Don't set new room alias before potential 403

changelog.d/10929.bugfix

@@ -0,0 +1 @@
+Newly-created public rooms are now only assigned an alias if the room's creation has not been blocked by permission settings. Contributed by @AndrewFerr.

synapse/handlers/room.py

@@ -752,6 +752,15 @@ async def create_room(
         if not allowed_by_third_party_rules:
             raise SynapseError(403, "Room visibility value not allowed.")
 
+        if is_public:
+            if not self.config.roomdirectory.is_publishing_room_allowed(
+                user_id, room_id, room_alias
+            ):
+                # Lets just return a generic message, as there may be all sorts of
+                # reasons why we said no. TODO: Allow configurable error messages
+                # per alias creation rule?
+                raise SynapseError(403, "Not allowed to publish room")
+
         directory_handler = self.hs.get_directory_handler()
         if room_alias:
             await directory_handler.create_association(
@@ -762,15 +771,6 @@ async def create_room(
                 check_membership=False,
             )
 
-        if is_public:
-            if not self.config.roomdirectory.is_publishing_room_allowed(
-                user_id, room_id, room_alias
-            ):
-                # Lets just return a generic message, as there may be all sorts of
-                # reasons why we said no. TODO: Allow configurable error messages
-                # per alias creation rule?
-                raise SynapseError(403, "Not allowed to publish room")
-
         preset_config = config.get(
             "preset",
             RoomCreationPreset.PRIVATE_CHAT