MDEV-28187 mariadb-backup doesn't utilise innodb-undo-log-directory (if specified as a relative path) during copy-back operation

Make absolute destination path from relative one, basing on mysql data
directory.

Reviewed by Alexander Barkov.

extra/mariabackup/backup_copy.cc

@@ -1816,13 +1816,28 @@ apply_log_finish()
 	return(true);
 }
 
+class Copy_back_dst_dir
+{
+  std::string buf;
+
+public:
+  const char *make(const char *path)
+  {
+    if (!path || !path[0])
+      return mysql_data_home;
+    if (is_absolute_path(path))
+      return path;
+    return buf.assign(mysql_data_home).append(path).c_str();
+  }
+};
+
 bool
 copy_back()
 {
 	bool ret = false;
 	datadir_iter_t *it = NULL;
 	datadir_node_t node;
-	char *dst_dir;
+	const char *dst_dir;
 
 	memset(&node, 0, sizeof(node));
 
@@ -1875,9 +1890,9 @@ copy_back()
 
 	/* copy undo tablespaces */
 
+	Copy_back_dst_dir dst_dir_buf;
 
-	dst_dir = (srv_undo_dir && *srv_undo_dir)
-			? srv_undo_dir : mysql_data_home;
+	dst_dir = dst_dir_buf.make(srv_undo_dir);
 
 	ds_data = ds_create(dst_dir, DS_TYPE_LOCAL);
 
@@ -1898,8 +1913,7 @@ copy_back()
 
 	/* copy redo logs */
 
-	dst_dir = (srv_log_group_home_dir && *srv_log_group_home_dir)
-		? srv_log_group_home_dir : mysql_data_home;
+	dst_dir = dst_dir_buf.make(srv_log_group_home_dir);
 
 	/* --backup generates a single ib_logfile0, which we must copy
 	if it exists. */
@@ -1926,8 +1940,7 @@ copy_back()
 
 	/* copy innodb system tablespace(s) */
 
-	dst_dir = (innobase_data_home_dir && *innobase_data_home_dir)
-				? innobase_data_home_dir : mysql_data_home;
+	dst_dir = dst_dir_buf.make(innobase_data_home_dir);
 
 	ds_data = ds_create(dst_dir, DS_TYPE_LOCAL);
 

mysql-test/suite/mariabackup/include/restart_and_restore.inc

@@ -4,5 +4,5 @@ echo # shutdown server;
 echo # remove datadir;
 rmdir $_datadir;
 echo # xtrabackup move back;
-exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --copy-back --datadir=$_datadir --target-dir=$targetdir --parallel=2 --throttle=1;
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --copy-back --datadir=$_datadir --target-dir=$targetdir --parallel=2 --throttle=1 $backup_opts;
 --source include/start_mysqld.inc

mysql-test/suite/mariabackup/relative_path.opt

@@ -0,0 +1 @@
+--innodb-undo-tablespaces=2

mysql-test/suite/mariabackup/relative_path.result

@@ -0,0 +1,20 @@
+CREATE TABLE t(i INT) ENGINE INNODB;
+INSERT INTO t VALUES(1);
+# xtrabackup backup
+# xtrabackup prepare
+# shutdown server
+# remove datadir
+# xtrabackup move back
+# restart
+# shutdown server
+# remove datadir
+# xtrabackup move back
+# restart
+# shutdown server
+# remove datadir
+# xtrabackup move back
+# restart
+SELECT * FROM t;
+i
+1
+DROP TABLE t;

mysql-test/suite/mariabackup/relative_path.test

@@ -0,0 +1,35 @@
+--source include/have_innodb.inc
+
+CREATE TABLE t(i INT) ENGINE INNODB;
+INSERT INTO t VALUES(1);
+
+echo # xtrabackup backup;
+let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
+--let $backup_log=$MYSQLTEST_VARDIR/tmp/backup.log
+
+--disable_result_log
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --target-dir=$targetdir > $backup_log 2>&1;
+--enable_result_log
+
+echo # xtrabackup prepare;
+--disable_result_log
+exec $XTRABACKUP  --prepare --target-dir=$targetdir;
+
+# If MDEV-28187 is not fixed, the following tries to copy backup to data
+# directory will fail, because their destination path will be the same as
+# their source path
+
+--let $backup_opts=--innodb_undo_directory=./
+--source include/restart_and_restore.inc
+
+--let $backup_opts=--innodb_log_group_home_dir=./
+--source include/restart_and_restore.inc
+
+--let $backup_opts=--innodb_data_home_dir=./
+--source include/restart_and_restore.inc
+
+--enable_result_log
+
+SELECT * FROM t;
+DROP TABLE t;
+rmdir $targetdir;