bugfix: model: Avoid creating empty messages from reaction events.

This updates an approach taken in the original reaction implementation.

Since locally cached messages are currently stored in a `defaultdict`,
direct indexing with a non-present message-id creates an empty message.
Such messages then generate `KeyError`s when flags are updated later
(eg. marking read, toggling star status), if the message has not been
fetched.

Tests updated to explicitly exclude generation of empty index for
reaction events.

Fixes #920.

Author: neiljp

tests/model/test_model.py

@@ -2041,9 +2041,7 @@ def reaction_event_index_factory(self):
         def _factory(msgs: MsgsType):
             return {
                 "messages": {
-                    message_id: {}
-                    if reactions is None
-                    else {
+                    message_id: {
                         "id": message_id,
                         "content": f"message content {message_id}",
                         "reactions": [
@@ -2061,6 +2059,7 @@ def _factory(msgs: MsgsType):
                         ],
                     }
                     for message_id, reactions in msgs
+                    if reactions is not None
                 }
             }
 
@@ -2082,7 +2081,7 @@ def test__handle_reaction_event_not_in_index(
         )
         model.index = reaction_event_index_factory(
             [
-                (unindexed_message_id, None),
+                (unindexed_message_id, None),  # explicitly exclude
                 (2, [(1, "unicode_emoji", "1232", "thumbs_up")]),
                 (3, []),
             ]

zulipterminal/model.py

@@ -1427,7 +1427,7 @@ def _handle_reaction_event(self, event: Event) -> None:
         assert event["type"] == "reaction"
         message_id = event["message_id"]
         # If the message is indexed
-        if self.index["messages"][message_id] != {}:
+        if message_id in self.index["messages"]:
 
             message = self.index["messages"][message_id]
             if event["op"] == "add":