Factor out an AsyncEventContext

The intention here is to make it clearer which fields we can expect to be
populated when: notably, that the _event_type etc aren't used for the
synchronous impl of EventContext.

synapse/events/snapshot.py

@@ -48,20 +48,6 @@ class EventContext:
             The current state map excluding the current event. None if outlier
             or we haven't fetched the state from DB yet.
             (type, state_key) -> event_id
-
-        _fetching_state_deferred (Deferred|None): Resolves when *_state_ids have
-            been calculated. None if we haven't started calculating yet
-
-        _event_type (str): The type of the event the context is associated with.
-            Only set when state has not been fetched yet.
-
-        _event_state_key (str|None): The state_key of the event the context is
-            associated with. Only set when state has not been fetched yet.
-
-        _prev_state_id (str|None): If the event associated with the context is
-            a state event, then `_prev_state_id` is the event_id of the state
-            that was replaced.
-            Only set when state has not been fetched yet.
     """
 
     state_group = attr.ib(default=None)
@@ -70,13 +56,8 @@ class EventContext:
     delta_ids = attr.ib(default=None)
     app_service = attr.ib(default=None)
 
-    _current_state_ids = attr.ib(default=None)
     _prev_state_ids = attr.ib(default=None)
-    _prev_state_id = attr.ib(default=None)
-
-    _event_type = attr.ib(default=None)
-    _event_state_key = attr.ib(default=None)
-    _fetching_state_deferred = attr.ib(default=None)
+    _current_state_ids = attr.ib(default=None)
 
     @staticmethod
     def with_state(
@@ -86,7 +67,6 @@ def with_state(
             current_state_ids=current_state_ids,
             prev_state_ids=prev_state_ids,
             state_group=state_group,
-            fetching_state_deferred=defer.succeed(None),
             prev_group=prev_group,
             delta_ids=delta_ids,
         )
@@ -136,7 +116,7 @@ def deserialize(store, input):
         Returns:
             EventContext
         """
-        context = EventContext(
+        context = AsyncEventContext(
             # We use the state_group and prev_state_id stuff to pull the
             # current_state_ids out of the DB and construct prev_state_ids.
             prev_state_id=input["prev_state_id"],
@@ -164,14 +144,7 @@ def get_current_state_ids(self, store):
                 Maps a (type, state_key) to the event ID of the state event matching
                 this tuple.
         """
-
-        if not self._fetching_state_deferred:
-            self._fetching_state_deferred = run_in_background(
-                self._fill_out_state, store
-            )
-
-        yield make_deferred_yieldable(self._fetching_state_deferred)
-
+        yield self._ensure_fetched(store)
         return self._current_state_ids
 
     @defer.inlineCallbacks
@@ -184,14 +157,7 @@ def get_prev_state_ids(self, store):
                 Maps a (type, state_key) to the event ID of the state event matching
                 this tuple.
         """
-
-        if not self._fetching_state_deferred:
-            self._fetching_state_deferred = run_in_background(
-                self._fill_out_state, store
-            )
-
-        yield make_deferred_yieldable(self._fetching_state_deferred)
-
+        yield self._ensure_fetched(store)
         return self._prev_state_ids
 
     def get_cached_current_state_ids(self):
@@ -205,6 +171,44 @@ def get_cached_current_state_ids(self):
 
         return self._current_state_ids
 
+    def _ensure_fetched(self, store):
+        return defer.succeed(None)
+
+
+@attr.s(slots=True)
+class AsyncEventContext(EventContext):
+    """
+    A version of EventContext which fetches _current_state_ids and _prev_state_ids
+    from the database on demand.
+
+    Attributes:
+
+        _fetching_state_deferred (Deferred|None): Resolves when *_state_ids have
+            been calculated. None if we haven't started calculating yet
+
+        _event_type (str): The type of the event the context is associated with.
+
+        _event_state_key (str): The state_key of the event the context is
+            associated with.
+
+        _prev_state_id (str|None): If the event associated with the context is
+            a state event, then `_prev_state_id` is the event_id of the state
+            that was replaced.
+    """
+
+    _prev_state_id = attr.ib(default=None)
+    _event_type = attr.ib(default=None)
+    _event_state_key = attr.ib(default=None)
+    _fetching_state_deferred = attr.ib(default=None)
+
+    def _ensure_fetched(self, store):
+        if not self._fetching_state_deferred:
+            self._fetching_state_deferred = run_in_background(
+                self._fill_out_state, store
+            )
+
+        return make_deferred_yieldable(self._fetching_state_deferred)
+
     @defer.inlineCallbacks
     def _fill_out_state(self, store):
         """Called to populate the _current_state_ids and _prev_state_ids