MDEV-33023 Crash in mariadb-backup --prepare --export after --prepare

mariadb-backup with --prepare option could result in empty redo log
file. When --prepare is followed by --prepare --export, we exit early
in srv_start function without opening the ibdata1 tablespace. Later
while trying to read rollback segment header page, we hit the debug
assert which claims that the system space should already have been
opened.

There are two assert cases here.

Issue-1: System tablespace object is not there in fil space hash i.e.
srv_sys_space.open_or_create() is not called.

Issue-2: The system tablespace data file ibdata1 is not opened i.e.
fil_system.sys_space->open() is not called.

Fix: For empty redo log and restore operation, open system tablespace
before returning.

mysql-test/suite/mariabackup/partial.result

@@ -14,6 +14,14 @@ ALTER TABLE t1 IMPORT TABLESPACE;
 SELECT * FROM t1;
 i
 1
+# MDEV-33023 Crash in mariadb-backup --prepare --export after --prepare
+t1.cfg
+t21.cfg
+ALTER TABLE t1 DISCARD TABLESPACE;
+ALTER TABLE t1 IMPORT TABLESPACE;
+SELECT * FROM t1;
+i
+1
 DROP TABLE t1;
 DROP TABLE t2;
 DROP TABLE t21;

mysql-test/suite/mariabackup/partial.test

@@ -54,6 +54,25 @@ copy_file $targetdir/test/t1.cfg $MYSQLD_DATADIR/test/t1.cfg;
 ALTER TABLE t1 IMPORT TABLESPACE;
 
 SELECT * FROM t1;
+
+--echo # MDEV-33023 Crash in mariadb-backup --prepare --export after --prepare
+--disable_result_log
+exec $XTRABACKUP --defaults-file=$server_cnf --defaults-group-suffix=.1 --prepare --target-dir=$targetdir;
+exec $XTRABACKUP --defaults-file=$server_cnf --defaults-group-suffix=.1 --prepare --export --target-dir=$targetdir;
+--enable_result_log
+
+list_files $targetdir/test *.cfg;
+# There must not be binary logs created on --prepare step
+list_files $targetdir/ mysqld-bin.*;
+
+let $MYSQLD_DATADIR= `select @@datadir`;
+ALTER TABLE t1 DISCARD TABLESPACE;
+copy_file $targetdir/test/t1.ibd $MYSQLD_DATADIR/test/t1.ibd;
+copy_file $targetdir/test/t1.cfg $MYSQLD_DATADIR/test/t1.cfg;
+ALTER TABLE t1 IMPORT TABLESPACE;
+
+SELECT * FROM t1;
+
 DROP TABLE t1;
 DROP TABLE t2;
 DROP TABLE t21;

storage/innobase/srv/srv0start.cc

@@ -1277,6 +1277,36 @@ dberr_t srv_start(bool create_new_db)
 		return srv_init_abort(DB_ERROR);
 	}
 
+	/* Open system tablespace and undo spaces. It is a special case for
+	restore with empty redo log file. We support restore in such case and
+	proceed after opening system tablespaces. */
+	ulint	sum_of_new_sizes;
+
+	auto open_system_spaces = [&]() {
+		ut_ad(!create_new_db);
+		ut_ad(srv_operation == SRV_OPERATION_RESTORE ||
+		      srv_operation == SRV_OPERATION_RESTORE_EXPORT);
+
+		/* Open system tablespace ibdata1. */
+		auto err = srv_sys_space.open_or_create(
+			false, false, &sum_of_new_sizes, &flushed_lsn);
+		if (err != DB_SUCCESS) {
+			return srv_init_abort(err);
+		}
+
+		/* Open data files for system tablespace. */
+		if (!fil_system.sys_space->open(false)) {
+			return srv_init_abort(DB_ERROR);
+		}
+
+		/* Open undo tablespaces. */
+		err = srv_undo_tablespaces_init(false);
+		if (err != DB_SUCCESS) {
+			return srv_init_abort(err);
+		}
+		return DB_SUCCESS;
+	};
+
 	std::string logfile0;
 	bool create_new_log = create_new_db;
 	if (create_new_db) {
@@ -1299,7 +1329,8 @@ dberr_t srv_start(bool create_new_db)
 		bool log_file_found;
 		if (dberr_t err = find_and_check_log_file(log_file_found)) {
 			if (err == DB_NOT_FOUND) {
-				return DB_SUCCESS;
+				/* For restore, we need to continue. */
+				return open_system_spaces();
 			}
 			return srv_init_abort(err);
 		}
@@ -1329,8 +1360,6 @@ dberr_t srv_start(bool create_new_db)
 	}
 
 	/* Open or create the data files. */
-	ulint	sum_of_new_sizes;
-
 	err = srv_sys_space.open_or_create(
 		false, create_new_db, &sum_of_new_sizes, &flushed_lsn);
 
@@ -1378,10 +1407,8 @@ dberr_t srv_start(bool create_new_db)
 		/* Suppress the message about
 		crash recovery. */
 		flushed_lsn = log_sys.get_lsn();
-		goto file_checked;
 	}
 
-file_checked:
 	/* Open data files in the systemtablespace: we keep
         them open until database shutdown */
 	ut_d(fil_system.sys_space->recv_size = srv_sys_space_size_debug);