MDEV-38041: MariaBackup fails during rollback of inplace FTS alter table

Problem:
========
When an inplace ALTER operation is rolled back, InnoDB drops
intermediate tables and their associated FTS internal tables.
However, MariaBackup's DDL tracking can incorrectly report
this as a backup failure.

The issue occurs because backup_set_alter_copy_lock() downgrades the
MDL_BACKUP_DDL lock before the inplace phase of ALTER,
allowing FTS internal tables to be dropped during the
later phases of backup when DDL tracking is still active.

Solution:
========
backup_file_op_fail(): Ignore delete operations on FTS internal
tables when not using --no-lock option, preventing false
positive backup failures.

Author: Thirunarayanan

extra/mariabackup/backup_copy.cc

@@ -1462,6 +1462,7 @@ bool backup_start(ds_ctxt *ds_data, ds_ctxt *ds_meta,
 
 	corrupted_pages.backup_fix_ddl(ds_data, ds_meta);
 
+	DBUG_MARIABACKUP_EVENT("after_backup_fix_ddl", {});
 	// There is no need to stop slave thread before coping non-Innodb data when
 	// --no-lock option is used because --no-lock option requires that no DDL or
 	// DML to non-transaction tables can occur.

extra/mariabackup/xtrabackup.cc

@@ -1072,7 +1072,19 @@ static void backup_file_op_fail(uint32_t space_id, int type,
 				filename_to_spacename(name, len).c_str());
 		msg("DDL tracking : delete %" PRIu32 " \"%.*s\"",
 			space_id, int(len), name);
-                error= "delete";
+		if (fail && !opt_no_lock &&
+		    check_if_fts_table(
+		      filename_to_spacename(name, len).c_str())) {
+			/* Ignore the FTS internal table because InnoDB does
+			drop intermediate table and their associative FTS
+			internal table as a part of inplace rollback operation.
+			backup_set_alter_copy_lock() downgrades the
+			MDL_BACKUP_DDL before inplace phase of alter.
+			This leads to the FTS internal table being
+			dropped in the late phase of backup. */
+			fail = false;
+		}
+		error= "delete";
 		break;
 	default:
 		ut_ad(0);

mysql-test/suite/mariabackup/alter_copy_race.result

@@ -22,3 +22,21 @@ SELECT * FROM t1;
 i
 1
 DROP TABLE t1;
+#
+#  MDEV-38041 MariaBackup fails during rollback of
+#		inplace FTS alter table
+#
+CREATE TABLE t1(f1 int, f2 char(100), fulltext(f2)) ENGINE=InnoDB;
+INSERT into t1 values(1, "MariaDB"), (1, "MySQL");
+connect  con2, localhost, root,,;
+set lock_wait_timeout=1;
+SET debug_sync='copy_data_between_tables_before_reset_backup_lock  SIGNAL go WAIT_FOR after_backup_stage_block_commit';
+SET debug_sync='alter_table_after_temp_table_drop SIGNAL temp_table_dropped';
+SET debug_sync='now WAIT_FOR after_backup_stage_start';ALTER TABLE test.t1 ADD PRIMARY KEY(f1), algorithm=COPY;|
+connection default;
+connection con2;
+ERROR 23000: Duplicate entry '1' for key 'PRIMARY'
+SET debug_sync='RESET';
+disconnect con2;
+connection default;
+DROP TABLE t1;

mysql-test/suite/mariabackup/alter_copy_race.test

@@ -49,3 +49,42 @@ exec $XTRABACKUP  --prepare --target-dir=$targetdir;
 SELECT * FROM t1;
 DROP TABLE t1;
 rmdir $targetdir;
+
+--echo #
+--echo #  MDEV-38041 MariaBackup fails during rollback of
+--echo #		inplace FTS alter table
+--echo #
+
+let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
+
+CREATE TABLE t1(f1 int, f2 char(100), fulltext(f2)) ENGINE=InnoDB;
+INSERT into t1 values(1, "MariaDB"), (1, "MySQL");
+
+connect  con2, localhost, root,,;
+set lock_wait_timeout=1;
+SET debug_sync='copy_data_between_tables_before_reset_backup_lock  SIGNAL go WAIT_FOR after_backup_stage_block_commit';
+SET debug_sync='alter_table_after_temp_table_drop SIGNAL temp_table_dropped';
+DELIMITER |;
+send  SET debug_sync='now WAIT_FOR after_backup_stage_start';ALTER TABLE test.t1 ADD PRIMARY KEY(f1), algorithm=COPY;|
+DELIMITER ;|
+connection default;
+
+# setup mariabackup events
+let backup_fix_ddl=;
+let after_backup_stage_start=SET debug_sync='now SIGNAL after_backup_stage_start WAIT_FOR go';
+let after_backup_stage_block_commit=SET debug_sync='now SIGNAL after_backup_stage_block_commit';
+let after_backup_fix_ddl=SET debug_sync='now WAIT_FOR temp_table_dropped';
+let $backup_log=$MYSQLTEST_VARDIR/tmp/backup.log;
+--disable_result_log
+exec  $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf  --backup  --target-dir=$targetdir --dbug=+d,mariabackup_events > $backup_log 2>&1;
+--enable_result_log
+
+connection con2;
+--error ER_DUP_ENTRY
+reap;
+SET debug_sync='RESET';
+disconnect con2;
+
+connection default;
+DROP TABLE t1;
+rmdir $targetdir;