Raise exception on failed lock's CM acquire fail

In lock's context manager we are not checking that acquire returns true
before entering managed code.

Even though we don't except acquire to return False, we should still
confirm that the lock has been really acquired and we can proceed, and
raise an exception if it failed.

This patch modified metaclass Lock to raise new exception
LockAcquireFailed if it didn't acquire the lock.

Change-Id: I6d6f14ffd9ea10dd1ca203a92c987334e8a00416

tooz/coordination.py

@@ -436,6 +436,10 @@ class OperationTimedOut(ToozError):
     """Exception raised when an operation times out."""
 
 
+class LockAcquireFailed(ToozError):
+    """Exception raised when a lock acquire fails in a context manager."""
+
+
 class GroupNotCreated(ToozError):
     """Exception raised when the caller request a group which does
     not exist.

tooz/locking.py

@@ -19,6 +19,8 @@ import six
 import threading
 import weakref
 
+from tooz import coordination
+
 
 @six.add_metaclass(abc.ABCMeta)
 class Lock(object):
@@ -32,7 +34,12 @@ class Lock(object):
         return self._name
 
     def __enter__(self):
-        self.acquire()
+        acquired = self.acquire()
+        if not acquired:
+            msg = u'Acquiring lock %s failed' % self.name
+            raise coordination.LockAcquireFailed(msg)
+
+        return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):
         self.release()

tooz/tests/test_coordination.py

@@ -21,6 +21,7 @@ import uuid
 
 from concurrent import futures
 import fixtures
+import mock
 import testscenarios
 from testtools import matchers
 from testtools import testcase
@@ -682,6 +683,22 @@ class TestAPI(testscenarios.TestWithScenarios,
         with lock1:
             self.assertFalse(lock2.acquire(blocking=False))
 
+    def test_get_lock_context_fails(self):
+        name = self._get_random_uuid()
+        lock1 = self._coord.get_lock(name)
+        lock2 = self._coord.get_lock(name)
+        with mock.patch.object(lock2, 'acquire', return_value=False):
+            with lock1:
+                self.assertRaises(
+                    tooz.coordination.LockAcquireFailed,
+                    lock2.__enter__)
+
+    def test_get_lock_context_check_value(self):
+        name = self._get_random_uuid()
+        lock = self._coord.get_lock(name)
+        with lock as returned_lock:
+            self.assertEqual(lock, returned_lock)
+
     def test_get_lock_locked_twice(self):
         name = self._get_random_uuid()
         lock = self._coord.get_lock(name)