Merge pull request 'daemon: AV_CODEC_FLAG_LOW_DELAY for H.264 — implements #11 part (2)' (#12) from noether/daemon-low-delay-h264 into main

Reviewed-on: #12

daemon/src/decoder.c

@@ -132,6 +132,32 @@ static int decoder_open_codec(struct daedalus_decoder *dec, uint32_t codec_id,
 	ctx = fm->avcodec_alloc_context3(codec);
 	if (!ctx)
 		return -ENOMEM;
+
+	/*
+	 * H.264-only: force libavcodec to emit frames in DECODE order
+	 * (one frame per send_packet, no internal display-order reorder
+	 * queue).  V4L2 stateless decoder protocol expects each OUTPUT
+	 * bitstream packet to produce one CAPTURE buffer with that
+	 * packet's slice-decoded pixels — regardless of display order.
+	 * ffmpeg-vaapi's H.264 decoder (which is what consumes our
+	 * CAPTURE buffers via libva-v4l2-request-fourier) does its own
+	 * POC-based display reorder upstream, so producing decode-order
+	 * output is correct.
+	 *
+	 * AV_CODEC_FLAG_LOW_DELAY forces `low_delay = 1` inside
+	 * libavcodec's H.264 decoder — `h264_select_output_frame` emits
+	 * the just-decoded picture immediately instead of holding it
+	 * for the display-order DPB output queue.  DPB management for
+	 * reference frames (short_ref / long_ref) is unaffected; B-frame
+	 * decoding correctness is preserved.
+	 *
+	 * Closes daedalus-v4l2#11 part (2).  Skipped for VP9 / AV1 —
+	 * those formats don't internally reorder, so the flag would be
+	 * a no-op but adds no value.
+	 */
+	if (codec_id == DAEDALUS_CODEC_H264)
+		ctx->flags |= AV_CODEC_FLAG_LOW_DELAY;
+
 	rc = fm->avcodec_open2(ctx, codec, NULL);
 	if (rc < 0) {
 		log_err("decoder: avcodec_open2 failed: %d", rc);