MDEV-13201 Assertion `srv_undo_sources || ...` failed on shutdown during DDL operation

dict_stats_exec_sql(): Refuse the operation if shutdown has been
initiated.

The real fix would be to update the persistent statistics as part
of the data dictionary transactions. To do this, we should move the
storage of InnoDB persistent statistics to the InnoDB data files,
and maybe also remove the InnoDB data dictionary.

mysql-test/suite/innodb/r/truncate_restart.result

+call mtr.add_suppression("InnoDB: Cannot save table statistics for table `test`\\.`t1`: Persistent statistics do not exist");
+SET GLOBAL innodb_stats_persistent= ON;
+CREATE TABLE t1 (t TEXT) ENGINE=InnoDB;
+connect  con1,localhost,root,,test;
+SET DEBUG_SYNC='ib_trunc_table_trunc_completing SIGNAL committed WAIT_FOR ever';
+TRUNCATE TABLE t1;
+connection default;
+SET DEBUG_SYNC='now WAIT_FOR committed';
+disconnect con1;
+SELECT COUNT(*) FROM t1;
+COUNT(*)
+0
+DROP TABLE t1;

mysql-test/suite/innodb/t/truncate_restart.test

+--source include/have_innodb.inc
+--source include/have_debug.inc
+--source include/have_debug_sync.inc
+
+call mtr.add_suppression("InnoDB: Cannot save table statistics for table `test`\\.`t1`: Persistent statistics do not exist");
+
+SET GLOBAL innodb_stats_persistent= ON;
+CREATE TABLE t1 (t TEXT) ENGINE=InnoDB;
+--connect (con1,localhost,root,,test)
+SET DEBUG_SYNC='ib_trunc_table_trunc_completing SIGNAL committed WAIT_FOR ever';
+--send
+TRUNCATE TABLE t1;
+--connection default
+SET DEBUG_SYNC='now WAIT_FOR committed';
+--source include/restart_mysqld.inc
+--disconnect con1
+SELECT COUNT(*) FROM t1;
+DROP TABLE t1;

storage/innobase/dict/dict0stats.cc

@@ -293,7 +293,10 @@ dict_stats_exec_sql(
 	ut_ad(rw_lock_own(dict_operation_lock, RW_LOCK_X));
 	ut_ad(mutex_own(&dict_sys->mutex));
 
-	if (!dict_stats_persistent_storage_check(true)) {
+	extern bool dict_stats_start_shutdown;
+
+	if (dict_stats_start_shutdown
+	    || !dict_stats_persistent_storage_check(true)) {
 		pars_info_free(pinfo);
 		return(DB_STATS_DO_NOT_EXIST);
 	}