# 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 _imp as imp
import os
import importlib
import sys
import time
import shutil
import unittest
from test.support import (
verbose, import_module, run_unittest, TESTFN, reap_threads,
forget, unlink, rmtree, start_threads)
threading = import_module('threading')
def task(N, done, done_tasks, errors):
try:
# We don't use modulefinder but still import it in order to stress
# importing of different modules from several threads.
if len(done_tasks) % 2:
import modulefinder
import random
else:
import random
import modulefinder
# This will fail if random is not completely initialized
x = random.randrange(1, 3)
except Exception as e:
errors.append(e.with_traceback(None))
finally:
done_tasks.append(threading.get_ident())
finished = len(done_tasks) == N
if finished:
done.set()
# Create a circular import structure: A -> C -> B -> D -> A
# NOTE: `time` is already loaded and therefore doesn't threaten to deadlock.
circular_imports_modules = {
'A': """if 1:
import time
time.sleep(%(delay)s)
x = 'a'
import C
""",
'B': """if 1:
import time
time.sleep(%(delay)s)
x = 'b'
import D
""",
'C': """import B""",
'D': """import A""",
}
class Finder:
"""A dummy finder to detect concurrent access to its find_spec()
method."""
def __init__(self):
self.numcalls = 0
self.x = 0
self.lock = threading.Lock()
def find_spec(self, name, path=None, target=None):
# Simulate some thread-unsafe behaviour. If calls to find_spec()
# are properly serialized, `x` will end up the same as `numcalls`.
# Otherwise not.
assert imp.lock_held()
with self.lock:
self.numcalls += 1
x = self.x
time.sleep(0.01)
self.x = x + 1
class FlushingFinder:
"""A dummy finder which flushes sys.path_importer_cache when it gets
called."""
def find_spec(self, name, path=None, target=None):
sys.path_importer_cache.clear()
class ThreadedImportTests(unittest.TestCase):
def setUp(self):
self.old_random = sys.modules.pop('random', None)
def tearDown(self):
# If the `random` module was already initialized, we restore the
# old module at the end so that pickling tests don't fail.
# See http://bugs.python.org/issue3657#msg110461
if self.old_random is not None:
sys.modules['random'] = self.old_random
def check_parallel_module_init(self):
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 = threading.Event()
for N in (20, 50) * 3:
if verbose:
print("Trying", N, "threads ...", end=' ')
# Make sure that random and modulefinder get reimported freshly
for modname in ['random', 'modulefinder']:
try:
del sys.modules[modname]
except KeyError:
pass
errors = []
done_tasks = []
done.clear()
t0 = time.monotonic()
with start_threads(threading.Thread(target=task,
args=(N, done, done_tasks, errors,))
for i in range(N)):
pass
completed = done.wait(10 * 60)
dt = time.monotonic() - t0
if verbose:
print("%.1f ms" % (dt*1e3), flush=True, end=" ")
dbg_info = 'done: %s/%s' % (len(done_tasks), N)
self.assertFalse(errors, dbg_info)
self.assertTrue(completed, dbg_info)
if verbose:
print("OK.")
def test_parallel_module_init(self):
self.check_parallel_module_init()
def test_parallel_meta_path(self):
finder = Finder()
sys.meta_path.insert(0, finder)
try:
self.check_parallel_module_init()
self.assertGreater(finder.numcalls, 0)
self.assertEqual(finder.x, finder.numcalls)
finally:
sys.meta_path.remove(finder)
def test_parallel_path_hooks(self):
# Here the Finder instance is only used to check concurrent calls
# to path_hook().
finder = Finder()
# In order for our path hook to be called at each import, we need
# to flush the path_importer_cache, which we do by registering a
# dedicated meta_path entry.
flushing_finder = FlushingFinder()
def path_hook(path):
finder.find_spec('')
raise ImportError
sys.path_hooks.insert(0, path_hook)
sys.meta_path.append(flushing_finder)
try:
# Flush the cache a first time
flushing_finder.find_spec('')
numtests = self.check_parallel_module_init()
self.assertGreater(finder.numcalls, 0)
self.assertEqual(finder.x, finder.numcalls)
finally:
sys.meta_path.remove(flushing_finder)
sys.path_hooks.remove(path_hook)
def test_import_hangers(self):
# In case this test is run again, make sure the helper module
# gets loaded from scratch again.
try:
del sys.modules['test.threaded_import_hangers']
except KeyError:
pass
import test.threaded_import_hangers
self.assertFalse(test.threaded_import_hangers.errors)
def test_circular_imports(self):
# The goal of this test is to exercise implementations of the import
# lock which use a per-module lock, rather than a global lock.
# In these implementations, there is a possible deadlock with
# circular imports, for example:
# - thread 1 imports A (grabbing the lock for A) which imports B
# - thread 2 imports B (grabbing the lock for B) which imports A
# Such implementations should be able to detect such situations and
# resolve them one way or the other, without freezing.
# NOTE: our test constructs a slightly less trivial import cycle,
# in order to better stress the deadlock avoidance mechanism.
delay = 0.5
os.mkdir(TESTFN)
self.addCleanup(shutil.rmtree, TESTFN)
sys.path.insert(0, TESTFN)
self.addCleanup(sys.path.remove, TESTFN)
for name, contents in circular_imports_modules.items():
contents = contents % {'delay': delay}
with open(os.path.join(TESTFN, name + ".py"), "wb") as f:
f.write(contents.encode('utf-8'))
self.addCleanup(forget, name)
importlib.invalidate_caches()
results = []
def import_ab():
import A
results.append(getattr(A, 'x', None))
def import_ba():
import B
results.append(getattr(B, 'x', None))
t1 = threading.Thread(target=import_ab)
t2 = threading.Thread(target=import_ba)
t1.start()
t2.start()
t1.join()
t2.join()
self.assertEqual(set(results), {'a', 'b'})
def test_side_effect_import(self):
code = """if 1:
import threading
def target():
import random
t = threading.Thread(target=target)
t.start()
t.join()
t = None"""
sys.path.insert(0, os.curdir)
self.addCleanup(sys.path.remove, os.curdir)
filename = TESTFN + ".py"
with open(filename, "wb") as f:
f.write(code.encode('utf-8'))
self.addCleanup(unlink, filename)
self.addCleanup(forget, TESTFN)
self.addCleanup(rmtree, '__pycache__')
importlib.invalidate_caches()
__import__(TESTFN)
del sys.modules[TESTFN]
@reap_threads
def test_main():
old_switchinterval = None
try:
old_switchinterval = sys.getswitchinterval()
sys.setswitchinterval(1e-5)
except AttributeError:
pass
try:
run_unittest(ThreadedImportTests)
finally:
if old_switchinterval is not None:
sys.setswitchinterval(old_switchinterval)
if __name__ == "__main__":
test_main()