MDEV-32640 Reset thd->lex->mi.connection_name.str towards the end of mysql_execute_command

Reset the connection_name to contain a null string, if the pointer
points to the same space as that of the system variable
default_master_connection.

We do this because the system variable may be updated which could free
the pointer and create a new one, causing use-after-free for
re-execution of prepared statements and stored procedures where the
LEX may be reused.

This allows connection_name to be set again be to the system variable
pointer in the next call of this function (see earlier in this
function), after any possible updates to the system variable.

mysql-test/suite/sys_vars/r/mdev_32640.result

+PREPARE s_1 FROM 'SHOW RELAYLOG EVENTS';
+/* 1 */ SET default_master_connection='MASTER';
+/* 1 */ EXECUTE s_1;
+ERROR HY000: There is no master connection 'MASTER'
+/* 2 */ SET default_master_connection='MASTER';
+/* 2 */ EXECUTE s_1;
+ERROR HY000: There is no master connection 'MASTER'
+create procedure p() SHOW RELAYLOG EVENTS;
+/* 1 */ SET default_master_connection='MASTER';
+/* 1 */ call p;
+ERROR HY000: There is no master connection 'MASTER'
+/* 2 */ SET default_master_connection='MASTER';
+/* 2 */ call p;
+ERROR HY000: There is no master connection 'MASTER'
+drop procedure p;

mysql-test/suite/sys_vars/t/mdev_32640.test

+--source include/not_embedded.inc
+
+PREPARE s_1 FROM 'SHOW RELAYLOG EVENTS';
+/* 1 */ SET default_master_connection='MASTER';
+--error WARN_NO_MASTER_INFO
+/* 1 */ EXECUTE s_1;
+/* 2 */ SET default_master_connection='MASTER';
+--error WARN_NO_MASTER_INFO
+/* 2 */ EXECUTE s_1;
+
+create procedure p() SHOW RELAYLOG EVENTS;
+/* 1 */ SET default_master_connection='MASTER';
+--error WARN_NO_MASTER_INFO
+/* 1 */ call p;
+/* 2 */ SET default_master_connection='MASTER';
+--error WARN_NO_MASTER_INFO
+/* 2 */ call p;
+drop procedure p;

sql/sql_parse.cc

@@ -5993,6 +5993,24 @@ mysql_execute_command(THD *thd, bool is_called_from_prepared_stmt)
   thd->wsrep_PA_safe= true;
 #endif /* WITH_WSREP */
 
+  /*
+    Reset the connection_name to contain a null string, if the
+    pointer points to the same space as that of the system variable
+    default_master_connection.
+
+    We do this because the system variable may be updated which could
+    free the pointer and create a new one, causing use-after-free for
+    re-execution of prepared statements and stored procedures where
+    the LEX may be reused.
+
+    This allows connection_name to be set again be to the system
+    variable pointer in the next call of this function (see earlier in
+    this function), after any possible updates to the system variable.
+  */
+  if (thd->lex->mi.connection_name.str ==
+      thd->variables.default_master_connection.str)
+    thd->lex->mi.connection_name= null_clex_str;
+
   if (lex->sql_command != SQLCOM_SET_OPTION)
     DEBUG_SYNC(thd, "end_of_statement");
   DBUG_RETURN(res || thd->is_error());