The `native_thread_id` field of `PyThreadState` is not updated after fork

[P403n1x87]

Bug report

The native_thread_id field of the PyThreadState object is not updated after a fork on Linux (at least). This means that child processes spawned by the main thread of the parent process will have a main thread with the parent thread ID.

The native_thread_id is meant to be consumed by tools like Austin and therefore the behaviour is easily observed with these tools. One way to reproduce this is to profile this with Austin

import multiprocessing


def fact(n):
    f = 1
    for i in range(1, n + 1):
        f *= i
    return f


def do(N):
    n = 1
    for _ in range(N):
        fact(n)
        n += 1


if __name__ == "__main__":
    import sys

    try:
        nproc = int(sys.argv[1])
    except Exception:
        nproc = 2

    processes = []
    for _ in range(nproc):
        process = multiprocessing.Process(target=do, args=(3000,))
        process.start()
        processes.append(process)

    for process in processes:
        process.join(timeout=5)

and observe that the reported thread IDs coincide with the parent's PID.

Your environment

• CPython versions tested on: 3.11.1
• Operating system and architecture: Ubuntu 22.04 (amd64)

Linked PRs

• gh-100649: Update native_thread_id after fork #100650
• [3.11] gh-100649: Update native_thread_id after fork (gh-100650) #100660