cpython/Lib/test/test_threaded_import.py

# This is a variant of the very old (early 90's) file
# Demo/threads/bug.py.  It simply provokes a number of threads into
# trying to import the same module "at the same time".
# There are no pleasant failure modes -- most likely is that Python
# complains several times about module random having no attribute
# randrange, and then Python hangs.

import unittest
from test.support import verbose, TestFailed, import_module
thread = import_module('_thread')

critical_section = thread.allocate_lock()
done = thread.allocate_lock()

def task():
    global N, critical_section, done
    import random
    x = random.randrange(1, 3)
    critical_section.acquire()
    N -= 1
    # Must release critical_section before releasing done, else the main
    # thread can exit and set critical_section to None as part of global
    # teardown; then critical_section.release() raises AttributeError.
    finished = N == 0
    critical_section.release()
    if finished:
        done.release()

def test_import_hangers():
    import sys
    if verbose:
        print("testing import hangers ...", end=' ')

    import test.threaded_import_hangers
    try:
        if test.threaded_import_hangers.errors:
            raise TestFailed(test.threaded_import_hangers.errors)
        elif verbose:
            print("OK.")
    finally:
        # In case this test is run again, make sure the helper module
        # gets loaded from scratch again.
        del sys.modules['test.threaded_import_hangers']

# Tricky:  When regrtest imports this module, the thread running regrtest
# grabs the import lock and won't let go of it until this module returns.
# All other threads attempting an import hang for the duration.  Since
# this test spawns threads that do little *but* import, we can't do that
# successfully until after this module finishes importing and regrtest
# regains control.  To make this work, a special case was added to
# regrtest to invoke a module's "test_main" function (if any) after
# importing it.

def test_main():        # magic name!  see above
    global N, done

    import imp
    if imp.lock_held():
        # This triggers on, e.g., from test import autotest.
        raise unittest.SkipTest("can't run when import lock is held")

    done.acquire()
    for N in (20, 50) * 3:
        if verbose:
            print("Trying", N, "threads ...", end=' ')
        for i in range(N):
            thread.start_new_thread(task, ())
        done.acquire()
        if verbose:
            print("OK.")
    done.release()

    test_import_hangers()

if __name__ == "__main__":
    test_main()