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.

1 files changed, 14 insertions, 2 deletions

diff --git a/extra/mariabackup/xtrabackup.cc b/extra/mariabackup/xtrabackup.cc
index c1b2a1a9c85..ada63e1e882 100644
--- a/extra/mariabackup/xtrabackup.cc
+++ b/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);