Better error messages from `get_create_event_for_room` (#11638)

"Unknown room" can mean a multitude of things here. To help with debugging, add
some more words to the exception text.

2 files changed, 6 insertions, 1 deletions

diff --git a/changelog.d/11638.misc b/changelog.d/11638.misc
new file mode 100644

index 0000000000..76dfb56bda
--- /dev/null
+++ b/changelog.d/11638.misc
@@ -0,0 +1 @@
+Improve the error messages from  `get_create_event_for_room`.
diff --git a/synapse/storage/databases/main/state.py b/synapse/storage/databases/main/state.py

index 7e5a6aae18..2fb3e65192 100644
--- a/synapse/storage/databases/main/state.py
+++ b/synapse/storage/databases/main/state.py
@@ -177,11 +177,15 @@ class StateGroupWorkerStore(EventsWorkerStore, SQLBaseStore):
             NotFoundError if the room is unknown
         """
         state_ids = await self.get_current_state_ids(room_id)
+
+        if not state_ids:
+            raise NotFoundError(f"Current state for room {room_id} is empty")
+
         create_id = state_ids.get((EventTypes.Create, ""))
 
         # If we can't find the create event, assume we've hit a dead end
         if not create_id:
-            raise NotFoundError("Unknown room %s" % (room_id,))
+            raise NotFoundError(f"No create event in current state for room {room_id}")
 
         # Retrieve the room's create event and return
         create_event = await self.get_event(create_id)