Bug#835645: libavformat: users of new AVStream.codecpar field do not get tighter dependencies

[James Cowgill]

Package: libavformat57
Version: 7:3.1.2-1
Severity: important
Control: affects -1 mpv

Hi,

Currently if mpv is installed with an old (3.0) version of
libavformat57, it segfaults. This appears to be due to mpv using the new
AVStream.codecpar field.

In relation to the recent ffmpeg ABI issues the timeline is:
3.1.0:
codecpar was added in the public API section of AVStream

3.1.1:
codecpar was moved to the end of AVStream as a workaround to fix (other)
applications using various private fields from AVStream. Adding the
field to the middle of the struct broke applications here.

Since it was originally added to the public API section, it seems like
it should be treated as such. Any application which uses the new
codecpar field should therefore get tighter dependencies (>= 3.1.1).

On a related note, two other fields were moved to fix ABI issues, but
since these fields are not public, I don't think anything needs updating
in the ffmpeg package to handle them.
AVFilterLink.hw_frames_ctx in 1eb43af1a0e542ad83dcbf327197785d815fc42d
AVFrame.hw_frames_ctx in 77473002898f1dce18761c8a9bca48a8fe888d2e

On implementing this, maybe you could bump these symbols to 3.1.1, which
seem to be the only legal ways of obtaining an AVStream? This does seem
a little fragile though - symbol files aren't very convenient for
handling changes to structs.
 avformat_alloc_context
 avformat_alloc_output_context2
 avformat_new_stream
 avformat_open_input

After this is done, mpv will need binNMUing.

Upstream MPV bug where this issue was originally reported:
https://github.com/mpv-player/mpv/issues/3463

FFmpeg 3.1.1 ABI bug for reference:
https://trac.ffmpeg.org/ticket/5676

Thanks,
James

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