[FFmpeg-devel] [PATCH 3/3] avformat/webpenc: Check seeks

[James Almer]

On 4/10/2021 10:21 PM, Andreas Rheinhardt wrote:
> James Almer:
>> On 4/10/2021 9:43 PM, Andreas Rheinhardt wrote:
>>> When writing the trailer, the WebP muxer unconditionally seeks back
>>> to the front to update some elements. Yet this doesn't work if
>>> the seek failed, so check for this.
>>>
>>> (This has been mentioned in ticket #9179.)
>>>
>>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
>>> ---
>>>  Â  libavformat/webpenc.c | 8 ++++----
>>>  Â  1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/libavformat/webpenc.c b/libavformat/webpenc.c
>>> index 3962986c32..a24920d181 100644
>>> --- a/libavformat/webpenc.c
>>> +++ b/libavformat/webpenc.c
>>> @@ -174,8 +174,8 @@ static int webp_write_trailer(AVFormatContext *s)
>>>  Â  Â Â Â Â Â  if (w->using_webp_anim_encoder) {
>>>  Â Â Â Â Â Â Â Â Â  if ((w->frame_count > 1) && w->loop) {Â  // Write loop count.
>>> -Â Â Â Â Â Â Â Â Â Â Â  avio_seek(s->pb, 42, SEEK_SET);
>>> -Â Â Â Â Â Â Â Â Â Â Â  avio_wl16(s->pb, w->loop);
>>> +Â Â Â Â Â Â Â Â Â Â Â  if (avio_seek(s->pb, 42, SEEK_SET) == 42)
>>
>> I think it's better if you also check for (s->pb->seekable &
>> AVIO_SEEKABLE_NORMAL) before calling avio_seek() + avio_w* to begin with.
>>
> 
> Actually I intentionally didn't do that because the seek might work even
> when said flag is not set (when the destination is still in the
> AVIOContext's buffer).

Ok, patch LGTM then.

Carl argued this value should not be set in webp_write_trailer() to 
begin with, but that's a separate change.

> 
>>> +Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  avio_wl16(s->pb, w->loop);
>>>  Â Â Â Â Â Â Â Â Â  }
>>>  Â Â Â Â Â  } else {
>>>  Â Â Â Â Â Â Â Â Â  int ret;
>>> @@ -183,8 +183,8 @@ static int webp_write_trailer(AVFormatContext *s)
>>>  Â Â Â Â Â Â Â Â Â Â Â Â Â  return ret;
>>>  Â  Â Â Â Â Â Â Â Â Â  filesize = avio_tell(s->pb);
>>> -Â Â Â Â Â Â Â  avio_seek(s->pb, 4, SEEK_SET);
>>> -Â Â Â Â Â Â Â  avio_wl32(s->pb, filesize - 8);
>>> +Â Â Â Â Â Â Â  if (avio_seek(s->pb, 4, SEEK_SET) == 4)
>>> +Â Â Â Â Â Â Â Â Â Â Â  avio_wl32(s->pb, filesize - 8);
>>>  Â Â Â Â Â Â Â Â Â  // Note: without the following, avio only writes 8 bytes to
>>> the file.
>>>  Â Â Â Â Â Â Â Â Â  avio_seek(s->pb, filesize, SEEK_SET);
>>>  Â Â Â Â Â  }
>>>
>>
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