[core] Warn user when FFmpeg is missing

[seproDev]

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Warns the user when the selected format on the default format selection differs due to FFmpeg missing.

As this is part of the _default_format_spec function we don't need to check if the user has passed any custom format selection.
To suppress this warning, the user can pass -f best/bestvideo+bestaudio.

Fіxes #9843

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[pukkandan]

LGTM, but I'd like a second set of eyes on this
cc @bashonly @Grub4K

[bashonly]

This could trigger an unnecessary and misleading warning if the user is passing -o - and doesn't have ffmpeg installed. Since the default format selection becomes best/bestvideo+bestaudio when piping to stdout, the warning would be incorrect: installing ffmpeg will not change the quality of what's piped. And I think it's fair to assume that a user who is piping to stdout knows what they're doing and does not need any warning here.

Maybe we could do this?

	to_stdout = self.params['outtmpl']['default'] == '-'

        def evaluate_formats(format_spec):
            formats = self._get_formats(info_dict)
            return self._select_formats(formats, self.build_format_selector(format_spec))
        if not can_merge() and not to_stdout and evaluate_formats('b/bv+ba') != evaluate_formats('bv*+ba/b'):
            self.report_warning('ffmpeg not found. The downloaded format is not the highest available quality. '
                                'Installing ffmpeg is strongly recommended: https://github.com/yt-dlp/yt-dlp#dependencies')

        prefer_best = (
            not self.params.get('simulate')
            and download
            and (
                not can_merge()
                or info_dict.get('is_live') and not self.params.get('live_from_start')
                or to_stdout)))

[pukkandan]

shouldn't we also check for is_live etc then?

[bashonly]

Yeah I think we need to as well

[pukkandan]

Might as well clean up the function then? Something like:

    def _default_format_spec(self, info_dict, download=True):
        download = download and not self.params.get('simulate')
        prefer_best = download and (
            self.params['outtmpl']['default'] == '-'
            or info_dict.get('is_live') and not self.params.get('live_from_start'))

        def can_merge():
            merger = FFmpegMergerPP(self)
            can_merge = merger.available and merger.can_merge()

        if not prefer_best and download and not can_merge():
            prefer_best = True
            formats = self._get_formats(info_dict)
            evaluate_formats = lambda spec: self._select_formats(formats, self.build_format_selector(spec))
            if evaluate_formats('b/bv+ba') != evaluate_formats('bv*+ba/b'):
                self.report_warning('ffmpeg not found. The downloaded format is not the highest available quality. '
                                    'Installing ffmpeg is strongly recommended: https://github.com/yt-dlp/yt-dlp#dependencies')

        compat = (self.params.get('allow_multiple_audio_streams')
                  or 'format-spec' in self.params['compat_opts'])

        return ('best/bestvideo+bestaudio' if prefer_best
                else 'bestvideo+bestaudio/best' if compat
                else 'bestvideo*+bestaudio/best')

Note: can_merge is a function to avoid initializing ffmpeg if not needed

[bashonly]

Side note: I think having simulate result in a different default format spec should be considered a bug. A dry-run should be no different than the real thing, right?

It looks like we can trace this logic back to ytdl-org/youtube-dl@0017d9a

Back then, upstream's default format spec was only best if ffmpeg was not available. So a simulated run would result in a "requested formats not available" error if ffmpeg was not available and there was no combined video+audio format available. This simulate check seems to be added so that you could print json without having to manually pass -f bv+ba or -f bv etc in this scenario -- see the linked upstream PR

(sepro and I started digging into this when I was questioning if simulate should result in a warning or not)

[bashonly]

(sepro and I started digging into this when I was questioning if simulate should result in a warning or not)

And if we don't change the default format spec behavior with simulate, then currently the warning is wrongly raised since we aren't preferring best

[pukkandan]

I don't remember anyone having complained, and I am hesitant to change long standing behavior without reason

And if we don't change the default format spec behavior with simulate, then currently the warning is wrongly raised since we aren't preferring best

Edited #9805 (comment) to address this

[seproDev]

Well, consider this me complaining lol
As bash mentioned, I think a simulate/dry run option is only really useful if you actually simulate what will happen. If a user wants to see what formats will be selected when downloading, the result would be wrong without FFmpeg installed.
This behaviour is currently also not documented.

[bashonly]

I was bitten by this undocumented behavior the other day when I was troubleshooting for someone on discord. I also would like to see it fixed or at the very least documented

[pukkandan]

Still, I don't want to make this change in this PR. Could you open a new issue/PR? We should discuss with dirkf as well.