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252 lines
8.5 KiB
Python
252 lines
8.5 KiB
Python
# This is a variant of the very old (early 90's) file
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# Demo/threads/bug.py. It simply provokes a number of threads into
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# trying to import the same module "at the same time".
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# There are no pleasant failure modes -- most likely is that Python
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# complains several times about module random having no attribute
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# randrange, and then Python hangs.
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import _imp as imp
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import os
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import importlib
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import sys
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import time
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import shutil
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import unittest
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from test.support import (
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verbose, import_module, run_unittest, TESTFN, reap_threads,
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forget, unlink, rmtree, start_threads)
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threading = import_module('threading')
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def task(N, done, done_tasks, errors):
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try:
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# We don't use modulefinder but still import it in order to stress
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# importing of different modules from several threads.
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if len(done_tasks) % 2:
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import modulefinder
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import random
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else:
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import random
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import modulefinder
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# This will fail if random is not completely initialized
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x = random.randrange(1, 3)
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except Exception as e:
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errors.append(e.with_traceback(None))
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finally:
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done_tasks.append(threading.get_ident())
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finished = len(done_tasks) == N
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if finished:
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done.set()
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# Create a circular import structure: A -> C -> B -> D -> A
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# NOTE: `time` is already loaded and therefore doesn't threaten to deadlock.
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circular_imports_modules = {
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'A': """if 1:
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import time
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time.sleep(%(delay)s)
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x = 'a'
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import C
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""",
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'B': """if 1:
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import time
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time.sleep(%(delay)s)
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x = 'b'
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import D
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""",
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'C': """import B""",
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'D': """import A""",
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}
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class Finder:
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"""A dummy finder to detect concurrent access to its find_spec()
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method."""
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def __init__(self):
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self.numcalls = 0
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self.x = 0
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self.lock = threading.Lock()
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def find_spec(self, name, path=None, target=None):
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# Simulate some thread-unsafe behaviour. If calls to find_spec()
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# are properly serialized, `x` will end up the same as `numcalls`.
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# Otherwise not.
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assert imp.lock_held()
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with self.lock:
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self.numcalls += 1
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x = self.x
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time.sleep(0.01)
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self.x = x + 1
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class FlushingFinder:
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"""A dummy finder which flushes sys.path_importer_cache when it gets
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called."""
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def find_spec(self, name, path=None, target=None):
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sys.path_importer_cache.clear()
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class ThreadedImportTests(unittest.TestCase):
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def setUp(self):
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self.old_random = sys.modules.pop('random', None)
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def tearDown(self):
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# If the `random` module was already initialized, we restore the
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# old module at the end so that pickling tests don't fail.
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# See http://bugs.python.org/issue3657#msg110461
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if self.old_random is not None:
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sys.modules['random'] = self.old_random
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def check_parallel_module_init(self):
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if imp.lock_held():
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# This triggers on, e.g., from test import autotest.
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raise unittest.SkipTest("can't run when import lock is held")
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done = threading.Event()
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for N in (20, 50) * 3:
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if verbose:
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print("Trying", N, "threads ...", end=' ')
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# Make sure that random and modulefinder get reimported freshly
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for modname in ['random', 'modulefinder']:
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try:
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del sys.modules[modname]
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except KeyError:
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pass
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errors = []
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done_tasks = []
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done.clear()
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t0 = time.monotonic()
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with start_threads(threading.Thread(target=task,
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args=(N, done, done_tasks, errors,))
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for i in range(N)):
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pass
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completed = done.wait(10 * 60)
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dt = time.monotonic() - t0
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if verbose:
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print("%.1f ms" % (dt*1e3), flush=True, end=" ")
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dbg_info = 'done: %s/%s' % (len(done_tasks), N)
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self.assertFalse(errors, dbg_info)
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self.assertTrue(completed, dbg_info)
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if verbose:
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print("OK.")
