Fix locking via RedLock

Fixes #1777

util/locking.py

@@ -7,6 +7,11 @@ from app import app
 
 logger = logging.getLogger(__name__)
 
+
+class LockNotAcquiredException(Exception):
+  """ Exception raised if a GlobalLock could not be acquired. """
+
+
 class GlobalLock(object):
   """ A lock object that blocks globally via Redis. Note that Redis is not considered a tier-1
       service, so this lock should not be used for any critical code paths.
@@ -20,17 +25,22 @@ class GlobalLock(object):
     self._redlock = None
 
   def __enter__(self):
-    return self.lock()
+    if not self.acquire():
+      raise LockNotAcquiredException()
 
-  def __exit__(self ,type, value, traceback):
-    self.unlock()
+  def __exit__(self, type, value, traceback):
+    self.release()
 
-  def lock(self):
+  def acquire(self):
     logger.debug('Acquiring global lock %s', self._lock_name)
     try:
       redlock = RedLock(self._lock_name, connection_details=[self._redis_info],
-                              ttl=self._lock_ttl)
-      redlock.acquire()
+                        ttl=self._lock_ttl)
+      acquired = redlock.acquire()
+      if not acquired:
+        logger.debug('Was unable to not acquire lock %s', self._lock_name)
+        return False
+
       self._redlock = redlock
       logger.debug('Acquired lock %s', self._lock_name)
       return True
@@ -41,10 +51,15 @@ class GlobalLock(object):
       logger.debug('Could not connect to Redis for lock %s: %s', self._lock_name, re)
       return False
 
-
-  def unlock(self):
+  def release(self):
     if self._redlock is not None:
       logger.debug('Releasing lock %s', self._lock_name)
-      self._redlock.release()
+      try:
+        self._redlock.release()
+      except RedLockError:
+        logger.debug('Could not release lock %s', self._lock_name)
+      except RedisError as re:
+        logger.debug('Could not connect to Redis for releasing lock %s: %s', self._lock_name, re)
+
       logger.debug('Released lock %s', self._lock_name)
       self._redlock = None