[FFmpeg-devel] [PATCH] common ID3v2 support for all formats

[Reimar Döffinger]

On Wed, Nov 03, 2010 at 11:05:41PM +0100, Reimar D?ffinger wrote:
> On Wed, Nov 03, 2010 at 10:40:30PM +0100, Anton Khirnov wrote:
> > On Fri, Oct 01, 2010 at 08:21:07PM +0200, Reimar D?ffinger wrote:
> > > Hello,
> > > considering issue 2258 I think it is a valid conclusion that people
> > > will prepend a ID3v2 header to anything that can't run aways fast enough
> > > (and then give it a .mp3 extension...).
> > > So I propose attached patch that moves that parsing to common code.
> > > Note that is assume that due to buffering the ID3 parsing attempt
> > > will not cause any issues even on non-seekable input.
> > > Note that I have no idea WTF the mpc demuxer code is doing there,
> > > my conclusion was that it should be safe to just throw it all away,
> > > but I'm not 100% sure I don't miss some subtlety.
> > > Mostly unrelated, but one issue with this whole ID3 handling is
> > > that AFAICT it will use the file-specific metadata conversion
> > > instead of using always the ID3 one.
> > This change broke detection of mp3 files with really big id3v2 headers
> > in MPD, which uses a small 64kb buffer. This is probably MPD's fault,
> > but perhaps there should be a way for av_probe_input_format to signal
> > that "the stream looks useful, need more data" -- which would be done e.g.
> > when an id3 header is found which is bigger than the probe data buffer.
> 
> I'd suggest to just set score_max, something like below.
> Of course doesn't work so well for anyone calling it with
> *score_max == AVPROBE_SCORE_MAX/4 from the start.

On second thought though I have to say that I can't think of a use
case. Any app should retry with larger buffers until either there's
a match with score > AVPROBE_SCORE_MAX/4 or there is a reason to not
provide more data.
You can't expect "if I pass at least n bytes I'll get at least some
indication if this is a valid format" unless n is the whole size...
I also don't think that the previous code was really better, I suspect
that MPD would have detected all formats with a large ID3v2 header as
MP3, even if it was actually a wav or other file.