Shim the macOS futex API

[joboet]

Since macOS 14.4, the Apple platforms have a public futex-like interface, which we'd very much like to use in std (see rust-lang/rust#122408), but we're still missing the miri shims for it. In typical Apple fashion, they have failed to upload the full documentation to their website and have not included it in their man-pages either. Still, it exists and can be accessed either from the header file (os/os_sync_wait_on_address.h) or from its upload to their public-source repository.

[RalfJung]

Is this the same as / a replacement of #2589?

[joboet]

This would make that other issue redundant, yes, although the API is different.

[RalfJung]

Well let us know then which of the APIs t-libs plans to use, ideally we won't have to implement both of them. :)

[RalfJung]

Looking at rust-lang/rust#122408, there are quite a few functions to be supported here:

pub fn os_sync_wait_on_address(*mut c_void, u64, usize, u32) -> c_int
pub fn os_sync_wait_on_address_with_timeout(*mut c_void, u64, usize, u32, u32, u64) -> c_int
pub fn os_sync_wake_by_address_any(*mut c_void, usize, u32) -> c_int
pub fn os_sync_wake_by_address_all(*mut c_void, usize, u32) -> c_int
pub fn __ulock_wait2(u32, *mut c_void, u64, u64, u64) -> c_int
pub fn __ulock_wait(u32, *mut c_void, u64, u32) -> c_int
pub fn __ulock_wake(u32, *mut c_void, u64) -> c_int

But it also seems there's a sequence of fallbacks so we probably don't need all of them? I'll have to check how weak symbols work on macOS and how Miri can provide them -- on Linux it's fairly easy, we just have global statics of the name and fill them with function pointers. On macOS it seems to boil down to dlsym? That should be easy as well then.

So then it seems like we need just the first four ones, the ones with the "nice" names (not starting with __).

[RalfJung]

What would help is having a little test program that invokes these APIs and checks basic functionality, with the flags std needs. (Can be direct calls, it's easy to then also make them available via dlsym). Usually I use the standard library to drive the testing but that won't work this time.

We have such a test for futexes at tests/pass-dep/concurrency/linux-futex.rs