MSCXXXX: Automatically forgetting rooms on leave

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

proposals/XXXX-auto-forget.md

@@ -0,0 +1,70 @@
+# MSCXXXX: Automatically forgetting rooms on leave
+
+Matrix discriminates between "leaving" and "forgetting" a room.
+[`/_matrix/client/v3/rooms/{roomId}/leave`] stops a user participating in a room
+but still allows them to retrieve history. A subsequent call to
+[`/_matrix/client/v3/rooms/{roomId}/forget`] then stops the user from being able
+to retrieve history.
+
+Clients don't always differentiate these two operations in their UI and may only
+offer leaving without forgetting a room. Thus, some servers automatically forget
+rooms when a user leaves them in order to more aggressively free up their
+resources. One example of this is the [`forget_rooms_on_leave`] config option in
+Synapse.
+
+The present proposal seeks to standaradize this proprietary behaviour.
+
+## Proposal
+
+When a user leaves a room, either through
+[`/_matrix/client/v3/rooms/{roomId}/leave`] or by being kicked or banned,
+Servers MAY automatically forget the room – as if the user had called
+[`/_matrix/client/v3/rooms/{roomId}/forget`] themselves.
+
+This can limit clients' options to maintain an archive of historic rooms (such
+that they have left *without* forgetting them). Therefore, servers that
+auto-forget rooms MUST advertise that they do so via an `m.leave_without_forget`
+capability.
+
+``` json5
+{
+  "capabilities": {
+    "m.leave_without_forget": {
+      "enabled": true
+    }
+  }
+}
+```
+
+A value of `false` means that the server performs auto-forget so that the client
+cannot leave rooms without also forgetting them. A value of `true` means that
+rooms will only be forgotten when the clients calls
+[`/_matrix/client/v3/rooms/{roomId}/forget`].
+
+When the capability is missing, clients SHOULD assume that the server does not
+auto-forget.
+
+## Potential issues
+
+None.
+
+## Alternatives
+
+None.
+
+## Security considerations
+
+None.
+
+## Unstable prefix
+
+While this proposal is unstable, clients should refer to
+`m.leave_without_forget` as `org.matrix.mscXXXX.leave_without_forget`.
+
+## Dependencies
+
+None.
+
+  [`/_matrix/client/v3/rooms/{roomId}/leave`]: https://spec.matrix.org/v1.13/client-server-api/#post_matrixclientv3roomsroomidleave
+  [`/_matrix/client/v3/rooms/{roomId}/forget`]: https://spec.matrix.org/v1.13/client-server-api/#post_matrixclientv3roomsroomidforget
+  [`forget_rooms_on_leave`]: https://github.com/element-hq/synapse/blob/12dc6b102f071eb2eb84f2cff4cf92903276ffbb/synapse/config/room.py#L88