Handle room upgrades for spaces (#10774)

By copying the `room_type` field of the create event and
migrating any non-empty `m.space.child` events to the
new room that is created.

synapse/handlers/room.py

@@ -33,6 +33,7 @@ from synapse.api.constants import (
     Membership,
     RoomCreationPreset,
     RoomEncryptionAlgorithms,
+    RoomTypes,
 )
 from synapse.api.errors import (
     AuthError,
@@ -397,7 +398,7 @@ class RoomCreationHandler(BaseHandler):
         initial_state = {}
 
         # Replicate relevant room events
-        types_to_copy = (
+        types_to_copy: List[Tuple[str, Optional[str]]] = [
             (EventTypes.JoinRules, ""),
             (EventTypes.Name, ""),
             (EventTypes.Topic, ""),
@@ -408,7 +409,16 @@ class RoomCreationHandler(BaseHandler):
             (EventTypes.ServerACL, ""),
             (EventTypes.RelatedGroups, ""),
             (EventTypes.PowerLevels, ""),
-        )
+        ]
+
+        # If the old room was a space, copy over the room type and the rooms in
+        # the space.
+        if (
+            old_room_create_event.content.get(EventContentFields.ROOM_TYPE)
+            == RoomTypes.SPACE
+        ):
+            creation_content[EventContentFields.ROOM_TYPE] = RoomTypes.SPACE
+            types_to_copy.append((EventTypes.SpaceChild, None))
 
         old_room_state_ids = await self.store.get_filtered_current_state_ids(
             old_room_id, StateFilter.from_types(types_to_copy)
@@ -419,6 +429,11 @@ class RoomCreationHandler(BaseHandler):
         for k, old_event_id in old_room_state_ids.items():
             old_event = old_room_state_events.get(old_event_id)
             if old_event:
+                # If the event is an space child event with empty content, it was
+                # removed from the space and should be ignored.
+                if k[0] == EventTypes.SpaceChild and not old_event.content:
+                    continue
+
                 initial_state[k] = old_event.content
 
         # deep-copy the power-levels event before we start modifying it