Remove unused `room_alias` field from `/createRoom` response (#15093)

* Change `create_room` return type

* Don't return room alias from /createRoom

* Update other callsites

* Fix up mypy complaints

It looks like new_room_user_id is None iff new_room_id is None. It's a
shame we haven't expressed this in a way that mypy can understand.

* Changelog

10 files changed, 40 insertions, 39 deletions

diff --git a/changelog.d/15093.bugfix b/changelog.d/15093.bugfix
new file mode 100644
index 0000000000..00f1c19391
--- /dev/null
+++ b/changelog.d/15093.bugfix
@@ -0,0 +1 @@
+Remove the unspecced `room_alias` field from the [`/createRoom`](https://spec.matrix.org/v1.6/client-server-api/#post_matrixclientv3createroom) response.
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index c611efb760..e4e506e62c 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -476,7 +476,7 @@ class RegistrationHandler:
                     # create room expects the localpart of the room alias
                     config["room_alias_name"] = room_alias.localpart
 
-                    info, _ = await room_creation_handler.create_room(
+                    room_id, _, _ = await room_creation_handler.create_room(
                         fake_requester,
                         config=config,
                         ratelimit=False,
@@ -490,7 +490,7 @@ class RegistrationHandler:
                                 user_id, authenticated_entity=self._server_name
                             ),
                             target=UserID.from_string(user_id),
-                            room_id=info["room_id"],
+                            room_id=room_id,
                             # Since it was just created, there are no remote hosts.
                             remote_room_hosts=[],
                             action="join",
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 837dabb3b7..37c87c8351 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -690,13 +690,14 @@ class RoomCreationHandler:
         config: JsonDict,
         ratelimit: bool = True,
         creator_join_profile: Optional[JsonDict] = None,
-    ) -> Tuple[dict, int]:
+    ) -> Tuple[str, Optional[RoomAlias], int]:
         """Creates a new room.
 
         Args:
-            requester:
-                The user who requested the room creation.
-            config : A dict of configuration options.
+            requester: The user who requested the room creation.
+            config: A dict of configuration options. This will be the body of
+                a /createRoom request; see
+                https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3createroom
             ratelimit: set to False to disable the rate limiter
 
             creator_join_profile:
@@ -707,14 +708,17 @@ class RoomCreationHandler:
                 `avatar_url` and/or `displayname`.
 
         Returns:
-                First, a dict containing the keys `room_id` and, if an alias
-                was, requested, `room_alias`. Secondly, the stream_id of the
-                last persisted event.
+            A 3-tuple containing:
+                - the room ID;
+                - if requested, the room alias, otherwise None; and
+                - the `stream_id` of the last persisted event.
         Raises:
-            SynapseError if the room ID couldn't be stored, 3pid invitation config
-            validation failed, or something went horribly wrong.
-            ResourceLimitError if server is blocked to some resource being
-            exceeded
+            SynapseError:
+                if the room ID couldn't be stored, 3pid invitation config
+                validation failed, or something went horribly wrong.
+            ResourceLimitError:
+                if server is blocked to some resource being
+                exceeded
         """
         user_id = requester.user.to_string()
 
@@ -1024,11 +1028,6 @@ class RoomCreationHandler:
             last_sent_event_id = member_event_id
             depth += 1
 
-        result = {"room_id": room_id}
-
-        if room_alias:
-            result["room_alias"] = room_alias.to_string()
-
         # Always wait for room creation to propagate before returning
         await self._replication.wait_for_stream_position(
             self.hs.config.worker.events_shard_config.get_instance(room_id),
@@ -1036,7 +1035,7 @@ class RoomCreationHandler:
             last_stream_id,
         )
 
-        return result, last_stream_id
+        return room_id, room_alias, last_stream_id
 
     async def _send_events_for_new_room(
         self,
@@ -1825,7 +1824,7 @@ class RoomShutdownHandler:
                 new_room_user_id, authenticated_entity=requester_user_id
             )
 
-            info, stream_id = await self._room_creation_handler.create_room(
+            new_room_id, _, stream_id = await self._room_creation_handler.create_room(
                 room_creator_requester,
                 config={
                     "preset": RoomCreationPreset.PUBLIC_CHAT,
@@ -1834,7 +1833,6 @@ class RoomShutdownHandler:
                 },
                 ratelimit=False,
             )
-            new_room_id = info["room_id"]
 
             logger.info(
                 "Shutting down room %r, joining to new room: %r", room_id, new_room_id
@@ -1887,6 +1885,7 @@ class RoomShutdownHandler:
 
                 # Join users to new room
                 if new_room_user_id:
+                    assert new_room_id is not None
                     await self.room_member_handler.update_membership(
                         requester=target_requester,
                         target=target_requester.user,
@@ -1919,6 +1918,7 @@ class RoomShutdownHandler:
 
             aliases_for_room = await self.store.get_aliases_for_room(room_id)
 
+            assert new_room_id is not None
             await self.store.update_aliases_for_room(
                 room_id, new_room_id, requester_user_id
             )
diff --git a/synapse/module_api/__init__.py b/synapse/module_api/__init__.py
index d22dd19d38..1964276a54 100644
--- a/synapse/module_api/__init__.py
+++ b/synapse/module_api/__init__.py
@@ -1576,14 +1576,14 @@ class ModuleApi:
             )
 
         requester = create_requester(user_id)
-        room_id_and_alias, _ = await self._hs.get_room_creation_handler().create_room(
+        room_id, room_alias, _ = await self._hs.get_room_creation_handler().create_room(
             requester=requester,
             config=config,
             ratelimit=ratelimit,
             creator_join_profile=creator_join_profile,
         )
-
-        return room_id_and_alias["room_id"], room_id_and_alias.get("room_alias", None)
+        room_alias_str = room_alias.to_string() if room_alias else None
+        return room_id, room_alias_str
 
     async def set_displayname(
         self,
diff --git a/synapse/rest/client/room.py b/synapse/rest/client/room.py
index d0db85cca7..14b04810a1 100644
--- a/synapse/rest/client/room.py
+++ b/synapse/rest/client/room.py
@@ -160,11 +160,11 @@ class RoomCreateRestServlet(TransactionRestServlet):
     async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
         requester = await self.auth.get_user_by_req(request)
 
