[FFmpeg-devel] [RESEND PATCH] avcodec/v4l2_context: always log POLLERR when buffers are uninitialized

[Marton Balint]

On Fri, 21 Jul 2023, Richard Acayan wrote:

> Will this patch be applied or receive any comments? I have been waiting
> more than 2 weeks since the original submission
> (https://ffmpeg.org/pipermail/ffmpeg-devel/2023-July/311580.html) and
> have not received a response since.
>
> I am trying to get mpv to work again with the v4l2_m2m codec, as it
> started emitting segmentation faults. This is partly due to some
> incorrect usage of libavcodec in mpv, and partly due to the
> error/warning handling in libavcodec. An mpv maintainer dismissed this
> as a bug in ffmpeg, and this change is simpler than what mpv would need
> for a fix, so I am hoping that this patch goes in.
>
> I don't have write access so I can't apply it myself, even if there are
> no objections.
>
> On Tue, Jul 18, 2023 at 06:00:17PM -0400, Richard Acayan wrote:
>> The error handler for POLLERRs assumes that the timeout is only zero
>> when v4l2_dequeue_v4l2buf is called by v4l2_getfree_v4l2buf. This
>> assumption is incorrect, as ff_v4l2_context_dequeue_frame also calls
>> this function with a timeout of zero. Do not check for unavailable
>> buffers if the buffers are uninitialized.
>>
>> See https://trac.ffmpeg.org/ticket/9957 for the original bug report.
>>
>> Signed-off-by: Richard Acayan <mailingradian at gmail.com>
>> ---
>>  libavcodec/v4l2_context.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/libavcodec/v4l2_context.c b/libavcodec/v4l2_context.c
>> index a40be94690..69ddf80723 100644
>> --- a/libavcodec/v4l2_context.c
>> +++ b/libavcodec/v4l2_context.c
>> @@ -327,7 +327,7 @@ start:
>>      if (pfd.revents & POLLERR) {
>>          /* if we are trying to get free buffers but none have been queued yet
>>             no need to raise a warning */
>> -        if (timeout == 0) {
>> +        if (timeout == 0 && ctx->buffers) {

This would warn if no buffers are allocated yet, but this is not a warning 
condition as far as I understand the issue, simply a different API usage 
pattern. So maybe it is better to simply do a

if (!ctx->buffers)
    return NULL;

in the beginning of the if (timeout == 0) block.

Thanks,
Marton

>>              for (i = 0; i < ctx->num_buffers; i++) {
>>                  if (ctx->buffers[i].status != V4L2BUF_AVAILABLE)
>>                      av_log(logger(ctx), AV_LOG_WARNING, "%s POLLERR\n", ctx->name);
>> --
>> 2.41.0
>>
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