Enforce type, sender, state_key and room_id lengths using byt…

…es rather than codepoints (#338)

This effectively reverts the change made in
[5f66df0](5f66df0)
to bytes instead of codepoints, since Synapse will now enforce the same
after matrix-org/synapse#13710.

History here is that Synapse originally calculated bytes in Python 2.x,
started counting codepoints in Python 3.x pretty much by accident and
then the spec was ambiguous after the fact (hence
matrix-org/matrix-spec#1001).

Rationale is that bytes are probably easier for implementations to
manage and less likely to generate huge indexes for client-side
databases (especially where limits might exist like LMDB).

cc @reivilibre

event.go

@@ -727,18 +727,18 @@ func (e *Event) CheckFields() error { // nolint: gocyclo
 		}
 	}
 
-	if l := len([]rune(fields.Type)); l > maxIDLength {
+	if l := len(fields.Type); l > maxIDLength {
 		return EventValidationError{
 			Code:    EventValidationTooLarge,
-			Message: fmt.Sprintf("gomatrixserverlib: event type is too long, length %d codepoints > maximum %d codepoints", l, maxIDLength),
+			Message: fmt.Sprintf("gomatrixserverlib: event type is too long, length %d bytes > maximum %d bytes", l, maxIDLength),
 		}
 	}
 
 	if fields.StateKey != nil {
-		if l := len([]rune(*fields.StateKey)); l > maxIDLength {
+		if l := len(*fields.StateKey); l > maxIDLength {
 			return EventValidationError{
 				Code:    EventValidationTooLarge,
-				Message: fmt.Sprintf("gomatrixserverlib: state key is too long, length %d codepoints > maximum %d codepoints", l, maxIDLength),
+				Message: fmt.Sprintf("gomatrixserverlib: state key is too long, length %d bytes > maximum %d bytes", l, maxIDLength),
 			}
 		}
 	}
@@ -765,10 +765,10 @@ func checkID(id, kind string, sigil byte) (err error) {
 		)
 		return
 	}
-	if l := len([]rune(id)); l > maxIDLength {
+	if l := len(id); l > maxIDLength {
 		err = EventValidationError{
 			Code:    EventValidationTooLarge,
-			Message: fmt.Sprintf("gomatrixserverlib: %s ID is too long, length %d codepoints > maximum %d codepoints", kind, l, maxIDLength),
+			Message: fmt.Sprintf("gomatrixserverlib: %s ID is too long, length %d bytes > maximum %d bytes", kind, l, maxIDLength),
 		}
 		return
 	}