Merged revisions 74841 via svnmerge from

svn+ssh://pythondev@svn.python.org/python/trunk

........
  r74841 | thomas.wouters | 2009-09-16 14:55:54 -0500 (Wed, 16 Sep 2009) | 23 lines


  Fix issue #1590864, multiple threads and fork() can cause deadlocks, by
  acquiring the import lock around fork() calls. This prevents other threads
  from having that lock while the fork happens, and is the recommended way of
  dealing with such issues. There are two other locks we care about, the GIL
  and the Thread Local Storage lock. The GIL is obviously held when calling
  Python functions like os.fork(), and the TLS lock is explicitly reallocated
  instead, while also deleting now-orphaned TLS data.

  This only fixes calls to os.fork(), not extension modules or embedding
  programs calling C's fork() directly. Solving that requires a new set of API
  functions, and possibly a rewrite of the Python/thread_*.c mess. Add a
  warning explaining the problem to the documentation in the mean time.

  This also changes behaviour a little on AIX. Before, AIX (but only AIX) was
  getting the import lock reallocated, seemingly to avoid this very same
  problem. This is not the right approach, because the import lock is a
  re-entrant one, and reallocating would do the wrong thing when forking while
  holding the import lock.

  Will backport to 2.6, minus the tiny AIX behaviour change.
........

1 files changed, 41 insertions, 0 deletions

diff --git a/Lib/test/test_fork1.py b/Lib/test/test_fork1.py
index e036661..88f9fe9 100644
--- a/Lib/test/test_fork1.py
+++ b/Lib/test/test_fork1.py
@@ -1,8 +1,14 @@
 """This test checks for correct fork() behavior.
 """
 
+import errno
+import imp
 import os
+import signal
+import sys
 import time
+import threading
+
 from test.fork_wait import ForkWait
 from test.support import run_unittest, reap_children, get_attribute
 
@@ -23,6 +29,41 @@ class ForkTest(ForkWait):
         self.assertEqual(spid, cpid)
         self.assertEqual(status, 0, "cause = %d, exit = %d" % (status&0xff, status>>8))
 
+    def test_import_lock_fork(self):
+        import_started = threading.Event()
+        fake_module_name = "fake test module"
+        partial_module = "partial"
+        complete_module = "complete"
+        def importer():
+            imp.acquire_lock()
+            sys.modules[fake_module_name] = partial_module
+            import_started.set()
+            time.sleep(0.01) # Give the other thread time to try and acquire.
+            sys.modules[fake_module_name] = complete_module
+            imp.release_lock()
+        t = threading.Thread(target=importer)
+        t.start()
+        import_started.wait()
+        pid = os.fork()
+        try:
+            if not pid:
+                m = __import__(fake_module_name)
+                if m == complete_module:
+                    os._exit(0)
+                else:
+                    os._exit(1)
+            else:
+                t.join()
+                # Exitcode 1 means the child got a partial module (bad.) No
+                # exitcode (but a hang, which manifests as 'got pid 0')
+                # means the child deadlocked (also bad.)
+                self.wait_impl(pid)
+        finally:
+            try:
+                os.kill(pid, signal.SIGKILL)
+            except OSError:
+                pass
+
 def test_main():
     run_unittest(ForkTest)
     reap_children()