Allow StackLock to be used as a context manager

We initially didn't implement this class as a context manager because it
was being used exclusively to lock operations that were happening in an
asycnronous thread (hence the thread_lock() context manager). However, we
now have some uses for it in the same thread (see
Ifa48b179723a2100fff548467db9e162bc669d13), so it makes sense to be able to
use the lock object itself as a context manager.

Change-Id: Ib90df8f8a149b57b0588d4fc9951f7f035ad7567

heat/engine/stack_lock.py

@@ -124,6 +124,14 @@ class StackLock(object):
                       "%(stack)s" % {'engine': self.engine_id,
                                      'stack': self.stack_id})
 
+    def __enter__(self):
+        self.acquire()
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self.release()
+        return False
+
     @contextlib.contextmanager
     def thread_lock(self, retry=True):
         """Acquire a lock and release it only if there is an exception.

heat/tests/test_stack_lock.py

@@ -161,6 +161,32 @@ class StackLockTest(common.HeatTestCase):
         mock_steal.assert_has_calls(
             [mock.call(self.stack_id, 'fake-engine-id', self.engine_id)] * 2)
 
+    def test_context_mgr_exception(self):
+        stack_lock_object.StackLock.create = mock.Mock(return_value=None)
+        stack_lock_object.StackLock.release = mock.Mock(return_value=None)
+        slock = stack_lock.StackLock(self.context, self.stack_id,
+                                     self.engine_id)
+
+        def check_lock():
+            with slock:
+                self.assertEqual(1,
+                                 stack_lock_object.StackLock.create.call_count)
+                raise self.TestThreadLockException
+        self.assertRaises(self.TestThreadLockException, check_lock)
+        self.assertEqual(1, stack_lock_object.StackLock.release.call_count)
+
+    def test_context_mgr_noexception(self):
+        stack_lock_object.StackLock.create = mock.Mock(return_value=None)
+        stack_lock_object.StackLock.release = mock.Mock(return_value=None)
+        slock = stack_lock.StackLock(self.context, self.stack_id,
+                                     self.engine_id)
+
+        with slock:
+            self.assertEqual(1,
+                             stack_lock_object.StackLock.create.call_count)
+
+        self.assertEqual(1, stack_lock_object.StackLock.release.call_count)
+
     def test_thread_lock_context_mgr_exception_acquire_success(self):
         stack_lock_object.StackLock.create = mock.Mock(return_value=None)
         stack_lock_object.StackLock.release = mock.Mock(return_value=None)