[FFmpeg-devel] [PATCH] avformat/oggparsevorbis: Fix oggvorbis duration parsing for small files

[Guangyu Sun <sunguangyucn@gmail.com>]

The decoded Data is not returned from the first frame; it must be used
to 'prime' the decode engine. The duration of the first packet should
be ignored also by the container, to make sure the total duration is
trimmed correctly.

If the file has multiple pages for the data packets. The duration of the
last packet can be correctly adjusted without being affected by the first
packet. But if the first packet and the last packet are located in the
same page, the duration of the last packet will have an unwanted offset.

This commit fixes https://trac.ffmpeg.org/ticket/6367.

dd if=/dev/zero of=./silence.raw count=1 bs=500
oggenc --raw silence.raw --output=silence.ogg
oggdec --raw --output silence.oggdec.raw silence.ogg
ffmpeg -codec:a libvorbis -i silence.ogg -f s16le -codec:a pcm_s16le silence.libvorbis.ffmpeg.raw
ffmpeg -i silence.ogg -f s16le -codec:a pcm_s16le silence.native.ffmpeg.raw
ls -l *.raw

Signed-off-by: Guangyu Sun <gsun@roblox.com>
---
 libavformat/oggparsevorbis.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/libavformat/oggparsevorbis.c b/libavformat/oggparsevorbis.c
index 289900f7cf..9a486c9fe8 100644
--- a/libavformat/oggparsevorbis.c
+++ b/libavformat/oggparsevorbis.c
@@ -412,6 +412,7 @@ static int vorbis_packet(AVFormatContext *s, int idx)
     struct ogg_stream *os = ogg->streams + idx;
     struct oggvorbis_private *priv = os->private;
     int duration, flags = 0;
+    int first_page;
 
     if (!priv->vp)
         return AVERROR_INVALIDDATA;
@@ -420,7 +421,8 @@ static int vorbis_packet(AVFormatContext *s, int idx)
      * here we parse the duration of each packet in the first page and compare
      * the total duration to the page granule to find the encoder delay and
      * set the first timestamp */
-    if ((!os->lastpts || os->lastpts == AV_NOPTS_VALUE) && !(os->flags & OGG_FLAG_EOS) && (int64_t)os->granule>=0) {
+    first_page = !os->lastpts || os->lastpts == AV_NOPTS_VALUE;
+    if (first_page && !(os->flags & OGG_FLAG_EOS) && (int64_t)os->granule>=0) {
         int seg, d;
         uint8_t *last_pkt  = os->buf + os->pstart;
         uint8_t *next_pkt  = last_pkt;
@@ -479,6 +481,13 @@ static int vorbis_packet(AVFormatContext *s, int idx)
             flags = 0;
         }
         os->pduration = duration;
+        /* Data is not returned from the first frame; it must be used to 'prime'
+         * the decode engine. The duration of the first packet should be ignored
+         * also by the container, to make sure the total duration is trimmed
+         * correctly.
+         */
+        if (first_page && os->segp == 1)
+            os->pduration = 0;
     }
 
     /* final packet handling
-- 
2.30.1

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