Detect types.MissingStateError in CheckServerAllowedToSeeEvent (#2667)

This will hopefully stop some 500 errors on `/event` where there is no state-before known.

roomserver/internal/helpers/helpers.go

@@ -254,8 +254,15 @@ func CheckServerAllowedToSeeEvent(
 			return false, err
 		}
 	default:
-		// Something else went wrong
-		return false, err
+		switch err.(type) {
+		case types.MissingStateError:
+			// If there's no state then we assume it's open visibility, as Synapse does:
+			// https://github.com/matrix-org/synapse/blob/aec87a0f9369a3015b2a53469f88d1de274e8b71/synapse/visibility.py#L654-L655
+			return true, nil
+		default:
+			// Something else went wrong
+			return false, err
+		}
 	}
 	return auth.IsServerAllowed(serverName, isServerInRoom, stateAtEvent), nil
 }

roomserver/storage/postgres/events_table.go

@@ -346,7 +346,7 @@ func (s *eventStatements) BulkSelectStateAtEventByID(
 		// Genuine create events are the only case where it's OK to have no previous state.
 		isCreate := result.EventTypeNID == types.MRoomCreateNID && result.EventStateKeyNID == 1
 		if result.BeforeStateSnapshotNID == 0 && !isCreate {
-			return nil, types.MissingEventError(
+			return nil, types.MissingStateError(
 				fmt.Sprintf("storage: missing state for event NID %d", result.EventNID),
 			)
 		}

roomserver/storage/sqlite3/events_table.go

@@ -362,7 +362,7 @@ func (s *eventStatements) BulkSelectStateAtEventByID(
 		// Genuine create events are the only case where it's OK to have no previous state.
 		isCreate := result.EventTypeNID == types.MRoomCreateNID && result.EventStateKeyNID == 1
 		if result.BeforeStateSnapshotNID == 0 && !isCreate {
-			return nil, types.MissingEventError(
+			return nil, types.MissingStateError(
 				fmt.Sprintf("storage: missing state for event NID %d", result.EventNID),
 			)
 		}