[FFmpeg-devel] [PATCH] avcodec: Vorbis decode: don't use a flag to determine if frames have been output

[jyrkive at nekonyansoft.com]

From: Jyrki Vesterinen <jyrkive at nekonyansoft.com>

If a developer using FFmpeg libraries seeks into an earlier position and calls
avcodec_flush_buffers() afterwards as recommended, the Vorbis decoder will drop
the next frame, since buffer flushing clears the first_frame flag. As a result,
the audio samples the calling code receives may be ahead of the requested seek
position, which is unacceptable in some use cases such as playing a looping
sound effect.

This commit removes the first_frame flag entirely and instead uses the
presentation timestamp to determine if it's the first frame.
---
 libavcodec/vorbisdec.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/libavcodec/vorbisdec.c b/libavcodec/vorbisdec.c
index 4d03947c49..d4b030d7b9 100644
--- a/libavcodec/vorbisdec.c
+++ b/libavcodec/vorbisdec.c
@@ -130,7 +130,6 @@ typedef struct vorbis_context_s {
     AVFloatDSPContext *fdsp;
 
     FFTContext mdct[2];
-    uint8_t       first_frame;
     uint32_t      version;
     uint8_t       audio_channels;
     uint32_t      audio_samplerate;
@@ -1845,8 +1844,7 @@ static int vorbis_decode_frame(AVCodecContext *avctx, AVFrame *frame,
     if ((len = vorbis_parse_audio_packet(vc, channel_ptrs)) <= 0)
         return len;
 
-    if (!vc->first_frame) {
-        vc->first_frame = 1;
+    if (frame->pts < 0) {
         *got_frame_ptr = 0;
         av_frame_unref(frame);
         return buf_size;
@@ -1881,7 +1879,6 @@ static av_cold void vorbis_decode_flush(AVCodecContext *avctx)
                              sizeof(*vc->saved));
     }
     vc->previous_window = -1;
-    vc->first_frame = 0;
 }
 
 const FFCodec ff_vorbis_decoder = {
-- 
2.37.2.windows.2