Issue #9260: A finer-grained import lock.

Most of the import sequence now uses per-module locks rather than the
global import lock, eliminating well-known issues with threads and imports.

Lib/importlib/test/test_locks.py

@@ -0,0 +1,115 @@
+from importlib import _bootstrap
+import time
+import unittest
+import weakref
+
+from test import support
+
+try:
+    import threading
+except ImportError:
+    threading = None
+else:
+    from test import lock_tests
+
+
+LockType = _bootstrap._ModuleLock
+DeadlockError = _bootstrap._DeadlockError
+
+
+if threading is not None:
+    class ModuleLockAsRLockTests(lock_tests.RLockTests):
+        locktype = staticmethod(lambda: LockType("some_lock"))
+
+        # _is_owned() unsupported
+        test__is_owned = None
+        # acquire(blocking=False) unsupported
+        test_try_acquire = None
+        test_try_acquire_contended = None
+        # `with` unsupported
+        test_with = None
+        # acquire(timeout=...) unsupported
+        test_timeout = None
+        # _release_save() unsupported
+        test_release_save_unacquired = None
+
+else:
+    class ModuleLockAsRLockTests(unittest.TestCase):
+        pass
+
+
+@unittest.skipUnless(threading, "threads needed for this test")
+class DeadlockAvoidanceTests(unittest.TestCase):
+
+    def run_deadlock_avoidance_test(self, create_deadlock):
+        NLOCKS = 10
+        locks = [LockType(str(i)) for i in range(NLOCKS)]
+        pairs = [(locks[i], locks[(i+1)%NLOCKS]) for i in range(NLOCKS)]
+        if create_deadlock:
+            NTHREADS = NLOCKS
+        else:
+            NTHREADS = NLOCKS - 1
+        barrier = threading.Barrier(NTHREADS)
+        results = []
+        def _acquire(lock):
+            """Try to acquire the lock. Return True on success, False on deadlock."""
+            try:
+                lock.acquire()
+            except DeadlockError:
+                return False
+            else:
+                return True
+        def f():
+            a, b = pairs.pop()
+            ra = _acquire(a)
+            barrier.wait()
+            rb = _acquire(b)
+            results.append((ra, rb))
+            if rb:
+                b.release()
+            if ra:
+                a.release()
+        lock_tests.Bunch(f, NTHREADS).wait_for_finished()
+        self.assertEqual(len(results), NTHREADS)
+        return results
+
+    def test_deadlock(self):
+        results = self.run_deadlock_avoidance_test(True)
+        # One of the threads detected a potential deadlock on its second
+        # acquire() call.
+        self.assertEqual(results.count((True, False)), 1)
+        self.assertEqual(results.count((True, True)), len(results) - 1)
+
+    def test_no_deadlock(self):
+        results = self.run_deadlock_avoidance_test(False)
+        self.assertEqual(results.count((True, False)), 0)
+        self.assertEqual(results.count((True, True)), len(results))
+
+
+class LifetimeTests(unittest.TestCase):
+
+    def test_lock_lifetime(self):
+        name = "xyzzy"
+        self.assertNotIn(name, _bootstrap._module_locks)
+        lock = _bootstrap._get_module_lock(name)
+        self.assertIn(name, _bootstrap._module_locks)
+        wr = weakref.ref(lock)
+        del lock
+        support.gc_collect()
+        self.assertNotIn(name, _bootstrap._module_locks)
+        self.assertIs(wr(), None)
+
+    def test_all_locks(self):
+        support.gc_collect()
+        self.assertEqual(0, len(_bootstrap._module_locks))
+
+
+@support.reap_threads
+def test_main():
+    support.run_unittest(ModuleLockAsRLockTests,
+                         DeadlockAvoidanceTests,
+                         LifetimeTests)
+
+
+if __name__ == '__main__':
+    test_main()