Implement unstable support for MSC4491 (#19874)

Signed-off-by: timedout <git@nexy7574.co.uk>

changelog.d/19874.feature

@@ -0,0 +1 @@
+Add experimental support for [MSC4491: Invite reasons in room creation](https://github.com/matrix-org/matrix-spec-proposals/pull/4491).

synapse/config/experimental.py

@@ -622,3 +622,6 @@ class ExperimentalConfig(Config):
         # MSC4455: Preview URL capability
         # Tracked in: https://github.com/element-hq/synapse/issues/19719
         self.msc4452_enabled: bool = experimental.get("msc4452_enabled", False)
+
+        # MSC4491: Invite reasons in room creation
+        self.msc4491_enabled: bool = experimental.get("msc4491_enabled", False)

synapse/handlers/room.py

@@ -1345,6 +1345,11 @@ class RoomCreationHandler:
             if is_direct:
                 content["is_direct"] = is_direct
 
+            if self.config.experimental.msc4491_enabled:
+                reason = config.get("uk.timedout.msc4491.invite_reason")
+                if reason and isinstance(reason, str):
+                    content["reason"] = reason
+
             for invitee in invite_list:
                 (
                     member_event_id,

synapse/rest/client/versions.py

@@ -208,6 +208,7 @@ class VersionsRestServlet(RestServlet):
                     "org.matrix.msc4380.stable": True,
                     # MSC4445: Sync timeline order
                     "org.matrix.msc4445.initial_sync_timeline_topological_ordering": True,
+                    "uk.timedout.msc4491.create_room_invite_reasons": self.config.experimental.msc4491_enabled,
                 },
             },
         )