support 3.13t free-threaded python

[davidhewitt]

First pass at adding CI and tests for freethreaded Python.

Locally I get some test failures, so I will have to investigate further before this is ready even if CI is green. cc @ngoldbaum

failures:

---- borrow::shared::tests::borrow_multiple_views stdout ----
thread 'borrow::shared::tests::borrow_multiple_views' panicked at src/borrow/shared.rs:836:17:
assertion `left == right` failed
  left: 3
 right: 1

---- borrow::shared::tests::borrow_multiple_arrays stdout ----
thread 'borrow::shared::tests::borrow_multiple_arrays' panicked at src/borrow/shared.rs:786:17:
assertion `left == right` failed
  left: 2
 right: 1


failures:
    borrow::shared::tests::borrow_multiple_arrays
    borrow::shared::tests::borrow_multiple_views

test result: FAILED. 27 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.13s

[davidhewitt]

Choice to use Mutex here makes me want to add a MutexExt trait to PyO3 similar to OnceLockExt which we already added.

Given that writes to this should be infrequent (it's a global cache, as far as I can tell), I also wonder if RwLock is appropriate here. Readers are extremely short-lived so the analysis in https://blog.nelhage.com/post/rwlock-contention/ seems to be a non-issue (cc @alex)

... in which case I want RwLockExt too 😂

[davidhewitt]

Based on the failures here, I propose rust-numpy 0.23.0 releases without free-threading support, and we seek to follow up in 0.23.1.

[Icxolu]

I did a bit of investigating here. I believe the fundamental problem is that the borrow checking API relies on the GIL for synchronization of exclusive access

rust-numpy/src/borrow/shared.rs

Lines 45 to 48 in b34bc29

	unsafe extern "C" fn acquire_shared(flags: *mut c_void, array: *mut PyArrayObject) -> c_int {
	// SAFETY: GIL must be held when calling `acquire_shared`.
	let py = Python::assume_gil_acquired();
	let flags = &mut *(flags as *mut BorrowFlags);

On the free-threaded build this is not true anymore, and therefore unsound.

An easy (but probably not optimal) solution is to wrap BorrowFlagsInner into a Mutex (I testet with parking_lot to not have to deal with poisoning) and change everything to shared access. I believe that should be enough to make it sound. The tests still don't pass, but I think that is related to their implicit assumption of running serially. The GIL enforced each tests running to completion before the next one can start, but on the free-threaded build the can run interleaved, and some assertions do not hold because of that.