Re: [FFmpeg-devel] [PATCH] lavf/matroskadec: stop mapping text/plain attachments to AV_CODEC_ID_TEXT

[Anton Khirnov <anton@khirnov.net>]

Quoting Soft Works (2022-06-08 05:45:38)
> You might allow me the question whether we can be sure that
> this is the only case which is subject to the regression?
> 
> Besides from what I reported above (and you might probably come up
> with a new codec ID for discriminating text subs vs. text
> attachments), this surely fixes the specific case I reported,
> but I wonder whether other cases could exist?
> (it's meant to be a normal question - I just don't know)

We can never be truly sure of anything except our own existence.

As far as I can tell, only two bits of code in lavf can export
AVMEDIA_TYPE_ATTACHMENT: apetag and matroskadec. apetag does not set
codec id at all, while matroskadec will now only export codec ids that
do not (and most likely will not) have decoders: fonts and generic
binary data.

So after this patch, to the best of my knowledge, there should never be
a case where an AVMEDIA_TYPE_ATTACHMENT stream has a decodable codec id.
Then again this does not exclude all possible cases of a mismatch
between a stream's codec type and id.

Overall I'd say this just strengthens the case for my original lavc
commit, since it is clearly helpful in exposing bugs in other code.

> Here's another thought that might be worth consideration:
> What turned this from a minor into a major issue (from my pov),
> is that it is causing ffprobe to fail and exit with error
> and incomplete output.
> What I'm wondering about in this context, is whether it
> even has to be like that?
> 
> I mean, an unknown codec doesn't cause ffprobe to error-exit,
> does it need to do so when avcodec_open2() returns error?
> 
> I would find that behavior ok and consistent when the same
> would happen when running ffmpeg (ffprobe fails <=> ffmpeg fails).
> But ffmpeg doesn't fail (unless you use the stream), so does
> ffprobe even need to fail in these cases?

I suppose it can make sense to log an error and continue when opening
the codec fails. This could be useful also for probing genuinely broken
streams where e.g. extradata parsing fails.

There could also be an option like ffmpeg's -xerror that would make
ffprobe exit on failure.

-- 
Anton Khirnov
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