MDEV-18875 Assertion `thd->transaction.stmt.ha_list == __null || trans == &thd->transaction.stmt' failed or bogus ER_DUP_ENTRY upon ALTER TABLE with versioning

Cause:
* when autocommit=0 (or transaction is issued by user),
 `ha_commit_trans` is called twice on ALTER TABLE, causing a duplicated
 insert into `transaction_registry` (ER_DUP_ENTRY).

Solution:
* ALTER TABLE makes an implicit commit by a second call. We actually
 need to make an insert only when it is a real commit. So is_real
 variable is additionally checked.

Author: FooBarrior

mysql-test/suite/versioning/r/trx_id.result

@@ -476,3 +476,16 @@ SET @@SYSTEM_VERSIONING_ALTER_HISTORY=ERROR;
 SELECT count(*) from mysql.transaction_registry where begin_timestamp>=commit_timestamp;
 count(*)
 0
+# MDEV-18875 Assertion `thd->transaction.stmt.ha_list == __null ||
+# trans == &thd->transaction.stmt' failed or bogus ER_DUP_ENTRY upon
+# ALTER TABLE with versioning
+create or replace table t (x int) engine=innodb;
+set autocommit= 0;
+alter table t
+algorithm=copy,
+add column row_start bigint unsigned as row start,
+add column row_end bigint unsigned as row end,
+add period for system_time(row_start,row_end),
+with system versioning;
+set autocommit= 1;
+create or replace database test;

mysql-test/suite/versioning/t/trx_id.test

@@ -498,3 +498,18 @@ DROP TABLE t;
 SET @@SYSTEM_VERSIONING_ALTER_HISTORY=ERROR;
 
 SELECT count(*) from mysql.transaction_registry where begin_timestamp>=commit_timestamp;
+
+--echo # MDEV-18875 Assertion `thd->transaction.stmt.ha_list == __null ||
+--echo # trans == &thd->transaction.stmt' failed or bogus ER_DUP_ENTRY upon
+--echo # ALTER TABLE with versioning
+create or replace table t (x int) engine=innodb;
+set autocommit= 0;
+alter table t
+  algorithm=copy,
+  add column row_start bigint unsigned as row start,
+  add column row_end bigint unsigned as row end,
+  add period for system_time(row_start,row_end),
+  with system versioning;
+set autocommit= 1;
+
+create or replace database test;

sql/handler.cc

@@ -1446,7 +1446,8 @@ int ha_commit_trans(THD *thd, bool all)
 
 #if 1 // FIXME: This should be done in ha_prepare().
   if (rw_trans || (thd->lex->sql_command == SQLCOM_ALTER_TABLE &&
-                   thd->lex->alter_info.flags & ALTER_ADD_SYSTEM_VERSIONING))
+                   thd->lex->alter_info.flags & ALTER_ADD_SYSTEM_VERSIONING &&
+                   is_real_trans))
   {
     ulonglong trx_start_id= 0, trx_end_id= 0;
     for (Ha_trx_info *ha_info= trans->ha_list; ha_info; ha_info= ha_info->next())