Bulk small improvements to room upgrades

proposals/3385-refresh-room-upgrades.md

@@ -0,0 +1,49 @@
+# MSC3385: Bulk small improvements to room upgrades
+
+[Room upgrades](https://matrix.org/docs/spec/client_server/r0.6.1#id160) presently rely on client
+and server implementations to determine what information, if any, gets copied to new rooms. The
+spec already specifies that core parts of the room (power levels, name, etc) *should* be transferred,
+though this does not account for more modern features like Spaces and other functionality.
+
+**TODO**: Words to explain in more detail the rationale to keep the room representation the same.
+
+## Proposal
+
+The set of copied information is updated to say that:
+
+* Servers must transfer "copiable" state from the original room to the new room. Copiable state is anything
+  in the `m.*` namespace that isn't a membership event and is not the create event (more on that in the
+  next point). This should allow for rooms to maintain future feature sets between upgrades easily.
+
+  Some future features might be sensitive to the sender of the event, though the current spec already says
+  that Matrix-namespaced events should be copied, so this is not perceived to be a problem.
+
+* Servers must copy the `content` of the old room's `m.room.create` event before applying the `predecessor`
+  and `room_version` properties. This ensures details such as the room type (eg: spaces) and federatable
+  status are copied to the new room accurately.
+
+* When the server sees an upgrade, and can validate that the upgrade happened legally (the tombstone and
+  predecessor point at each other's rooms), the server must copy per-room account data from the old room
+  to the new room. This is not generally done during the `/upgrade` call as rooms can be upgraded over
+  federation.
+
+... Others TBD
+
+**TODO**: Mention how Synapse already does the last two points above.
+
+## Potential issues
+
+TODO
+
+## Alternatives
+
+TODO
+
+## Security considerations
+
+TODO
+
+## Unstable prefix
+
+This MSC does not require an unstable prefix as it is codifying behaviour in an area of the spec which permits
+implementation-specific behaviour.