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

1 files changed, 5 insertions, 4 deletions

diff --git a/synapse/state/__init__.py b/synapse/state/__init__.py

index cad3b42640..54e41d5375 100644
--- a/synapse/state/__init__.py
+++ b/synapse/state/__init__.py
@@ -130,6 +130,7 @@ class StateHandler:
         self.state_store = hs.get_storage().state
         self.hs = hs
         self._state_resolution_handler = hs.get_state_resolution_handler()
+        self._storage = hs.get_storage()
 
     @overload
     async def get_current_state(
@@ -361,10 +362,10 @@ class StateHandler:
 
         if not event.is_state():
             return EventContext.with_state(
+                storage=self._storage,
                 state_group_before_event=state_group_before_event,
                 state_group=state_group_before_event,
-                current_state_ids=state_ids_before_event,
-                prev_state_ids=state_ids_before_event,
+                state_delta_due_to_event={},
                 prev_group=state_group_before_event_prev_group,
                 delta_ids=deltas_to_state_group_before_event,
                 partial_state=partial_state,
@@ -393,10 +394,10 @@ class StateHandler:
         )
 
         return EventContext.with_state(
+            storage=self._storage,
             state_group=state_group_after_event,
             state_group_before_event=state_group_before_event,
-            current_state_ids=state_ids_after_event,
-            prev_state_ids=state_ids_before_event,
+            state_delta_due_to_event=delta_ids,
             prev_group=state_group_before_event,
             delta_ids=delta_ids,
             partial_state=partial_state,