MDEV-20354 All but last insert ignored in InnoDB tables when table locked

mysql_insert() first opens all affected tables (which implicitly
starts a transaction in InnoDB), then stat tables.
A failure to open a stat table caused open_tables() to abort
the current stmt transaction (trans_rollback_stmt()). So, from the
server point of view the following ha_write_row()-s happened outside
of a transactions, and the server didn't bother to commit them.

The server has a mechanism to prevent a transaction being
unexpectedly committed or rolled back in the middle of a statement -
if an operation takes place _in a sub-statement_ it cannot change
the transaction state. Operations on stat tables are exactly that -
they are not allowed to change a transaction state. Put them in
a sub-statement to make sure they don't.

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

+rename table mysql.table_stats to mysql.table_stats_save;
+flush tables;
+set use_stat_tables= PREFERABLY;
+create table t1 (a int) engine=InnoDB;
+start transaction;
+insert t1 values (1);
+insert t1 values (2);
+commit;
+select * from t1;
+a
+1
+2
+drop table t1;
+rename table mysql.table_stats_save to mysql.table_stats;
+flush tables;

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

+source include/have_innodb.inc;
+
+#
+# MDEV-20354 All but last insert ignored in InnoDB tables when table locked
+#
+rename table mysql.table_stats to mysql.table_stats_save;
+flush tables;
+set use_stat_tables= PREFERABLY;
+create table t1 (a int) engine=InnoDB;
+start transaction;
+insert t1 values (1);
+insert t1 values (2);
+commit;
+select * from t1;
+drop table t1;
+rename table mysql.table_stats_save to mysql.table_stats;
+flush tables;

sql/sql_class.h

@@ -1444,12 +1444,16 @@ class Open_tables_backup: public Open_tables_state
 /**
   @class Sub_statement_state
   @brief Used to save context when executing a function or trigger
+
+  operations on stat tables aren't technically a sub-statement, but they are
+  similar in a sense that they cannot change the transaction status.
 */
 
 /* Defines used for Sub_statement_state::in_sub_stmt */
 
 #define SUB_STMT_TRIGGER 1
 #define SUB_STMT_FUNCTION 2
+#define SUB_STMT_STAT_TABLES 4
 
 
 class Sub_statement_state

sql/sql_statistics.cc

@@ -268,7 +268,9 @@ inline int open_stat_tables(THD *thd, TABLE_LIST *tables,
   thd->push_internal_handler(&deh);
   init_table_list_for_stat_tables(tables, for_write);
   init_mdl_requests(tables);
+  thd->in_sub_stmt|= SUB_STMT_STAT_TABLES;
   rc= open_system_tables_for_read(thd, tables, backup);
+  thd->in_sub_stmt&= ~SUB_STMT_STAT_TABLES;
   thd->pop_internal_handler();