MDEV-30492 Crash when use mariabackup.exe with config 'innodb_flush_method=async_unbuffered'

Normalize innodb_flush_method, the same as the service, before
attempting to print it.

extra/mariabackup/xtrabackup.cc

@@ -1896,6 +1896,17 @@ xb_get_one_option(int optid,
     break;
 
   case OPT_INNODB_FLUSH_METHOD:
+#ifdef _WIN32
+    /* From: storage/innobase/handler/ha_innodb.cc:innodb_init_params */
+    switch (srv_file_flush_method) {
+    case SRV_ALL_O_DIRECT_FSYNC + 1 /* "async_unbuffered"="unbuffered" */:
+      srv_file_flush_method= SRV_ALL_O_DIRECT_FSYNC;
+      break;
+    case SRV_ALL_O_DIRECT_FSYNC + 2 /* "normal"="fsync" */:
+      srv_file_flush_method= SRV_FSYNC;
+      break;
+    }
+#endif
     ut_a(srv_file_flush_method
 	 <= IF_WIN(SRV_ALL_O_DIRECT_FSYNC, SRV_O_DIRECT_NO_FSYNC));
     ADD_PRINT_PARAM_OPT(innodb_flush_method_names[srv_file_flush_method]);

mysql-test/suite/mariabackup/full_backup_win.result

@@ -0,0 +1,12 @@
+#
+# MDEV-30492 Crash when use mariabackup.exe with config 'innodb_flush_method=async_unbuffered'
+#
+# xtrabackup backup
+# xtrabackup prepare
+# shutdown server
+# remove datadir
+# xtrabackup move back
+# restart
+#
+# End of 10.4 tests
+#

mysql-test/suite/mariabackup/full_backup_win.test

@@ -0,0 +1,24 @@
+--source include/windows.inc
+
+let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
+
+--echo #
+--echo # MDEV-30492 Crash when use mariabackup.exe with config 'innodb_flush_method=async_unbuffered'
+--echo #
+
+echo # xtrabackup backup;
+--disable_result_log
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --innodb_flush_method=normal --backup --target-dir=$targetdir;
+--enable_result_log
+
+echo # xtrabackup prepare;
+--disable_result_log
+exec $XTRABACKUP  --prepare --innodb-flush-method=async_unbuffered --target-dir=$targetdir;
+-- source include/restart_and_restore.inc
+--enable_result_log
+
+rmdir $targetdir;
+
+--echo #
+--echo # End of 10.4 tests
+--echo #