Move decoder frame to decoder

The video buffer held 3 frames:
 - the producer frame (for decoding)
 - the pending frame (to exchange between the producer and consumer)
 - the consumer frame (for rendering)

It worked well, but it prevented video buffers to be chained, because
the consumer frame of the first video buffer must be the same as the
producer frame of the second video buffer.

To solve this problem, make the decoder handle its decoding frame, and
keep only the pending and consumer frames in the video_buffer.

    decoder -> pending -> consumer -> pending -> consumer
             |---------------------||---------------------|
                  video_buffer 1         video_buffer 2

This paves the way to support asynchronous swscale.

app/src/decoder.c

@@ -30,11 +30,19 @@ decoder_open(struct decoder *decoder, const AVCodec *codec) {
         return false;
     }
 
+    decoder->frame = av_frame_alloc();
+    if (!decoder->frame) {
+        avcodec_close(decoder->codec_ctx);
+        avcodec_free_context(&decoder->codec_ctx);
+        return false;
+    }
+
     return true;
 }
 
 void
 decoder_close(struct decoder *decoder) {
+    av_frame_free(&decoder->frame);
     avcodec_close(decoder->codec_ctx);
     avcodec_free_context(&decoder->codec_ctx);
 }
@@ -49,11 +57,11 @@ decoder_push(struct decoder *decoder, const AVPacket *packet) {
         LOGE("Could not send video packet: %d", ret);
         return false;
     }
-    ret = avcodec_receive_frame(decoder->codec_ctx,
-                                decoder->video_buffer->producer_frame);
+    ret = avcodec_receive_frame(decoder->codec_ctx, decoder->frame);
     if (!ret) {
         // a frame was received
-        video_buffer_producer_offer_frame(decoder->video_buffer);
+        video_buffer_producer_offer_frame(decoder->video_buffer,
+                                          &decoder->frame);
     } else if (ret != AVERROR(EAGAIN)) {
         LOGE("Could not receive video frame: %d", ret);
         return false;
@@ -61,7 +69,7 @@ decoder_push(struct decoder *decoder, const AVPacket *packet) {
 #else
     int got_picture;
     int len = avcodec_decode_video2(decoder->codec_ctx,
-                                    decoder->video_buffer->decoding_frame,
+                                    decoder->frame,
                                     &got_picture,
                                     packet);
     if (len < 0) {

app/src/decoder.h

@@ -12,6 +12,7 @@ struct decoder {
     struct video_buffer *video_buffer;
 
     AVCodecContext *codec_ctx;
+    AVFrame *frame;
 };
 
 void

app/src/video_buffer.c

@@ -8,24 +8,19 @@
 
 bool
 video_buffer_init(struct video_buffer *vb, bool wait_consumer) {
-    vb->producer_frame = av_frame_alloc();
-    if (!vb->producer_frame) {
-        goto error_0;
-    }
-
     vb->pending_frame = av_frame_alloc();
     if (!vb->pending_frame) {
-        goto error_1;
+        goto error_0;
     }
 
     vb->consumer_frame = av_frame_alloc();
     if (!vb->consumer_frame) {
-        goto error_2;
+        goto error_1;
     }
 
     bool ok = sc_mutex_init(&vb->mutex);
     if (!ok) {
-        goto error_3;
+        goto error_2;
     }
 
     vb->wait_consumer = wait_consumer;
@@ -49,12 +44,10 @@ video_buffer_init(struct video_buffer *vb, bool wait_consumer) {
 
     return true;
 
-error_3:
-    av_frame_free(&vb->consumer_frame);
 error_2:
-    av_frame_free(&vb->pending_frame);
+    av_frame_free(&vb->consumer_frame);
 error_1:
-    av_frame_free(&vb->producer_frame);
+    av_frame_free(&vb->pending_frame);
 error_0:
     return false;
 }
@@ -67,7 +60,6 @@ video_buffer_destroy(struct video_buffer *vb) {
     sc_mutex_destroy(&vb->mutex);
     av_frame_free(&vb->consumer_frame);
     av_frame_free(&vb->pending_frame);
-    av_frame_free(&vb->producer_frame);
 }
 
 static inline void
@@ -89,7 +81,7 @@ video_buffer_set_consumer_callbacks(struct video_buffer *vb,
 }
 
 void
-video_buffer_producer_offer_frame(struct video_buffer *vb) {
+video_buffer_producer_offer_frame(struct video_buffer *vb, AVFrame **pframe) {
     assert(vb->cbs);
 
     sc_mutex_lock(&vb->mutex);
@@ -101,7 +93,7 @@ video_buffer_producer_offer_frame(struct video_buffer *vb) {
     }
 
     av_frame_unref(vb->pending_frame);
-    swap_frames(&vb->producer_frame, &vb->pending_frame);
+    swap_frames(pframe, &vb->pending_frame);
 
     bool skipped = !vb->pending_frame_consumed;
     vb->pending_frame_consumed = false;

app/src/video_buffer.h

@@ -29,7 +29,6 @@ typedef struct AVFrame AVFrame;
  */
 
 struct video_buffer {
-    AVFrame *producer_frame;
     AVFrame *pending_frame;
     AVFrame *consumer_frame;
 
@@ -67,8 +66,9 @@ video_buffer_set_consumer_callbacks(struct video_buffer *vb,
                                     void *cbs_userdata);
 
 // set the producer frame as ready for consuming
+// the produced frame is exchanged with an unused allocated frame
 void
-video_buffer_producer_offer_frame(struct video_buffer *vb);
+video_buffer_producer_offer_frame(struct video_buffer *vb, AVFrame **pframe);
 
 // mark the consumer frame as consumed and return it
 // the frame is valid until the next call to this function