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

[Richard Acayan]

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) {
>              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
>