First of all, this never worked. Or if it ever did, it was in some
select few scenarios. c9474dc9ed is what
originally added support for the auto choice. However, that commit
worked by propagating a value to a fake option used internally. This
shouldn't have ever worked because the underlying m_config_cache was
never updated so the value shouldn't have been preserved when accessed
in sd_lavc. And indeed with some testing, the value there is always 0
unsurprisingly.
This was later rewritten in ba7cc07106
along with a lot of other sub changes, but with that, it was still
mostly broken. The reason is because one of the key parts of having to
hit this logic (prefer_forced) required `--no-subs-with-matching-audio`
to be set. If the audio language matches the subtitle language (the
requirement also excludes forced subs), the option makes no subtitle
selection in the first place so pick->forced_only_def is not set to true
and nothing even happens. Another way around this would be to attempt to
change your OS language (like with the LANG environment variable) so
that the subtitle track gets selected but then audio_matches mistakenly
becomes false because it compares the OS language to the audio language
which then make preferred_forced 0, so nothing happens. I don't think
there's a scenario where pick->forced_only_def is actually set to true
(thus meaning `auto` is useless), but maybe someone could contrive
something very strange. Regardless, it's definitely not something even
remotely common.
fbe8f99194 changed track selection again
but didn't consider this particular case. The net result is that DVD/PGS
subs become equivalent to --sub-forced-only being yes, so this a change
in behavior and probably not a good one. Note that I wasn't able to
actually observe any difference in a PGS sample. It still displayed
subtitles fine but that sample probably didn't have the right flags to
hit the sub-forced-only logic.
Anyways, the auto feature is extremely questionable at best and in my
view, not actually worth it. It is meant to be used with
`--no-subs-with-matching-audio` to display forced pictures in subtitle
tracks that are not marked as forced, but that contradicts that
particular option's purpose and description in the manual (secretly
selecting a track under certain conditions even though it says not to).
Instead of trying to shove all this logic into select_default_track
which is already insanely complicated as it is, recognize that this is a
trivial lua script. If you absolutely want to turn --sub-forced-only on
under these certain conditions (DVD/PGS subtitles, matching audio and
subtitle languages, etc.), just look at the current-tracks property and
do your thing. The very, very niche behavior that this option tried to
accomplish basically never worked, no user even knows what this option
does, and well it's just not worth supporting in core mpv code. Drop
all this code for sanity's sake and change --sub-forced-only back to a
bool.
4009e99b9c