Restoring a backup may result in garbage intermediate tables from DDL

MDEV-17567 and MDEV-25180 changed something in InnoDB startup. We forgot to adjust Mariabackup accordingly.

Specifically, the command mariabackup --prepare --export is supposed to purge the history of old transactions, and also apply all logs.

Starting with MDEV-25180, InnoDB no longer treats specially any tables whose names start with #sql. The only exception are table names that start with #sql-ib. It is the responsibility of the SQL layer's DDL log recovery to drop any such tables, in case the server was killed (or a backup was made) while a table-rebuilding DDL operation was in progress.

The following patch proves that a regression was introduced. If the function (which would start the InnoDB purge of transaction history) would be invoked during mariabackup --prepare --export, the execution would be aborted due to assertion failure.

diff --git a/storage/innobase/handler/ha_innodb.cc b/storage/innobase/handler/ha_innodb.cc

index db5c8524776..73f7a1aa248 100644

--- a/storage/innobase/handler/ha_innodb.cc

+++ b/storage/innobase/handler/ha_innodb.cc

@@ -1954,6 +1954,7 @@ static int innodb_check_version(handlerton *hton, const char *path,

 static void innodb_ddl_recovery_done(handlerton*)

 {

   ut_ad(!ddl_recovery_done);

+  ut_a(srv_operation != SRV_OPERATION_RESTORE_EXPORT);

   ut_d(ddl_recovery_done= true);

   if (!srv_read_only_mode && srv_operation == SRV_OPERATION_NORMAL &&

       srv_force_recovery < SRV_FORCE_NO_BACKGROUND)

We must ensure that ha_signal_ddl_recovery_done() will be invoked. Should it for some reason be unacceptable to invoke ddl_log_execute_recovery() in mariabackup --prepare --export, then we would have to invoke special code that would drop all #sql- tables from InnoDB, before executing ha_signal_ddl_recovery_done(). In any case, we must somehow compensate for the change that was made in MDEV-25666.

Note: Nothing must change in mariabackup outside --prepare --export. We do not want to ruin incremental backups by executing actions that would generate InnoDB redo log records.