Workaround issue with greenthreads and lockfiles

* Adds a GreenLockFile that always works in greenthreads
 * Adds test to verify that regular Lockfile is broken
 * Adds test to verify that GreenLockfile works
 * Adds note about limitation of external locks
 * Adds test showing limitation of nested locks
 * Fixes bug 956313

Change-Id: I11cd1206611aa4862dadd2fcc077c4c2e0f798f6

nova/tests/test_misc.py

@@ -22,6 +22,7 @@ import select
 
 from eventlet import greenpool
 from eventlet import greenthread
+import lockfile
 
 from nova import exception
 from nova import test
@@ -134,6 +135,21 @@ class LockTestCase(test.TestCase):
         self.assertEqual(saved_sem_num, len(utils._semaphores),
                          "Semaphore leak detected")
 
+    def test_nested_external_fails(self):
+        """We can not nest external syncs"""
+
+        @utils.synchronized('testlock1', external=True)
+        def outer_lock():
+
+            @utils.synchronized('testlock2', external=True)
+            def inner_lock():
+                pass
+            inner_lock()
+        try:
+            self.assertRaises(lockfile.NotMyLock, outer_lock)
+        finally:
+            utils.cleanup_file_locks()
+
     def test_synchronized_externally(self):
         """We can lock across multiple processes"""
         rpipe1, wpipe1 = os.pipe()