Make hashlib related modules thread-safe without the GIL

[colesbury]

Feature or enhancement

The hashlib based modules already have some locking to make some operations thread-safe (with the GIL), but the logic isn't sufficient if running with the GIL disabled.

Relevant files:

• Modules/_blake2/blake2b_impl.c
• Modules/_blake2/blake2s_impl.c
• Modules/_hashopenssl.c
• Modules/hashlib.h
• Modules/md5module.c
• Modules/sha1module.c
• Modules/sha2module.c
• Modules/sha3module.c

Basic idea:

1. Replace PyThread_type_lock lock with PyMutex. This should be both simpler and faster in general and avoid the need for dynamically assigning a lock, which can pose thread-safety issues without the GIL
2. Add a field bool use_mutex to indicate if the code should lock the mutex. This should always be set to true in Py_NOGIL. In the default build, we should dynamically set it to true in places where we previously allocated self->lock
3. Update ENTER_HASHLIB and EXIT_HASHLIB macros.

Linked PRs

• gh-111916: Make hashlib related modules thread-safe without the GIL #111981

[tomasr8]

Hey! I'd like to give this a try. I have a limited experience with cpython internals but I had a quick look at the code and it seems feasible to me 😅

[colesbury]

Great, thanks! Let me know if you have any questions or there is anything I can help with.