If the user specifies --backup with --delete (but without --backup-dir

and --delete-excluded), append a protect-filter rule onto the list of
filter rules.  We run this on both the client and server side so that
an older client talking to a newer server still gets the protection
rule.  (This rule will not cause compatibility problems with older
versions of rsync due to the graceful way that a protect rule turns
into an exclude rule if it needs to be sent to an older server.)

options.c

@@ -1207,6 +1207,10 @@ int parse_arguments(int *argc, const char ***argv, int frommain)
 		snprintf(err_buf, sizeof err_buf,
 			"--suffix cannot be a null string without --backup-dir\n");
 		return 0;
+	} else if (delete_mode && !delete_excluded) {
+		snprintf(backup_dir_buf, sizeof backup_dir_buf,
+			"P *%s", backup_suffix);
+		parse_rule(&filter_list, backup_dir_buf, 0, 0);
 	}
 	if (make_backups && !backup_dir)
 		omit_dir_times = 1;