Refactor `EventContext` (#12689)

Refactor how the `EventContext` class works, with the intention of reducing the amount of state we fetch from the DB during event processing.

The idea here is to get rid of the cached `current_state_ids` and `prev_state_ids` that live in the `EventContext`, and instead defer straight to the database (and its caching). 

One change that may have a noticeable effect is that we now no longer prefill the `get_current_state_ids` cache on a state change. However, that query is relatively light, since its just a case of reading a table from the DB (unlike fetching state at an event which is more heavyweight). For deployments with workers this cache isn't even used.


Part of #12684

3 files changed, 11 insertions, 7 deletions

diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 38dc5b1f6e..be5099b507 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -659,7 +659,7 @@ class FederationHandler:
         # in the invitee's sync stream. It is stripped out for all other local users.
         event.unsigned["knock_room_state"] = stripped_room_state["knock_state_events"]
 
-        context = EventContext.for_outlier()
+        context = EventContext.for_outlier(self.storage)
         stream_id = await self._federation_event_handler.persist_events_and_notify(
             event.room_id, [(event, context)]
         )
@@ -848,7 +848,7 @@ class FederationHandler:
             )
         )
 
-        context = EventContext.for_outlier()
+        context = EventContext.for_outlier(self.storage)
         await self._federation_event_handler.persist_events_and_notify(
             event.room_id, [(event, context)]
         )
@@ -877,7 +877,7 @@ class FederationHandler:
 
         await self.federation_client.send_leave(host_list, event)
 
-        context = EventContext.for_outlier()
+        context = EventContext.for_outlier(self.storage)
         stream_id = await self._federation_event_handler.persist_events_and_notify(
             event.room_id, [(event, context)]
         )
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index 6cf927e4ff..6d11b32b61 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -1423,7 +1423,7 @@ class FederationEventHandler:
                 # we're not bothering about room state, so flag the event as an outlier.
                 event.internal_metadata.outlier = True
 
-                context = EventContext.for_outlier()
+                context = EventContext.for_outlier(self._storage)
                 try:
                     validate_event_for_room_version(room_version_obj, event)
                     check_auth_rules_for_event(room_version_obj, event, auth)
@@ -1874,10 +1874,10 @@ class FederationEventHandler:
         )
 
         return EventContext.with_state(
+            storage=self._storage,
             state_group=state_group,
             state_group_before_event=context.state_group_before_event,
-            current_state_ids=current_state_ids,
-            prev_state_ids=prev_state_ids,
+            state_delta_due_to_event=state_updates,
             prev_group=prev_group,
             delta_ids=state_updates,
             partial_state=context.partial_state,
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index c28b792e6f..e47799e7f9 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -757,6 +757,10 @@ class EventCreationHandler:
             The previous version of the event is returned, if it is found in the
             event context. Otherwise, None is returned.
         """
+        if event.internal_metadata.is_outlier():
+            # This can happen due to out of band memberships
+            return None
+
         prev_state_ids = await context.get_prev_state_ids()
         prev_event_id = prev_state_ids.get((event.type, event.state_key))
         if not prev_event_id:
@@ -1001,7 +1005,7 @@ class EventCreationHandler:
         # after it is created
         if builder.internal_metadata.outlier:
             event.internal_metadata.outlier = True
-            context = EventContext.for_outlier()
+            context = EventContext.for_outlier(self.storage)
         elif (
             event.type == EventTypes.MSC2716_INSERTION
             and state_event_ids