Detect `types.MissingStateError` in `CheckServerAllowedToSeeEvent` (#2667)

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

1 files changed, 9 insertions, 2 deletions

diff --git a/roomserver/internal/helpers/helpers.go b/roomserver/internal/helpers/helpers.go
index 6091f8ec..cbd1561f 100644
--- a/roomserver/internal/helpers/helpers.go
+++ b/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
 }