A demonstration of GIL-releasing hashing leading to a data race in Python

test.py

#! /usr/bin/env python3

import hashlib
import threading


def hash_buf(buf):
    return hashlib.sha256(buf).hexdigest()


def hash_buf_on_bg_thread(buf):
    result = [None]

    def work():
        result[0] = hash_buf(buf)

    thread = threading.Thread(target=work)
    thread.start()
    return (thread, result)


def main():
    buf = bytearray(1_000_000)

    # Print the hash of the buffer while it's all zero.
    hash_before = hash_buf(buf)
    print("hash before:", hash_before)

    # Start a background thread to hash the buffer again.
    thread, result = hash_buf_on_bg_thread(buf)

    # Do two unsynchronized writes, one at the front of the buffer, and one at
    # the back. This is at the same time as the background thread is working.
    # It will likely miss the first write (either because it's already read the
    # front of the buffer, or because our CPU core won't flush its stores
    # immediately) but observe the second.
    buf[0] = 1
    buf[-1] = 1

    # Await the background thread and print its result.
    thread.join()
    hash_during = result[0]
    print("hash in bg: ", hash_during)

    # Print the hash of the buffer after both writes.
    hash_after = hash_buf(buf)
    print("hash after: ", hash_after)


if __name__ == "__main__":
    main()