Add `FrozenEvent.get_state_key` and use it in a couple of places (#11793)

This is more efficient, since we only have to look up `state_key` in the event
dict once, rather than three (!) times.

changelog.d/11793.misc

@@ -0,0 +1 @@
+Add `FrozenEvent.get_state_key` and use it in a couple of places.

synapse/events/__init__.py

@@ -315,10 +315,11 @@ class EventBase(metaclass=abc.ABCMeta):
     redacts: DefaultDictProperty[Optional[str]] = DefaultDictProperty("redacts", None)
     room_id: DictProperty[str] = DictProperty("room_id")
     sender: DictProperty[str] = DictProperty("sender")
-    # TODO state_key should be Optional[str], this is generally asserted in Synapse
+    # TODO state_key should be Optional[str]. This is generally asserted in Synapse
-    # by calling is_state() first (which ensures this), but it is hard (not possible?)
+    # by calling is_state() first (which ensures it is not None), but it is hard (not possible?)
     # to properly annotate that calling is_state() asserts that state_key exists
-    # and is non-None.
+    # and is non-None. It would be better to replace such direct references with
+    # get_state_key() (and a check for None).
     state_key: DictProperty[str] = DictProperty("state_key")
     type: DictProperty[str] = DictProperty("type")
     user_id: DictProperty[str] = DictProperty("sender")
@@ -332,7 +333,11 @@ class EventBase(metaclass=abc.ABCMeta):
         return self.content["membership"]
 
     def is_state(self) -> bool:
-        return hasattr(self, "state_key") and self.state_key is not None
+        return self.get_state_key() is not None
+
+    def get_state_key(self) -> Optional[str]:
+        """Get the state key of this event, or None if it's not a state event"""
+        return self._dict.get("state_key")
 
     def get_dict(self) -> JsonDict:
         d = dict(self._dict)

synapse/events/snapshot.py

@@ -163,7 +163,7 @@ class EventContext:
         return {
             "prev_state_id": prev_state_id,
             "event_type": event.type,
-            "event_state_key": event.state_key if event.is_state() else None,
+            "event_state_key": event.get_state_key(),
             "state_group": self._state_group,
             "state_group_before_event": self.state_group_before_event,
             "rejected": self.rejected,