Prepatory work for batching events to send (#13487)

This PR begins work on batching up events during the creation of a room. The PR splits out the creation and sending/persisting of the events. The first three events in the creation of the room-creating the room, joining the creator to the room, and the power levels event are sent sequentially, while the subsequent events are created and collected to be sent at the end of the function. This is currently done by appending them to a list and then iterating over the list to send, the next step (after this PR) would be to send and persist the collected events as a batch.

synapse/state/__init__.py

@@ -420,6 +420,69 @@ class StateHandler:
             partial_state=partial_state,
         )
 
+    async def compute_event_context_for_batched(
+        self,
+        event: EventBase,
+        state_ids_before_event: StateMap[str],
+        current_state_group: int,
+    ) -> EventContext:
+        """
+        Generate an event context for an event that has not yet been persisted to the
+        database. Intended for use with events that are created to be persisted in a batch.
+        Args:
+            event: the event the context is being computed for
+            state_ids_before_event: a state map consisting of the state ids of the events
+            created prior to this event.
+            current_state_group: the current state group before the event.
+        """
+        state_group_before_event_prev_group = None
+        deltas_to_state_group_before_event = None
+
+        state_group_before_event = current_state_group
+
+        # if the event is not state, we are set
+        if not event.is_state():
+            return EventContext.with_state(
+                storage=self._storage_controllers,
+                state_group_before_event=state_group_before_event,
+                state_group=state_group_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=False,
+            )
+
+        # otherwise, we'll need to create a new state group for after the event
+        key = (event.type, event.state_key)
+
+        if state_ids_before_event is not None:
+            replaces = state_ids_before_event.get(key)
+
+        if replaces and replaces != event.event_id:
+            event.unsigned["replaces_state"] = replaces
+
+        delta_ids = {key: event.event_id}
+
+        state_group_after_event = (
+            await self._state_storage_controller.store_state_group(
+                event.event_id,
+                event.room_id,
+                prev_group=state_group_before_event,
+                delta_ids=delta_ids,
+                current_state_ids=None,
+            )
+        )
+
+        return EventContext.with_state(
+            storage=self._storage_controllers,
+            state_group=state_group_after_event,
+            state_group_before_event=state_group_before_event,
+            state_delta_due_to_event=delta_ids,
+            prev_group=state_group_before_event,
+            delta_ids=delta_ids,
+            partial_state=False,
+        )
+
     @measure_func()
     async def resolve_state_groups_for_events(
         self, room_id: str, event_ids: Collection[str], await_full_state: bool = True