threads: remove unused recursive mutex support

[kasper93]

The only use of MP_MUTEX_RECURSIVE is in vdpau.c. If it is disabled, remove the unused code. This simplifies the win32 implementation by eliminating unnecessary critical section references.

[github-actions]

Download the artifacts for this pull request:

Windows

• mpv-i686-w64-mingw32
• mpv-x86_64-w64-mingw32
• mpv-x86_64-windows-msvc

macOS

• mpv-macos-13-intel
• mpv-macos-14-arm
• mpv-macos-15-arm

[na-na-hi]

Shouldn't win32 recursive mutex be removed completely instead of ifdef? I don't think vdpau is supported on win32.

[kasper93]

Shouldn't win32 recursive mutex be removed completely instead of ifdef? I don't think vdpau is supported on win32.

The idea is if in the feature anyone wants to use it the code is there. When I added it initially there was more users of recursive mutex. I also don't like those ifdefs, but not having feature parity between wrappers would be awkward.

[na-na-hi]

When I added it initially there was more users of recursive mutex. I also don't like those ifdefs, but not having feature parity between wrappers would be awkward.

It doesn't simplify the code when the recursive mutex isn't removed but instead adding more ifdefs on top of it. At best it saves a few bytes for the mutex struct. If you want to keep the recursive mutex implementation for future use, I don't think the ifdefs worth it, because if it is added in a mandatory part of mpv, then the recursive mutex becomes mandatory anyway so ifdefs aren't needed.

[Dudemanguy]

I'd rather just remove vdpau than this tbh. Not that I'm suggesting we need to.

[kasper93]

It doesn't simplify the code

Simplification is not the main objective of this PR.

At best it saves a few bytes for the mutex struct.

It saves 40 bytes from mp_mutex struct. Still not the main concern. The main difference is the bloated code by adding if (mutex->use_cs) with essentially dead branch of code. And I by design wanted to inline it, so it is optimized out, if possible. Also mp_mutex_destroy becomes noop, so there is not even need for function call.

because if it is added in a mandatory part of mpv, then the recursive mutex becomes mandatory anyway so ifdefs aren't needed.

That's a big "if". I would rather avoid recursive mutexes at all cost.

I'd rather just remove vdpau than this tbh. Not that I'm suggesting we need to.

Maybe I should read the code if this mutex is needed there.

[na-na-hi]

The main difference is the bloated code by adding if (mutex->use_cs) with essentially dead branch of code. And I by design wanted to inline it, so it is optimized out, if possible.

There is no need to put ifdef everywhere if this is what you wanted. Replace mutex->use_cs with a macro like MP_MUTEX_IS_CS(mutex) and ifdef that to be 0. The compiler will eliminate the if (0) branch.

[kasper93]

I will remove recursive mutex completely after #15997 is merged.

EDIT: Updated this PR too.