1 # This is a variant of the very old (early 90's) file 2 # Demo/threads/bug.py. It simply provokes a number of threads into 3 # trying to import the same module "at the same time". 4 # There are no pleasant failure modes -- most likely is that Python 5 # complains several times about module random having no attribute 6 # randrange, and then Python hangs. 7 8 import _imp as imp 9 import os 10 import importlib 11 import sys 12 import time 13 import shutil 14 import unittest 15 from test.support import ( 16 verbose, import_module, run_unittest, TESTFN, reap_threads, 17 forget, unlink, rmtree, start_threads) 18 threading = import_module('threading') 19 20 def task(N, done, done_tasks, errors): 21 try: 22 # We don't use modulefinder but still import it in order to stress 23 # importing of different modules from several threads. 24 if len(done_tasks) % 2: 25 import modulefinder 26 import random 27 else: 28 import random 29 import modulefinder 30 # This will fail if random is not completely initialized 31 x = random.randrange(1, 3) 32 except Exception as e: 33 errors.append(e.with_traceback(None)) 34 finally: 35 done_tasks.append(threading.get_ident()) 36 finished = len(done_tasks) == N 37 if finished: 38 done.set() 39 40 # Create a circular import structure: A -> C -> B -> D -> A 41 # NOTE: `time` is already loaded and therefore doesn't threaten to deadlock. 42 43 circular_imports_modules = { 44 'A': """if 1: 45 import time 46 time.sleep(%(delay)s) 47 x = 'a' 48 import C 49 """, 50 'B': """if 1: 51 import time 52 time.sleep(%(delay)s) 53 x = 'b' 54 import D 55 """, 56 'C': """import B""", 57 'D': """import A""", 58 } 59 60 class Finder: 61 """A dummy finder to detect concurrent access to its find_spec() 62 method.""" 63 64 def __init__(self): 65 self.numcalls = 0 66 self.x = 0 67 self.lock = threading.Lock() 68 69 def find_spec(self, name, path=None, target=None): 70 # Simulate some thread-unsafe behaviour. If calls to find_spec() 71 # are properly serialized, `x` will end up the same as `numcalls`. 72 # Otherwise not. 73 assert imp.lock_held() 74 with self.lock: 75 self.numcalls += 1 76 x = self.x 77 time.sleep(0.01) 78 self.x = x + 1 79 80 class FlushingFinder: 81 """A dummy finder which flushes sys.path_importer_cache when it gets 82 called.""" 83 84 def find_spec(self, name, path=None, target=None): 85 sys.path_importer_cache.clear() 86 87 88 class ThreadedImportTests(unittest.TestCase): 89 90 def setUp(self): 91 self.old_random = sys.modules.pop('random', None) 92 93 def tearDown(self): 94 # If the `random` module was already initialized, we restore the 95 # old module at the end so that pickling tests don't fail. 96 # See http://bugs.python.org/issue3657#msg110461 97 if self.old_random is not None: 98 sys.modules['random'] = self.old_random 99 100 def check_parallel_module_init(self): 101 if imp.lock_held(): 102 # This triggers on, e.g., from test import autotest. 103 raise unittest.SkipTest("can't run when import lock is held") 104 105 done = threading.Event() 106 for N in (20, 50) * 3: 107 if verbose: 108 print("Trying", N, "threads ...", end=' ') 109 # Make sure that random and modulefinder get reimported freshly 110 for modname in ['random', 'modulefinder']: 111 try: 112 del sys.modules[modname] 113 except KeyError: 114 pass 115 errors = [] 116 done_tasks = [] 117 done.clear() 118 t0 = time.monotonic() 119 with start_threads(threading.Thread(target=task, 120 args=(N, done, done_tasks, errors,)) 121 for i in range(N)): 122 pass 123 completed = done.wait(10 * 60) 124 dt = time.monotonic() - t0 125 if verbose: 126 print("%.1f ms" % (dt*1e3), flush=True, end=" ") 127 dbg_info = 'done: %s/%s' % (len(done_tasks), N) 128 self.assertFalse(errors, dbg_info) 129 self.assertTrue(completed, dbg_info) 130 if verbose: 131 print("OK.") 132 133 def test_parallel_module_init(self): 134 self.check_parallel_module_init() 135 136 def test_parallel_meta_path(self): 137 finder = Finder() 138 sys.meta_path.insert(0, finder) 139 try: 140 self.check_parallel_module_init() 141 self.assertGreater(finder.numcalls, 0) 142 self.assertEqual(finder.x, finder.numcalls) 143 finally: 144 sys.meta_path.remove(finder) 145 146 def test_parallel_path_hooks(self): 147 # Here the Finder instance is only used to check concurrent calls 148 # to path_hook(). 