-        info, _ = await self._room_creation_handler.create_room(
+        room_id, _, _ = await self._room_creation_handler.create_room(
             requester, self.get_room_config(request)
         )
 
-        return 200, info
+        return 200, {"room_id": room_id}
 
     def get_room_config(self, request: Request) -> JsonDict:
         user_supplied_config = parse_json_object_from_request(request)
diff --git a/synapse/server_notices/server_notices_manager.py b/synapse/server_notices/server_notices_manager.py
index 564e3705c2..9732dbdb6e 100644
--- a/synapse/server_notices/server_notices_manager.py
+++ b/synapse/server_notices/server_notices_manager.py
@@ -178,7 +178,7 @@ class ServerNoticesManager:
                 "avatar_url": self._config.servernotices.server_notices_mxid_avatar_url,
             }
 
-        info, _ = await self._room_creation_handler.create_room(
+        room_id, _, _ = await self._room_creation_handler.create_room(
             requester,
             config={
                 "preset": RoomCreationPreset.PRIVATE_CHAT,
@@ -188,7 +188,6 @@ class ServerNoticesManager:
             ratelimit=False,
             creator_join_profile=join_profile,
         )
-        room_id = info["room_id"]
 
         self.maybe_get_notice_room_for_user.invalidate((user_id,))
 
diff --git a/tests/storage/test_cleanup_extrems.py b/tests/storage/test_cleanup_extrems.py
index d570684c99..7de109966d 100644
--- a/tests/storage/test_cleanup_extrems.py
+++ b/tests/storage/test_cleanup_extrems.py
@@ -43,8 +43,9 @@ class CleanupExtremBackgroundUpdateStoreTestCase(HomeserverTestCase):
         # Create a test user and room
         self.user = UserID("alice", "test")
         self.requester = create_requester(self.user)
-        info, _ = self.get_success(self.room_creator.create_room(self.requester, {}))
-        self.room_id = info["room_id"]
+        self.room_id, _, _ = self.get_success(
+            self.room_creator.create_room(self.requester, {})
+        )
 
     def run_background_update(self) -> None:
         """Re run the background update to clean up the extremities."""
@@ -275,10 +276,9 @@ class CleanupExtremDummyEventsTestCase(HomeserverTestCase):
         self.user = UserID.from_string(self.register_user("user1", "password"))
         self.token1 = self.login("user1", "password")
         self.requester = create_requester(self.user)
-        info, _ = self.get_success(
+        self.room_id, _, _ = self.get_success(
             self.room_creator.create_room(self.requester, {"visibility": "public"})
         )
-        self.room_id = info["room_id"]
         self.event_creator = homeserver.get_event_creation_handler()
         homeserver.config.consent.user_consent_version = self.CONSENT_VERSION
 
diff --git a/tests/storage/test_event_metrics.py b/tests/storage/test_event_metrics.py
index a91411168c..6897addbd3 100644
--- a/tests/storage/test_event_metrics.py
+++ b/tests/storage/test_event_metrics.py
@@ -33,8 +33,7 @@ class ExtremStatisticsTestCase(HomeserverTestCase):
         events = [(3, 2), (6, 2), (4, 6)]
 
         for event_count, extrems in events:
-            info, _ = self.get_success(room_creator.create_room(requester, {}))
-            room_id = info["room_id"]
+            room_id, _, _ = self.get_success(room_creator.create_room(requester, {}))
 
             last_event = None
 
diff --git a/tests/storage/test_receipts.py b/tests/storage/test_receipts.py
index 12c17f1073..1b52eef23f 100644
--- a/tests/storage/test_receipts.py
+++ b/tests/storage/test_receipts.py
@@ -50,12 +50,14 @@ class ReceiptTestCase(HomeserverTestCase):
         self.otherRequester = create_requester(self.otherUser)
 
         # Create a test room
-        info, _ = self.get_success(self.room_creator.create_room(self.ourRequester, {}))
-        self.room_id1 = info["room_id"]
+        self.room_id1, _, _ = self.get_success(
+            self.room_creator.create_room(self.ourRequester, {})
+        )
 
         # Create a second test room
-        info, _ = self.get_success(self.room_creator.create_room(self.ourRequester, {}))
-        self.room_id2 = info["room_id"]
+        self.room_id2, _, _ = self.get_success(
+            self.room_creator.create_room(self.ourRequester, {})
+        )
 
         # Join the second user to the first room
         memberEvent, memberEventContext = self.get_success(
diff --git a/tests/test_federation.py b/tests/test_federation.py
index 82dfd88b99..46d2f99eac 100644
--- a/tests/test_federation.py
+++ b/tests/test_federation.py
@@ -47,7 +47,7 @@ class MessageAcceptTests(unittest.HomeserverTestCase):
             room_creator.create_room(
                 our_user, room_creator._presets_dict["public_chat"], ratelimit=False
             )
-        )[0]["room_id"]
+        )[0]
 
         self.store = self.hs.get_datastores().main