */
 int sch_add_mux(Scheduler *sch, SchThreadFunc func, int (*init)(void *),
                 void *ctx, int sdp_auto, unsigned thread_queue_size);
+
+/**
+ * Default size of a packet thread queue.  For muxing this can be overridden by
+ * the thread_queue_size option as passed to a call to sch_add_mux().
+ */
+#define DEFAULT_PACKET_THREAD_QUEUE_SIZE 8
+
+/**
+ * Default size of a frame thread queue.
+ */
+#define DEFAULT_FRAME_THREAD_QUEUE_SIZE 8
+
 /**
  * Add a muxed stream for a previously added muxer.
  *