Crash report

What happened?

Trying to iterate on a generator from multiple threads under the free-threaded build results in a segmentation fault.

Here is a minimal repro:

import concurrent.futures


def gen():
    while True:
        yield


it = gen()
with concurrent.futures.ThreadPoolExecutor() as executor:
    while True:
        _ = executor.submit(lambda: next(it))

The issue seems to be specific to generators as other kinds of iterators work well in parallel.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.14.0a0 experimental free-threading build (heads/main:c3b6dbff2c, Jun 10 2024, 16:54:16) [GCC 11.4.0]

Linked PRs

• gh-120321: Lock gen_send #120327
• gh-120321: Make gi_frame_state transitions atomic in FT build #142599
• gh-120321: Fix TSan reported race in gen_clear_frame #142995
• gh-120321: Avoid -Wunreachable-code warning on Clang #143022
• gh-120321: Make gen.gi_frame.clear() thread-safe #143112
• gh-120321: Fix TSan reported races on gi_frame_state #143128
• gh-120321: Add missing "return false" in gen_try_set_executing #144291
• gh-120321: Make gi_yieldfrom thread-safe in free-threading build #144292
• gh-120321: Add gi_state, cr_state, and ag_state attributes #144409