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def test_parallel_module_init(self):
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self.check_parallel_module_init()
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def test_parallel_meta_path(self):
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finder = Finder()
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sys.meta_path.insert(0, finder)
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try:
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self.check_parallel_module_init()
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self.assertGreater(finder.numcalls, 0)
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self.assertEqual(finder.x, finder.numcalls)
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finally:
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sys.meta_path.remove(finder)
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def test_parallel_path_hooks(self):
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# Here the Finder instance is only used to check concurrent calls
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# to path_hook().
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finder = Finder()
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# In order for our path hook to be called at each import, we need
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# to flush the path_importer_cache, which we do by registering a
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# dedicated meta_path entry.
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flushing_finder = FlushingFinder()
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def path_hook(path):
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finder.find_spec('')
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raise ImportError
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sys.path_hooks.insert(0, path_hook)
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sys.meta_path.append(flushing_finder)
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try:
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# Flush the cache a first time
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flushing_finder.find_spec('')
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numtests = self.check_parallel_module_init()
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self.assertGreater(finder.numcalls, 0)
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self.assertEqual(finder.x, finder.numcalls)
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finally:
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sys.meta_path.remove(flushing_finder)
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sys.path_hooks.remove(path_hook)
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def test_import_hangers(self):
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# In case this test is run again, make sure the helper module
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# gets loaded from scratch again.
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try:
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del sys.modules['test.threaded_import_hangers']
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except KeyError:
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pass
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import test.threaded_import_hangers
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self.assertFalse(test.threaded_import_hangers.errors)
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def test_circular_imports(self):
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# The goal of this test is to exercise implementations of the import
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# lock which use a per-module lock, rather than a global lock.
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# In these implementations, there is a possible deadlock with
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# circular imports, for example:
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# - thread 1 imports A (grabbing the lock for A) which imports B
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# - thread 2 imports B (grabbing the lock for B) which imports A
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# Such implementations should be able to detect such situations and
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# resolve them one way or the other, without freezing.
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# NOTE: our test constructs a slightly less trivial import cycle,
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# in order to better stress the deadlock avoidance mechanism.
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delay = 0.5
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os.mkdir(TESTFN)
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self.addCleanup(shutil.rmtree, TESTFN)
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sys.path.insert(0, TESTFN)
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self.addCleanup(sys.path.remove, TESTFN)
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for name, contents in circular_imports_modules.items():
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contents = contents % {'delay': delay}
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with open(os.path.join(TESTFN, name + ".py"), "wb") as f:
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f.write(contents.encode('utf-8'))
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self.addCleanup(forget, name)
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importlib.invalidate_caches()
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results = []
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def import_ab():
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import A
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results.append(getattr(A, 'x', None))
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def import_ba():
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import B
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results.append(getattr(B, 'x', None))
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t1 = threading.Thread(target=import_ab)
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t2 = threading.Thread(target=import_ba)
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t1.start()
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t2.start()
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t1.join()
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t2.join()
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self.assertEqual(set(results), {'a', 'b'})
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def test_side_effect_import(self):
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code = """if 1:
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import threading
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def target():
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import random
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t = threading.Thread(target=target)
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t.start()
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t.join()"""
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sys.path.insert(0, os.curdir)
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self.addCleanup(sys.path.remove, os.curdir)
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filename = TESTFN + ".py"
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with open(filename, "wb") as f:
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f.write(code.encode('utf-8'))
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self.addCleanup(unlink, filename)
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self.addCleanup(forget, TESTFN)
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self.addCleanup(rmtree, '__pycache__')
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importlib.invalidate_caches()
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__import__(TESTFN)
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@reap_threads
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def test_main():
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old_switchinterval = None
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try:
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old_switchinterval = sys.getswitchinterval()
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sys.setswitchinterval(1e-5)
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except AttributeError:
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pass
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try:
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run_unittest(ThreadedImportTests)
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finally:
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if old_switchinterval is not None:
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sys.setswitchinterval(old_switchinterval)
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if __name__ == "__main__":
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test_main()
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