From 78ed83da46b4f87e6e04a15f4d330ccb28274d03 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Charles-Fran=C3=A7ois=20Natali?= <neologix@free.fr>
Date: Mon, 19 Dec 2011 12:18:55 +0100
Subject: Issue #11867: Make test_mailbox.test_lock_conflict deterministic (and
 fix a race condition).

---
 Lib/test/test_mailbox.py | 30 +++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/Lib/test/test_mailbox.py b/Lib/test/test_mailbox.py
index 007c419..720eb6f 100644
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -17,6 +17,10 @@ try:
     import fcntl
 except ImportError:
     pass
+try:
+    import multiprocessing
+except ImportError:
+    multiprocessing = None
 
 
 class TestBase(unittest.TestCase):
@@ -994,28 +998,36 @@ class _TestMboxMMDF(TestMailbox):
             self.assertEqual(contents, f.read())
         self._box = self._factory(self._path)
 
+    @unittest.skipUnless(hasattr(os, 'fork'), "Test needs fork().")
+    @unittest.skipUnless(multiprocessing, "Test needs multiprocessing.")
     def test_lock_conflict(self):
-        # Fork off a subprocess that will lock the file for 2 seconds,
-        # unlock it, and then exit.
-        if not hasattr(os, 'fork'):
-            return
+        # Fork off a child process that will lock the mailbox temporarily,
+        # unlock it and exit.
+        ready = multiprocessing.Event()
+        done = multiprocessing.Event()
+
         pid = os.fork()
         if pid == 0:
-            # In the child, lock the mailbox.
+            # child
             try:
+                # lock the mailbox, and signal the parent it can proceed
                 self._box.lock()
-                time.sleep(2)
+                ready.set()
+
+                # wait until the parent is done, and unlock the mailbox
+                done.wait(5)
                 self._box.unlock()
             finally:
                 os._exit(0)
 
-        # In the parent, sleep a bit to give the child time to acquire
-        # the lock.
-        time.sleep(0.5)
+        # In the parent, wait until the child signals it locked the mailbox.
+        ready.wait(5)
         try:
             self.assertRaises(mailbox.ExternalClashError,
                               self._box.lock)
         finally:
+            # Signal the child it can now release the lock and exit.
+            done.set()
             # Wait for child to exit.  Locking should now succeed.
             exited_pid, status = os.waitpid(pid, 0)
 
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