Raises proper error when unwatching a group

This patch makes the coordinator code to raises a WatchCallbackNotFound
error when one try to unwatch a group with a callback that does not exist.
Prior to this patch ValueError was being returned with an unhelpful
message.

Closes-Bug: #1544029
Change-Id: Ibfc862fb7fe80d0fab5bd66939e6336a62b11b1d

tooz/coordination.py

@@ -194,7 +194,11 @@ class CoordinationDriver(object):
         :param callback: The function that was executed when a member joined
                          this group
         """
-        self._hooks_join_group[group_id].remove(callback)
+        try:
+            self._hooks_join_group[group_id].remove(callback)
+        except ValueError:
+            raise WatchCallbackNotFound(group_id, callback)
+
         if (not self._has_hooks_for_group(group_id) and
            group_id in self._group_members):
             del self._group_members[group_id]
@@ -221,7 +225,11 @@ class CoordinationDriver(object):
         :param callback: The function that was executed when a member left
                          this group
         """
-        self._hooks_leave_group[group_id].remove(callback)
+        try:
+            self._hooks_leave_group[group_id].remove(callback)
+        except ValueError:
+            raise WatchCallbackNotFound(group_id, callback)
+
         if (not self._has_hooks_for_group(group_id) and
            group_id in self._group_members):
             del self._group_members[group_id]
@@ -252,7 +260,11 @@ class CoordinationDriver(object):
                          group
 
         """
-        self._hooks_elected_leader[group_id].remove(callback)
+        try:
+            self._hooks_elected_leader[group_id].remove(callback)
+        except ValueError:
+            raise WatchCallbackNotFound(group_id, callback)
+
         if not self._hooks_elected_leader[group_id]:
             del self._hooks_elected_leader[group_id]
 
@@ -611,6 +623,21 @@ class GroupNotEmpty(ToozError):
         super(GroupNotEmpty, self).__init__("Group %s is not empty" % group_id)
 
 
+class WatchCallbackNotFound(ToozError):
+    """Exception raised when unwatching a group.
+
+    Raised when the caller tries to unwatch a group with a callback that
+    does not exist.
+
+    """
+    def __init__(self, group_id, callback):
+        self.group_id = group_id
+        self.callback = callback
+        super(WatchCallbackNotFound, self).__init__(
+            'Callback %s is not registered on group %s' %
+            (callback.__name__, group_id))
+
+
 class SerializationError(ToozError):
     "Exception raised when serialization or deserialization breaks."
 

tooz/tests/test_coordination.py

@@ -671,6 +671,24 @@ class TestAPI(testscenarios.TestWithScenarios,
         self._coord.unwatch_elected_as_leader(self.group_id, self._set_event)
         self.assertEqual(0, len(self._coord._hooks_elected_leader))
 
+    def test_unwatch_elected_as_leader_callback_not_found(self):
+        self._coord.create_group(self.group_id).get()
+        self.assertRaises(tooz.coordination.WatchCallbackNotFound,
+                          self._coord.unwatch_elected_as_leader,
+                          self.group_id, lambda x: None)
+
+    def test_unwatch_join_group_callback_not_found(self):
+        self._coord.create_group(self.group_id).get()
+        self.assertRaises(tooz.coordination.WatchCallbackNotFound,
+                          self._coord.unwatch_join_group,
+                          self.group_id, lambda x: None)
+
+    def test_unwatch_leave_group_callback_not_found(self):
+        self._coord.create_group(self.group_id).get()
+        self.assertRaises(tooz.coordination.WatchCallbackNotFound,
+                          self._coord.unwatch_leave_group,
+                          self.group_id, lambda x: None)
+
     def test_get_lock(self):
         lock = self._coord.get_lock(self._get_random_uuid())
         self.assertTrue(lock.acquire())