[FFmpeg-devel] [PATCH 2/2] avformat/id3v2: Check that decode_str() did advance

[softworkz .]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> Michael Niedermayer
> Sent: Mittwoch, 16. April 2025 03:34
> To: FFmpeg development discussions and patches <ffmpeg-
> devel at 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 client
 * application when demuxing. Conversely it allows a client application to
 * set metadata when muxing.
 *
 * Metadata is exported or set as pairs of key/value strings in the 'metadata'
 * fields of the AVFormatContext, AVStream, AVChapter and AVProgram structs
 * using the @ref lavu_dict "AVDictionary" API. Like all strings in FFmpeg,
 * 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. This is
 *    also meant semantically, i.e., a demuxer should not knowingly produce
 *    several keys that are literally different but semantically identical.
 *    E.g., key=Author5, key=Author6. 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=alice_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 order
 *    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 language
 *       is appended with the ISO 639-2/B 3-letter language code.
 *       For example: Author-ger=Michael, Author-eng=Mike
 *       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 for
 *       sorting will have '-sort' appended. E.g. artist="The Beatles",
 *       artist-sort="Beatles, The".


Especially:

*    E.g., key=Author5, key=Author6. 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.


Best,
sw