MDEV-7640: CHANGE MASTER TO doesn't work with prepared statements

When CHANGE MASTER was executed as a PS, its attributes were wrongly
getting reset toward the end of PREPARE. As a result, the subsequent
executions had no effect. Fixed by making sure that the CHANGE MASTER
attributes are preserved during the lifetime of the PS.

mysql-test/r/ps_change_master.result

+#
+# CHANGE MASTER TO doesn't work with prepared statements
+#
+CHANGE MASTER TO MASTER_HOST='host1', MASTER_USER='user1';
+# Master_Host : host1
+# Master_User : user1
+SET @s := "CHANGE MASTER TO MASTER_HOST='host2'";
+PREPARE stmt FROM @s;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+# Master_Host : host2
+# Master_User : user1
+SET @s := "CHANGE MASTER TO MASTER_USER='user2'";
+PREPARE stmt FROM @s;
+EXECUTE stmt;
+EXECUTE stmt;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+# Master_Host : host2
+# Master_User : user2
+CHANGE MASTER TO MASTER_HOST='127.0.0.1', MASTER_USER='root';
+# End of test

mysql-test/t/ps_change_master.test

+--source include/not_embedded.inc
+--source include/have_log_bin.inc
+
+--echo #
+--echo # CHANGE MASTER TO doesn't work with prepared statements
+--echo #
+
+CHANGE MASTER TO MASTER_HOST='host1', MASTER_USER='user1';
+
+let $master_host= query_get_value(SHOW SLAVE STATUS, Master_Host, 1);
+let $master_user= query_get_value(SHOW SLAVE STATUS, Master_User, 1);
+
+--echo # Master_Host : $master_host
+--echo # Master_User : $master_user
+
+SET @s := "CHANGE MASTER TO MASTER_HOST='host2'";
+PREPARE stmt FROM @s;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
+let $master_host= query_get_value(SHOW SLAVE STATUS, Master_Host, 1);
+let $master_user= query_get_value(SHOW SLAVE STATUS, Master_User, 1);
+
+--echo # Master_Host : $master_host
+--echo # Master_User : $master_user
+
+SET @s := "CHANGE MASTER TO MASTER_USER='user2'";
+PREPARE stmt FROM @s;
+EXECUTE stmt;
+# Multiple executions should not hurt.
+EXECUTE stmt;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
+let $master_host= query_get_value(SHOW SLAVE STATUS, Master_Host, 1);
+let $master_user= query_get_value(SHOW SLAVE STATUS, Master_User, 1);
+
+--echo # Master_Host : $master_host
+--echo # Master_User : $master_user
+
+
+# Reset
+CHANGE MASTER TO MASTER_HOST='127.0.0.1', MASTER_USER='root';
+
+--echo # End of test

sql/sql_lex.cc

@@ -545,6 +545,16 @@ void lex_end(LEX *lex)
   DBUG_ENTER("lex_end");
   DBUG_PRINT("enter", ("lex: 0x%lx", (long) lex));
 
+  lex_end_stage1(lex);
+  lex_end_stage2(lex);
+
+  DBUG_VOID_RETURN;
+}
+
+void lex_end_stage1(LEX *lex)
+{
+  DBUG_ENTER("lex_end_stage1");
+
   /* release used plugins */
   if (lex->plugins.elements) /* No function call and no mutex if no plugins. */
   {
@@ -556,6 +566,19 @@ void lex_end(LEX *lex)
   delete lex->sphead;
   lex->sphead= NULL;
 
+  DBUG_VOID_RETURN;
+}
+
+/*
+  MASTER INFO parameters (or state) is normally cleared towards the end
+  of a statement. But in case of PS, the state needs to be preserved during
+  its lifetime and should only be cleared on PS close or deallocation.
+*/
+void lex_end_stage2(LEX *lex)
+{
+  DBUG_ENTER("lex_end_stage2");
+
+  /* Reset LEX_MASTER_INFO */
   lex->mi.reset();
 
   DBUG_VOID_RETURN;

sql/sql_lex.h

@@ -2940,6 +2940,8 @@ extern void lex_init(void);
 extern void lex_free(void);
 extern void lex_start(THD *thd);
 extern void lex_end(LEX *lex);
+extern void lex_end_stage1(LEX *lex);
+extern void lex_end_stage2(LEX *lex);
 void end_lex_with_single_table(THD *thd, TABLE *table, LEX *old_lex);
 int init_lex_with_single_table(THD *thd, TABLE *table, LEX *lex);
 extern int MYSQLlex(union YYSTYPE *yylval, THD *thd);

sql/sql_prepare.cc

@@ -3416,7 +3416,8 @@ bool Prepared_statement::prepare(const char *packet, uint packet_len)
     thd->mdl_context.release_transactional_locks();
   }
 
-  lex_end(lex);
+  /* Preserve CHANGE MASTER attributes */
+  lex_end_stage1(lex);
   cleanup_stmt();
   thd->restore_backup_statement(this, &stmt_backup);
   thd->stmt_arena= old_stmt_arena;
@@ -3997,6 +3998,10 @@ void Prepared_statement::deallocate()
 {
   /* We account deallocate in the same manner as mysqld_stmt_close */
   status_var_increment(thd->status_var.com_stmt_close);
+
+  /* It should now be safe to reset CHANGE MASTER parameters */
+  lex_end_stage2(lex);
+
   /* Statement map calls delete stmt on erase */
   thd->stmt_map.erase(this);
 }