149 finder = Finder() 150 # In order for our path hook to be called at each import, we need 151 # to flush the path_importer_cache, which we do by registering a 152 # dedicated meta_path entry. 153 flushing_finder = FlushingFinder() 154 def path_hook(path): 155 finder.find_spec('') 156 raise ImportError 157 sys.path_hooks.insert(0, path_hook) 158 sys.meta_path.append(flushing_finder) 159 try: 160 # Flush the cache a first time 161 flushing_finder.find_spec('') 162 numtests = self.check_parallel_module_init() 163 self.assertGreater(finder.numcalls, 0) 164 self.assertEqual(finder.x, finder.numcalls) 165 finally: 166 sys.meta_path.remove(flushing_finder) 167 sys.path_hooks.remove(path_hook) 168 169 def test_import_hangers(self): 170 # In case this test is run again, make sure the helper module 171 # gets loaded from scratch again. 172 try: 173 del sys.modules['test.threaded_import_hangers'] 174 except KeyError: 175 pass 176 import test.threaded_import_hangers 177 self.assertFalse(test.threaded_import_hangers.errors) 178 179 def test_circular_imports(self): 180 # The goal of this test is to exercise implementations of the import 181 # lock which use a per-module lock, rather than a global lock. 182 # In these implementations, there is a possible deadlock with 183 # circular imports, for example: 184 # - thread 1 imports A (grabbing the lock for A) which imports B 185 # - thread 2 imports B (grabbing the lock for B) which imports A 186 # Such implementations should be able to detect such situations and 187 # resolve them one way or the other, without freezing. 188 # NOTE: our test constructs a slightly less trivial import cycle, 189 # in order to better stress the deadlock avoidance mechanism. 190 delay = 0.5 191 os.mkdir(TESTFN) 192 self.addCleanup(shutil.rmtree, TESTFN) 193 sys.path.insert(0, TESTFN) 194 self.addCleanup(sys.path.remove, TESTFN) 195 for name, contents in circular_imports_modules.items(): 196 contents = contents % {'delay': delay} 197 with open(os.path.join(TESTFN, name + ".py"), "wb") as f: 198 f.write(contents.encode('utf-8')) 199 self.addCleanup(forget, name) 200 201 importlib.invalidate_caches() 202 results = [] 203 def import_ab(): 204 import A 205 results.append(getattr(A, 'x', None)) 206 def import_ba(): 207 import B 208 results.append(getattr(B, 'x', None)) 209 t1 = threading.Thread(target=import_ab) 210 t2 = threading.Thread(target=import_ba) 211 t1.start() 212 t2.start() 213 t1.join() 214 t2.join() 215 self.assertEqual(set(results), {'a', 'b'}) 216 217 def test_side_effect_import(self): 218 code = """if 1: 219 import threading 220 def target(): 221 import random 222 t = threading.Thread(target=target) 223 t.start() 224 t.join()""" 225 sys.path.insert(0, os.curdir) 226 self.addCleanup(sys.path.remove, os.curdir) 227 filename = TESTFN + ".py" 228 with open(filename, "wb") as f: 229 f.write(code.encode('utf-8')) 230 self.addCleanup(unlink, filename) 231 self.addCleanup(forget, TESTFN) 232 self.addCleanup(rmtree, '__pycache__') 233 importlib.invalidate_caches() 234 __import__(TESTFN) 235 236 237 @reap_threads 238 def test_main(): 239 old_switchinterval = None 240 try: 241 old_switchinterval = sys.getswitchinterval() 242 sys.setswitchinterval(1e-5) 243 except AttributeError: 244 pass 245 try: 246 run_unittest(ThreadedImportTests) 247 finally: 248 if old_switchinterval is not None: 249 sys.setswitchinterval(old_switchinterval) 250 251 if __name__ == "__main__": 252 test_main() 253
