Merged revisions 84258,84268 via svnmerge from

svn+ssh://pythondev@svn.python.org/python/branches/py3k

........
  r84258 | antoine.pitrou | 2010-08-22 12:18:36 +0200 (dim., 22 août 2010) | 4 lines

  Test that calls to path hooks and meta_path entries are serialized by the import lock.
  (part of issue #9251)
........
  r84268 | antoine.pitrou | 2010-08-22 22:43:26 +0200 (dim., 22 août 2010) | 4 lines

  Add an import lock test for multithreaded circular imports.
  (part of #9657)
........

Lib/test/test_threaded_import.py

@@ -5,15 +5,26 @@
 # complains several times about module random having no attribute
 # randrange, and then Python hangs.
 
+import os
 import imp
 import sys
+import time
+import shutil
 import unittest
-from test.support import verbose, TestFailed, import_module, run_unittest
+from test.support import verbose, import_module, run_unittest, TESTFN
 thread = import_module('_thread')
+threading = import_module('threading')
 
 def task(N, done, done_tasks, errors):
     try:
-        import random
+        # 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:
@@ -24,6 +35,52 @@ def task(N, done, done_tasks, errors):
         if finished:
             done.release()
 
+# 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_module()
+    method."""
+
+    def __init__(self):
+        self.numcalls = 0
+        self.x = 0
+        self.lock = thread.allocate_lock()
+
+    def find_module(self, name, path=None):
+        # Simulate some thread-unsafe behaviour. If calls to find_module()
+        # are properly serialized, `x` will end up the same as `numcalls`.
+        # Otherwise not.
+        with self.lock:
+            self.numcalls += 1
+        x = self.x
+        time.sleep(0.1)
+        self.x = x + 1
+
+class FlushingFinder:
+    """A dummy finder which flushes sys.path_importer_cache when it gets
+    called."""
+
+    def find_module(self, name, path=None):
+        sys.path_importer_cache.clear()
+
 
 class ThreadedImportTests(unittest.TestCase):
 
@@ -37,7 +94,7 @@ def tearDown(self):
         if self.old_random is not None:
             sys.modules['random'] = self.old_random
 
-    def test_parallel_module_init(self):
+    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")
@@ -47,11 +104,12 @@ def test_parallel_module_init(self):
         for N in (20, 50) * 3:
             if verbose:
                 print("Trying", N, "threads ...", end=' ')
-            # Make sure that random gets reimported freshly
-            try:
-                del sys.modules['random']
-            except KeyError:
-                pass
+            # 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 = []
             for i in range(N):
@@ -62,6 +120,42 @@ def test_parallel_module_init(self):
                 print("OK.")
         done.release()
 
+    def test_parallel_module_init(self):
+        self.check_parallel_module_init()
+
+    def test_parallel_meta_path(self):
+        finder = Finder()
+        sys.meta_path.append(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_module('')
+            raise ImportError
+        sys.path_hooks.append(path_hook)
+        sys.meta_path.append(flushing_finder)
+        try:
+            # Flush the cache a first time
+            flushing_finder.find_module('')
+            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.
@@ -72,10 +166,46 @@ def test_import_hangers(self):
         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(sys.modules.pop, name, None)
+
+        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_main():
     run_unittest(ThreadedImportTests)
 
-
 if __name__ == "__main__":
     test_main()