MDEV-13821 : mariabackup sometimes could lose ib_logf(FATAL) messages,

The messages are getting lost because they are written with fprintf()
but without fflush(), so abort() would lose buffered text.

Applied fix from 10.2, which makes in_logf() use
sql_print_information(), which in turn does proper flush after each message

extra/mariabackup/xtrabackup.cc

@@ -95,6 +95,7 @@ Place, Suite 330, Boston, MA 02111-1307 USA
 #include <sql_plugin.h>
 #include <srv0srv.h>
 #include <crc_glue.h>
+#include <log.h>
 
 /* TODO: replace with appropriate macros used in InnoDB 5.6 */
 #define PAGE_ZIP_MIN_SIZE_SHIFT	10
@@ -6483,6 +6484,11 @@ int main(int argc, char **argv)
 	system_charset_info = &my_charset_utf8_general_ci;
 	key_map_full.set_all();
 
+	logger.init_base();
+	logger.set_handlers(LOG_FILE, LOG_NONE, LOG_NONE);
+	mysql_mutex_init(key_LOCK_error_log, &LOCK_error_log,
+			 MY_MUTEX_INIT_FAST);
+
 	handle_options(argc, argv, &client_defaults, &server_defaults);
 
 	int argc_server;

storage/xtradb/handler/ha_innodb.cc

@@ -21949,27 +21949,21 @@ ib_logf(
 	str = static_cast<char*>(malloc(BUFSIZ));
 	my_vsnprintf(str, BUFSIZ, format, args);
 #endif /* __WIN__ */
-	if (!IS_XTRABACKUP()) {
-		switch (level) {
-		case IB_LOG_LEVEL_INFO:
-			sql_print_information("InnoDB: %s", str);
-			break;
-		case IB_LOG_LEVEL_WARN:
-			sql_print_warning("InnoDB: %s", str);
-			break;
-		case IB_LOG_LEVEL_ERROR:
-			sql_print_error("InnoDB: %s", str);
-			sd_notifyf(0, "STATUS=InnoDB: Error: %s", str);
-			break;
-		case IB_LOG_LEVEL_FATAL:
-			sql_print_error("InnoDB: %s", str);
-			sd_notifyf(0, "STATUS=InnoDB: Fatal: %s", str);
-			break;
-		}
-	}
-	else {
-		/* Don't use server logger for XtraBackup, just print to stderr. */
-		fprintf(stderr, "InnoDB: %s\n", str);
+	switch (level) {
+	case IB_LOG_LEVEL_INFO:
+		sql_print_information("InnoDB: %s", str);
+		break;
+	case IB_LOG_LEVEL_WARN:
+		sql_print_warning("InnoDB: %s", str);
+		break;
+	case IB_LOG_LEVEL_ERROR:
+		sql_print_error("InnoDB: %s", str);
+		sd_notifyf(0, "STATUS=InnoDB: Error: %s", str);
+		break;
+	case IB_LOG_LEVEL_FATAL:
+		sql_print_error("InnoDB: %s", str);
+		sd_notifyf(0, "STATUS=InnoDB: Fatal: %s", str);
+		break;
 	}
 
 	va_end(args);