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Date: Wed, 16 Apr 2025 13:02:03 +0200
From: Michael Niedermayer <michael@niedermayer.cc>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
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Subject: Re: [FFmpeg-devel] [PATCH 2/2] avformat/id3v2: Check that
decode_str() did advance
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Hi Andreas
On Wed, Apr 16, 2025 at 12:57:27PM +0200, Andreas Rheinhardt wrote:
> Michael Niedermayer:
> > Hi softworkz
> >=20
> > On Wed, Apr 16, 2025 at 02:52:21AM +0000, softworkz . wrote:
> >>
> >>
> >>> -----Original Message-----
> >>> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
> >>> Michael Niedermayer
> >>> Sent: Mittwoch, 16. April 2025 03:34
> >>> To: FFmpeg development discussions and patches <ffmpeg-
> >>> devel@ffmpeg.org>
> >>> Subject: Re: [FFmpeg-devel] [PATCH 2/2] avformat/id3v2: Check that
> >>> decode_str() did advance
> >>>
> >>> On Wed, Apr 16, 2025 at 01:29:02AM +0000, softworkz . wrote:
> >>> [...]
> >>>>>> This will cause deserialization errors for many people in the
> >>> world
> >>>>>> who are processing FFprobe data.
> >>>>>
> >>>>> As said, ffprobe should not produce troublesome output
> >>
> >> First of all, any patch MUST NOT introduce behavior that goes
> >> against our own specifications.
> >>
> >> From avformat.h:
> >>
> >> /**
> >> * @defgroup metadata_api Public Metadata API
> >> * @{
> >> * @ingroup libavf
> >> * The metadata API allows libavformat to export metadata tags to a cl=
ient
> >> * application when demuxing. Conversely it allows a client applicatio=
n to
> >> * set metadata when muxing.
> >> *
> >> * Metadata is exported or set as pairs of key/value strings in the 'm=
etadata'
> >> * fields of the AVFormatContext, AVStream, AVChapter and AVProgram st=
ructs
> >> * using the @ref lavu_dict "AVDictionary" API. Like all strings in FF=
mpeg,
> >> * metadata is assumed to be UTF-8 encoded Unicode. Note that metadata
> >> * exported by demuxers isn't checked to be valid UTF-8 in most cases.
> >> *
> >> * Important concepts to keep in mind:
> >> * - Keys are unique; there can never be 2 tags with the same key. Th=
is is
> >> * also meant semantically, i.e., a demuxer should not knowingly pr=
oduce
> >> * several keys that are literally different but semantically ident=
ical.
> >> * E.g., key=3DAuthor5, key=3DAuthor6. In this example, all authors=
must be
> >> * placed in the same tag.
> >> * - Metadata is flat, not hierarchical; there are no subtags. If you
> >> * want to store, e.g., the email address of the child of producer =
Alice
> >> * and actor Bob, that could have key=3Dalice_and_bobs_childs_email=
_address.
> >> * - Several modifiers can be applied to the tag name. This is done by
> >> * appending a dash character ('-') and the modifier name in the or=
der
> >> * they appear in the list below -- e.g. foo-eng-sort, not foo-sort=
-eng.
> >> * - language -- a tag whose value is localized for a particular l=
anguage
> >> * is appended with the ISO 639-2/B 3-letter language code.
> >> * For example: Author-ger=3DMichael, Author-eng=3DMike
> >> * The original/default language is in the unqualified "Author" =
tag.
> >> * A demuxer should set a default if it sets any translated tag.
> >> * - sorting -- a modified version of a tag that should be used f=
or
> >> * sorting will have '-sort' appended. E.g. artist=3D"The Beatle=
s",
> >> * artist-sort=3D"Beatles, The".
> >>
> >>
> >> Especially:
> >>
> >> * E.g., key=3DAuthor5, key=3DAuthor6. In this example, all authors =
must be
> >> * placed in the same tag.
> >>
> >> I think, this tells very clearly how it's gotta be and how not.
> >=20
> > This is written by me 16 years ago
> > and it made sense at the time. Our APIs did not support multiple values=
per
> > key.
> > The API supports multiple values per key since 9 years. Using said API =
is
> > more convenient than having to parse and escape ";" around.
> > Thats for our code, its simpler for a muxer to iterate over a AVDiction=
ary key
> > than parse a ";" seperated string (with undefined escaping rules).
> > and easier to build a ";" string from a multi-value AVDictionary than t=
o assume
> > the internal ";" escaping rules (whatever they would be) match the targ=
et format.
>=20
> This is all true, but it does not change that we are bound by our API
> guarantees.
yes, as i said i will revert the patch, iam just a bit behind with what
i want to do and it happening in reality
[...]
--=20
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
The worst form of inequality is to try to make unequal things equal.
-- Aristotle
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