MDEV-28446 mariabackup prepare fails for incrementals if a new schema is created after full backup is taken

When "mariabackup --target-dir=$basedir --incremental-dir=$incremental_dir"
is running and is moving a new table file (e.g. `db1/t1.new`) from the
incremental directory to the base directory, it needs to verify that the base
backup database directory (e.g. `$basedir/db1`) really exists
(or create it otherwise).

The table `db1/t1` can come from a new database `db1` which
was created during the base mariabackup execution time.

In such case the directory `db1` exists only in the incremental directory,
but does not exist in the base directory.

extra/mariabackup/xtrabackup.cc

@@ -5441,11 +5441,23 @@ static ibool prepare_handle_new_files(const char *data_home_dir,
                                       const char *file_name, void *arg)
 {
 	const char *dest_dir = static_cast<const char *>(arg);
-	std::string src_path = std::string(data_home_dir) + '/' + std::string(db_name) + '/' + file_name;
+	std::string src_path = std::string(data_home_dir) + '/' + std::string(db_name) + '/';
 	/* Copy "*.new" files from incremental to base dir for incremental backup */
 	std::string dest_path=
 		dest_dir ? std::string(dest_dir) + '/' + std::string(db_name) +
-			'/' + file_name : src_path;
+			'/' : src_path;
+
+	/*
+	  A CREATE DATABASE could have happened during the base mariabackup run.
+	  In case if the current table file (e.g. `t1.new`) is from such
+	  a new database, the database directory may not exist yet in
+	  the base backup directory. Let's make sure to check if the directory
+	  exists (and create if needed).
+	*/
+	if (!directory_exists(dest_path.c_str(), true/*create if not exists*/))
+		return FALSE;
+	src_path+= file_name;
+	dest_path+= file_name;
 
 	size_t index = dest_path.find(".new");
 	DBUG_ASSERT(index != std::string::npos);

mysql-test/suite/mariabackup/incremental_newdb_while_backup.result

@@ -0,0 +1,24 @@
+call mtr.add_suppression("InnoDB: New log files created");
+#
+# Start of 10.2 tests
+#
+#
+# MDEV-28446 mariabackup prepare fails for incrementals if a new schema is created after full backup is taken
+#
+CREATE TABLE t1(i INT PRIMARY KEY) ENGINE INNODB;
+# Prepare full backup, apply incremental one
+# shutdown server
+# remove datadir
+# xtrabackup move back
+# restart server
+SELECT COUNT(*) FROM test.t1;
+COUNT(*)
+0
+SELECT COUNT(*) FROM test1.t1;
+COUNT(*)
+10000
+DROP TABLE t1;
+DROP DATABASE test1;
+#
+# End of 10.2 tests
+#

mysql-test/suite/mariabackup/incremental_newdb_while_backup.test

@@ -0,0 +1,47 @@
+--source include/have_innodb.inc
+--source include/have_debug.inc
+
+call mtr.add_suppression("InnoDB: New log files created");
+
+--echo #
+--echo # Start of 10.2 tests
+--echo #
+
+--echo #
+--echo # MDEV-28446 mariabackup prepare fails for incrementals if a new schema is created after full backup is taken
+--echo #
+
+--let $basedir=$MYSQLTEST_VARDIR/tmp/backup
+--let $incremental_dir=$MYSQLTEST_VARDIR/tmp/backup_inc1
+
+CREATE TABLE t1(i INT PRIMARY KEY) ENGINE INNODB;
+
+--disable_result_log
+--exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf  --backup --target-dir=$basedir
+--enable_result_log
+
+--let after_load_tablespaces=BEGIN NOT ATOMIC CREATE DATABASE test1; CREATE TABLE test1.t1 ENGINE=INNODB SELECT UUID() from test.seq_1_to_10000; END
+
+--exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf  --backup --target-dir=$incremental_dir --incremental-basedir=$basedir --dbug=+d,mariabackup_events,mariabackup_inject_code
+--let after_load_tablespaces=
+--disable_result_log
+--echo # Prepare full backup, apply incremental one
+--exec $XTRABACKUP --apply-log-only --prepare --target-dir=$basedir
+--exec $XTRABACKUP --prepare --target-dir=$basedir --incremental-dir=$incremental_dir
+--enable_result_log
+
+--let $targetdir=$basedir
+--source include/restart_and_restore.inc
+--enable_result_log
+
+SELECT COUNT(*) FROM test.t1;
+SELECT COUNT(*) FROM test1.t1;
+
+DROP TABLE t1;
+DROP DATABASE test1;
+--rmdir $basedir
+--rmdir $incremental_dir
+
+--echo #
+--echo # End of 10.2 tests
+--echo #