FFmpeg: Support FFmpeg 5.0

[rtandy]

Hello,

Debian's FFmpeg maintainers reported that mgba fails to compile using the newly released FFmpeg 5.0.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1004768

I have compiled, run mgba-cinema, and briefly tested video and GIF encoding with these changes on Debian unstable, with both FFmpeg 4.4 and 5.0.

I'm not sure about all the const additions. The FFmpeg 5.0 APIs return const pointers, but there are places with both versions where the pointers are passed to functions taking void *, which of course causes a new warning. I don't really know what's less bad here; at least const makes sure mGBA itself is sane?

[endrift]

The consts should be fine if they don't throw warnings in older versions. Can you check on older FFmpegs, e.g. the one in bionic or xenial? Also, what functions are passed these as void*?

[rtandy]

Thank you for the speedy review!

The consts should be fine if they don't throw warnings in older versions. Can you check on older FFmpegs, e.g. the one in bionic or xenial?

Same set of warnings on Debian bullseye (FFmpeg 4.3), Ubuntu bionic (FFmpeg 3.4), and xenial (FFmpeg 2.8):

/home/ryan/mgba/src/feature/ffmpeg/ffmpeg-encoder.c: In function ‘FFmpegEncoderSetVideo’:
/home/ryan/mgba/src/feature/ffmpeg/ffmpeg-encoder.c:219:30: warning: passing argument 1 of ‘av_opt_find’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  if (vbr < 0 && !av_opt_find(&codec->priv_class, "crf", NULL, 0, 0)) {
                              ^
In file included from /home/ryan/mgba/src/feature/ffmpeg/ffmpeg-encoder.c:28:0:
/usr/include/x86_64-linux-gnu/libavutil/opt.h:650:17: note: expected ‘void *’ but argument is of type ‘const AVClass * const* {aka const struct AVClass * const*}’
 const AVOption *av_opt_find(void *obj, const char *name, const char *unit,
                 ^
/home/ryan/mgba/src/feature/ffmpeg/ffmpeg-encoder.c: In function ‘FFmpegEncoderOpen’:
/home/ryan/mgba/src/feature/ffmpeg/ffmpeg-encoder.c:280:52: warning: passing argument 2 of ‘avformat_alloc_output_context2’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  avformat_alloc_output_context2(&encoder->context, oformat, 0, outfile);
                                                    ^
In file included from /home/ryan/mgba/src/feature/ffmpeg/ffmpeg-common.h:15:0,
                 from /home/ryan/mgba/src/feature/ffmpeg/ffmpeg-encoder.h:15,
                 from /home/ryan/mgba/src/feature/ffmpeg/ffmpeg-encoder.c:6:
/usr/include/x86_64-linux-gnu/libavformat/avformat.h:2013:5: note: expected ‘AVOutputFormat * {aka struct AVOutputFormat *}’ but argument is of type ‘const AVOutputFormat * {aka const struct AVOutputFormat *}’
 int avformat_alloc_output_context2(AVFormatContext **ctx, AVOutputFormat *oformat,
     ^

Also, what functions are passed these as void*?

Looks like just av_opt_find, actually.

[endrift]

Add an explicit cast to (void*) for the cases where needed. It looks like (frustratingly) libavutil casts them back to const inside that function, even though it's not declared as const void*. That should squelch that warning.

[rtandy]

Add an explicit cast to (void*) for the cases where needed.

Done.

Also added a cast to remove the const on the second argument to avformat_alloc_output_context2, which silences the other warning I pasted above (ffmpeg-encoder.c:280). Fingers crossed that's what you would have wanted.

Now compiling without warnings on 5.0 and 4.4, will go back and verify the older ones as well. Verified.

[rtandy]

Thank you!