From da20f270c6d6ca28192a6fa49f38241ce5a121e5 Mon Sep 17 00:00:00 2001
From: Neil Alexander <neilalexander@users.noreply.github.com>
Date: Tue, 26 May 2020 12:19:31 +0100
Subject: [PATCH] Return correct room ID from PerformJoin

---
 clientapi/routing/joinroom.go       | 2 +-
 roomserver/api/perform.go           | 1 +
 roomserver/internal/perform_join.go | 6 ++++++
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/clientapi/routing/joinroom.go b/clientapi/routing/joinroom.go
index 500df337a..a3d676532 100644
--- a/clientapi/routing/joinroom.go
+++ b/clientapi/routing/joinroom.go
@@ -75,6 +75,6 @@ func JoinRoomByIDOrAlias(
 		// TODO: Put the response struct somewhere internal.
 		JSON: struct {
 			RoomID string `json:"room_id"`
-		}{joinReq.RoomIDOrAlias},
+		}{joinRes.RoomID},
 	}
 }
diff --git a/roomserver/api/perform.go b/roomserver/api/perform.go
index c98f91fb4..f5afd67bf 100644
--- a/roomserver/api/perform.go
+++ b/roomserver/api/perform.go
@@ -24,6 +24,7 @@ type PerformJoinRequest struct {
 }
 
 type PerformJoinResponse struct {
+	RoomID string `json:"room_id"`
 }
 
 func (h *httpRoomserverInternalAPI) PerformJoin(
diff --git a/roomserver/internal/perform_join.go b/roomserver/internal/perform_join.go
index ce158f605..75dfdd19c 100644
--- a/roomserver/internal/perform_join.go
+++ b/roomserver/internal/perform_join.go
@@ -90,6 +90,12 @@ func (r *RoomserverInternalAPI) performJoinRoomByID(
 	req *api.PerformJoinRequest,
 	res *api.PerformJoinResponse, // nolint:unparam
 ) error {
+	// By this point, if req.RoomIDOrAlias contained an alias, then
+	// it will have been overwritten with a room ID by performJoinRoomByAlias.
+	// We should now include this in the response so that the CS API can
+	// return the right room ID.
+	res.RoomID = req.RoomIDOrAlias
+
 	// Get the domain part of the room ID.
 	_, domain, err := gomatrixserverlib.SplitID('!', req.RoomIDOrAlias)
 	if err != nil {