gh-127020: Make PyCode_GetCode thread-safe for free threading

[colesbury]

Some fields in PyCodeObject are lazily initialized. Use atomics and critical sections to make their initializations and accesses thread-safe.

• Issue: PyCode_GetCode is not thread-safe and causes assertion fail with Python 3.13td #127020

[vfdev-5]

Hi @colesbury , Lysandros pointed me out to your PR fixing a bug in FT mode.
I had another repro code showing randomly tsan warnings in get_cached_locals and init_co_cached :

import concurrent.futures
import threading
import inspect


def decorator(f):
    # Introspect the callable for optional features.
    sig = inspect.signature(f)
    for param in sig.parameters.values():
        pass

    def emit_call_op(*call_args):
        pass

    wrapped = emit_call_op
    return wrapped


def test_dialects_vector_repro_3():
    num_workers = 6
    num_runs = 10
    barrier = threading.Barrier(num_workers)

    def closure():
        barrier.wait()
        for _ in range(num_runs):

            @decorator
            def print_vector(arg):
                return 0

        barrier.wait()

    with concurrent.futures.ThreadPoolExecutor(
        max_workers=num_workers
    ) as executor:
        futures = []
        for _ in range(num_workers):
            futures.append(executor.submit(closure))
        # We should call future.result() to re-raise an exception if test has
        # failed
        assert len(list(f.result() for f in futures)) == num_workers


if __name__ == "__main__":
    test_dialects_vector_repro_3()

Outputs (cpython 3.13 built with TSAN):

==================
WARNING: ThreadSanitizer: data race (pid=30720)
  Write of size 8 at 0x7fedfa39de10 by thread T6:
    #0 init_co_cached /tmp/cpython-tsan/Objects/codeobject.c:309:26 (python3.13t+0x1e6441) (BuildId: f1122943b999261e166922e88bbc71af5ecf54a9)
    #1 _PyCode_GetVarnames /tmp/cpython-tsan/Objects/codeobject.c:1562:9 (python3.13t+0x1e6441)
    #2 code_getvarnames /tmp/cpython-tsan/Objects/codeobject.c:2107:12 (python3.13t+0x1e6441)
    #3 getset_get /tmp/cpython-tsan/Objects/descrobject.c:193:16 (python3.13t+0x1eb966) (BuildId: f1122943b999261e166922e88bbc71af5ecf54a9)

or

  Read of size 8 at 0x7efddc0e0068 by thread T2:
    #0 get_cached_locals /tmp/cpython-tsan/Objects/codeobject.c:1547:9 (python3.13t+0x1e0e8a) (BuildId: f1122943b999261e166922e88bbc71af5ecf54a9)
    #1 _PyCode_GetVarnames /tmp/cpython-tsan/Objects/codeobject.c:1565:12 (python3.13t+0x1e6484) (BuildId: f1122943b999261e166922e88bbc71af5ecf54a9)
    #2 code_getvarnames /tmp/cpython-tsan/Objects/codeobject.c:2107:12 (python3.13t+0x1e6484)
    #3 getset_get /tmp/cpython-tsan/Objects/descrobject.c:193:16 (python3.13t+0x1eb966) (BuildId: f1122943b999261e166922e88bbc71af5ecf54a9)

Can you please check and run it in your PR and see if this is also fixed? Thanks a lot!

[colesbury]

@vfdev-5, I verified that it fixed your repro as well.

[miss-islington-app]

Thanks @colesbury for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-127107 is a backport of this pull request to the 3.13 branch.