[FFmpeg-devel] [PATCH 1/8] lavc/avcodec: simplify codec id/type validity checking

[Anton Khirnov]

Quoting Soft Works (2022-06-05 09:54:51)
> 
> 
> > -----Original Message-----
> > From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of Anton
> > Khirnov
> > Sent: Sunday, June 5, 2022 9:01 AM
> > To: FFmpeg development discussions and patches <ffmpeg-devel at ffmpeg.org>
> > Subject: Re: [FFmpeg-devel] [PATCH 1/8] lavc/avcodec: simplify codec id/type
> > validity checking
> > 
> > Quoting Soft Works (2022-06-05 07:23:18)
> > > This is causing a regression in ffprobe.
> > >
> > > The commit removes the special-case check for AVMEDIA_TYPE_ATTACHMENT which
> > > was required for ffprobe and had been added with
> > e83c716e16c52fa56a78274408f7628e5dc719da.
> > >
> > > The demand from the commit message is not yet guaranteed to be fulfilled:
> > >
> > > > On entry to avcodec_open2(), the type and id either have to be
> > > > UNKNOWN/NONE or have to match the codec to be used.
> > >
> > > I have one verified example (maybe a second will follow), which is an MKV
> > with
> > > an attachment "stream" of type "text".
> > > The found codec will be textdec of type 'subtitle' even though the stream
> > type
> > > is attachment. Without the special condition for attachment streams, this
> > > is now causing ffprobe to error out with non-zero exit code and incomplete
> > > output.
> > >
> > >
> > > ------------------------------------------------------------------------
> > > Example:
> > >
> > >   [...]
> > >   Stream #0:9: Attachment: text
> > >     Metadata:
> > >       filename        : textfile.text
> > >       mimetype        : text/plain
> > > [text @ 000001AC32310340] Codec type or id mismatches
> > > Could not open codec for input stream 9
> > > ------------------------------------------------------------------------
> > 
> > This sounds very much like a bug in ffprobe. It makes no sense to call
> > avcodec_open2() with the AVMEDIA_TYPE_ATTACHMENT type.
> 
> You make a behavioral change to an API function that had this behavior 
> established and constant over more than 10 years, and when that change 
> breaks functionality, it's the callers' fault?
> How does this go together with all that peanut counting of major, minor
> and micro version numbers per library? What is this versioning good for,
> when you can make breaking changes and declare the breakage as bugs?

We maintain compatibility for valid API usage. We do not maintain bug
compatibility.

I fail to see how calling avcodec_open2() with AVMEDIA_TYPE_ATTACHMENT
is valid API usage. What do you expect it to do? There are no
AVMEDIA_TYPE_ATTACHMENT decoders.

More generally, arguments along the line of "change <X> is needed to
keep program <Y> working>" on their own sound very shady to me and
suggest that perhaps program <Y> should not be doing whatever it is
doing.

-- 
Anton Khirnov