Add `allow_departed_users` param to `check_in_room_or_world_readable`

... and set it everywhere it's called.

while we're here, rename it for consistency with `check_user_in_room` (and to
help check that I haven't missed any instances)

synapse/api/auth.py

@@ -625,10 +625,18 @@ class Auth(object):
             return query_params[0].decode("ascii")
 
     @defer.inlineCallbacks
-    def check_in_room_or_world_readable(self, room_id, user_id):
+    def check_user_in_room_or_world_readable(
+        self, room_id: str, user_id: str, allow_departed_users: bool = False
+    ):
         """Checks that the user is or was in the room or the room is world
         readable. If it isn't then an exception is raised.
 
+        Args:
+            room_id: room to check
+            user_id: user to check
+            allow_departed_users: if True, accept users that were previously
+                members but have now departed
+
         Returns:
             Deferred[tuple[str, str|None]]: Resolves to the current membership of
                 the user in the room and the membership event ID of the user. If
@@ -643,7 +651,7 @@ class Auth(object):
             #  * The user is a guest user, and has joined the room
             # else it will throw.
             member_event = yield self.check_user_in_room(
-                room_id, user_id, allow_departed_users=True
+                room_id, user_id, allow_departed_users=allow_departed_users
             )
             return member_event.membership, member_event.event_id
         except AuthError:
@@ -656,7 +664,9 @@ class Auth(object):
             ):
                 return Membership.JOIN, None
             raise AuthError(
-                403, "Guest access not allowed", errcode=Codes.GUEST_ACCESS_FORBIDDEN
+                403,
+                "User %s not in room %s, and room previews are disabled"
+                % (user_id, room_id),
             )
 
     @defer.inlineCallbacks

synapse/handlers/initial_sync.py

@@ -277,7 +277,9 @@ class InitialSyncHandler(BaseHandler):
         (
             membership,
             member_event_id,
-        ) = await self.auth.check_user_in_room_or_world_readable(room_id, user_id)
+        ) = await self.auth.check_user_in_room_or_world_readable(
+            room_id, user_id, allow_departed_users=True,
+        )
         is_peeking = member_event_id is None
 
         if membership == Membership.JOIN:

synapse/handlers/message.py

@@ -99,7 +99,9 @@ class MessageHandler(object):
         (
             membership,
             membership_event_id,
-        ) = yield self.auth.check_in_room_or_world_readable(room_id, user_id)
+        ) = yield self.auth.check_user_in_room_or_world_readable(
+            room_id, user_id, allow_departed_users=True
+        )
 
         if membership == Membership.JOIN:
             data = yield self.state.get_current_state(room_id, event_type, state_key)
@@ -177,7 +179,9 @@ class MessageHandler(object):
             (
                 membership,
                 membership_event_id,
-            ) = yield self.auth.check_in_room_or_world_readable(room_id, user_id)
+            ) = yield self.auth.check_user_in_room_or_world_readable(
+                room_id, user_id, allow_departed_users=True
+            )
 
             if membership == Membership.JOIN:
                 state_ids = yield self.store.get_filtered_current_state_ids(
@@ -216,8 +220,8 @@ class MessageHandler(object):
         if not requester.app_service:
             # We check AS auth after fetching the room membership, as it
             # requires us to pull out all joined members anyway.
-            membership, _ = yield self.auth.check_in_room_or_world_readable(
-                room_id, user_id
+            membership, _ = yield self.auth.check_user_in_room_or_world_readable(
+                room_id, user_id, allow_departed_users=True
             )
             if membership != Membership.JOIN:
                 raise NotImplementedError(

synapse/handlers/pagination.py

@@ -335,7 +335,9 @@ class PaginationHandler(object):
             (
                 membership,
                 member_event_id,
-            ) = await self.auth.check_in_room_or_world_readable(room_id, user_id)
+            ) = await self.auth.check_user_in_room_or_world_readable(
+                room_id, user_id, allow_departed_users=True
+            )
 
             if source_config.direction == "b":
                 # if we're going backwards, we might need to backfill. This

synapse/rest/client/v2_alpha/relations.py

@@ -142,8 +142,8 @@ class RelationPaginationServlet(RestServlet):
     ):
         requester = await self.auth.get_user_by_req(request, allow_guest=True)
 
-        await self.auth.check_in_room_or_world_readable(
-            room_id, requester.user.to_string()
+        await self.auth.check_user_in_room_or_world_readable(
+            room_id, requester.user.to_string(), allow_departed_users=True
         )
 
         # This gets the original event and checks that a) the event exists and
@@ -235,8 +235,8 @@ class RelationAggregationPaginationServlet(RestServlet):
     ):
         requester = await self.auth.get_user_by_req(request, allow_guest=True)
 
-        await self.auth.check_in_room_or_world_readable(
-            room_id, requester.user.to_string()
+        await self.auth.check_user_in_room_or_world_readable(
+            room_id, requester.user.to_string(), allow_departed_users=True,
         )
 
         # This checks that a) the event exists and b) the user is allowed to
@@ -313,8 +313,8 @@ class RelationAggregationGroupPaginationServlet(RestServlet):
     async def on_GET(self, request, room_id, parent_id, relation_type, event_type, key):
         requester = await self.auth.get_user_by_req(request, allow_guest=True)
 
-        await self.auth.check_in_room_or_world_readable(
-            room_id, requester.user.to_string()
+        await self.auth.check_user_in_room_or_world_readable(
+            room_id, requester.user.to_string(), allow_departed_users=True,
         )
 
         # This checks that a) the event exists and b) the user is allowed to