Don't pull out the full state when storing state

synapse/state/__init__.py

@@ -293,8 +293,11 @@ async def compute_event_context(
             deltas_to_state_group_before_event = entry.delta_ids
 
             # We make sure that we have a state group assigned to the state.
+            state_ids_before_event = None
             if entry.state_group is None:
-                state_ids_before_event = None
+                # store_state_group requires us to have either a previous state group
+                # (with deltas) or the complete state map. So, if we don't have a
+                # previous state group, load the complete state map now.
                 if state_group_before_event_prev_group is None:
                     state_ids_before_event = await entry.get_state(
                         self._state_storage_controller, StateFilter.all()
@@ -312,7 +315,6 @@ async def compute_event_context(
                 entry.state_group = state_group_before_event
             else:
                 state_group_before_event = entry.state_group
-                state_ids_before_event = None
 
         #
         # now if it's not a state event, we're done

synapse/storage/databases/state/store.py

@@ -404,6 +404,9 @@ async def store_state_group(
     ) -> int:
         """Store a new set of state, returning a newly assigned state group.
 
+        If `current_state_ids` is None then `prev_group` and `delta_ids` must
+        not be None.
+
         Args:
             event_id: The event ID for which the state was calculated
             room_id
@@ -427,7 +430,9 @@ async def store_state_group(
         def insert_delta_group_txn(
             txn: LoggingTransaction, prev_group: int, delta_ids: StateMap[str]
         ) -> Optional[int]:
-            """If we have a delta we try and persist the new group as a delta.
+            """Try and persist the new group as a delta.
+
+            Requires that we have the state as a delta from a previous state group.
 
             Returns:
                 The state group if successfully created, or None if the state
@@ -446,8 +451,9 @@ def insert_delta_group_txn(
                     % (prev_group,)
                 )
 
+            # if the chain of state group deltas is going too long, we fall back to
+            # persisting a complete state group.
             potential_hops = self._count_state_group_hops_txn(txn, prev_group)
-
             if potential_hops >= MAX_STATE_DELTA_HOPS:
                 return None
 
@@ -540,6 +546,9 @@ def insert_full_state_txn(
             if state_group is not None:
                 return state_group
 
+        # We're going to persist the state as a complete group rather than
+        # a delta, so first we need to ensure we have loaded the state map
+        # from the database.
         if current_state_ids is None:
             assert prev_group is not None
             assert delta_ids is not None