MDEV-6956:SET STATEMENT default_master_connection = ... has no effect

the problem was in assigning default value during parsing.

mysql-test/include/show_all_slaves_status.inc

+# ==== Usage ====
+#
+# --let $status_items= Column_Name[, Column_Name[, ...]]
+# --source include/show_all_slaves_status.inc
+#
+# Parameters:
+#   $status_items
+#     Set to the name of the column in the output of SHOW SLAVE STATUS
+#     that you want to display. Example:
+#
+#       --let $status_items= Master_SSL_Allowed
+#
+#     You can show multiple columns by setting $status_items to a
+#     comma-separated list. Example:
+#
+#       --let $status_items= Master_Log_File, Relay_Master_Log_File
+#
+#   $slave_field_result_replace
+#     If set, one or more regex patterns for replacing variable
+#     text in the error message. Syntax as --replace-regex
+#
+#   $slave_sql_mode
+#     If set, change the slave sql mode during this macro, reverting
+#     to the previous on exit. Default sql_mode is NO_BACKSLASH_ESCAPES
+#     to allow replace '\' by '/' making paths OS independent. Example:
+#
+#       --let $slave_sql_mode= NO_BACKSLASH_ESCAPES
+#
+
+
+--let $_show_slave_status_items=$status_items
+if (!$status_items)
+{
+  --die Bug in test case: The mysqltest variable $status_items is not set.
+}
+
+
+--let $_slave_sql_mode= NO_BACKSLASH_ESCAPES
+if ($slave_sql_mode)
+{
+  --let $_slave_sql_mode= $slave_sql_mode
+}
+--let $_previous_slave_sql_mode = `SELECT @@sql_mode`
+--disable_query_log
+eval SET sql_mode= '$_slave_sql_mode';
+--enable_query_log
+
+
+while ($_show_slave_status_items) 
+{
+  --let $_show_slave_status_name= `SELECT SUBSTRING_INDEX('$_show_slave_status_items', ',', 1)`
+  --let $_show_slave_status_items= `SELECT LTRIM(SUBSTRING('$_show_slave_status_items', LENGTH('$_show_slave_status_name') + 2))`
+
+  --replace_regex /\.[\\\/]master/master/
+  --let $_show_slave_status_value= query_get_value(SHOW ALL SLAVES STATUS, $_show_slave_status_name, 1)
+  --let $_slave_field_result_replace= /[\\\\]/\// $slave_field_result_replace
+  --replace_regex $_slave_field_result_replace
+  --let $_show_slave_status_value= `SELECT REPLACE("$_show_slave_status_value", '$MYSQL_TEST_DIR', 'MYSQL_TEST_DIR')`
+  --echo $_show_slave_status_name = '$_show_slave_status_value'
+}
+
+
+--disable_query_log
+eval SET sql_mode= '$_previous_slave_sql_mode';
+--enable_query_log

mysql-test/r/set_statement_notembedded.result

@@ -16,7 +16,3 @@ Max_statement_time_exceeded	1
 SELECT @@MAX_STATEMENT_TIME;
 @@MAX_STATEMENT_TIME
 0.000000
-set statement default_master_connection=default for select 1;
-ERROR 42000: The system variable default_master_connection cannot be set in SET STATEMENT.
-set statement default_master_connection=default for select 1;
-ERROR 42000: The system variable default_master_connection cannot be set in SET STATEMENT.

mysql-test/suite/rpl/r/rpl_set_statement_default_master.result

+include/master-slave.inc
+[connection master]
+include/stop_slave.inc
+RESET SLAVE ALL;
+# Does not work for CHANGE MASTER:
+SET STATEMENT default_master_connection = 'm1' FOR 
+CHANGE MASTER TO MASTER_HOST='127.0.0.1', MASTER_PORT=MASTER_MYPORT, MASTER_USER='root';
+#
+# The first field, Connection_name, should say 'm1'...
+#
+Connection_name = 'm1'
+RESET SLAVE ALL;
+CHANGE MASTER 'm1' TO MASTER_HOST='127.0.0.1', MASTER_PORT=MASTER_MYPORT, MASTER_USER='root';
+SET STATEMENT default_master_connection = 'm1' FOR START SLAVE;
+set default_master_connection = 'm1';
+stop slave;
+include/wait_for_slave_to_stop.inc
+reset slave all;
+set default_master_connection = '';

mysql-test/suite/rpl/t/rpl_set_statement_default_master.test

+--source include/master-slave.inc
+
+--connection slave
+
+--source include/stop_slave.inc
+RESET SLAVE ALL;
+
+--echo # Does not work for CHANGE MASTER:
+--replace_result $MASTER_MYPORT MASTER_MYPORT
+eval SET STATEMENT default_master_connection = 'm1' FOR 
+  CHANGE MASTER TO MASTER_HOST='127.0.0.1', MASTER_PORT=$MASTER_MYPORT, MASTER_USER='root';
+
+
+--echo #
+--echo # The first field, Connection_name, should say 'm1'...
+--echo #
+--let $status_items= Connection_name
+--source include/show_all_slaves_status.inc
+#query_vertical SHOW ALL SLAVES STATUS;
+
+
+RESET SLAVE ALL;
+
+--replace_result $MASTER_MYPORT MASTER_MYPORT
+eval CHANGE MASTER 'm1' TO MASTER_HOST='127.0.0.1', MASTER_PORT=$MASTER_MYPORT, MASTER_USER='root';
+SET STATEMENT default_master_connection = 'm1' FOR START SLAVE;
+
+set default_master_connection = 'm1';
+stop slave;
+--source include/wait_for_slave_to_stop.inc
+reset slave all;
+set default_master_connection = '';
+--disconnect slave
+--connection default

mysql-test/t/set_statement_notembedded.test

@@ -10,8 +10,4 @@ SET STATEMENT MAX_STATEMENT_TIME=2 FOR SELECT SLEEP(3);
 SHOW STATUS LIKE "max_statement_time_exceeded";
 SELECT @@MAX_STATEMENT_TIME;
 
---error ER_SET_STATEMENT_NOT_SUPPORTED
-set statement default_master_connection=default for select 1;
---error ER_SET_STATEMENT_NOT_SUPPORTED
-set statement default_master_connection=default for select 1;
 

sql/sql_parse.cc

@@ -2730,6 +2730,9 @@ mysql_execute_command(THD *thd)
     }
   }
 
+  if (thd->lex->mi.connection_name.str == NULL)
+      thd->lex->mi.connection_name= thd->variables.default_master_connection;
+
   /*
     Force statement logging for DDL commands to allow us to update
     privilege, system or statistic tables directly without the updates

sql/sql_yacc.yy

@@ -2443,7 +2443,7 @@ optional_connection_name:
           /* empty */
           {
             LEX *lex= thd->lex;
-            lex->mi.connection_name= thd->variables.default_master_connection;
+            lex->mi.connection_name= null_lex_str;
           }
         | connection_name;
         ;
@@ -12674,7 +12674,7 @@ show_param:
         | SLAVE STATUS_SYM
           {
             LEX *lex= thd->lex;
-            lex->mi.connection_name= thd->variables.default_master_connection;
+            lex->mi.connection_name= null_lex_str;
             lex->sql_command = SQLCOM_SHOW_SLAVE_STAT;
             lex->verbose= 0;
           }

sql/sys_vars.cc

@@ -993,7 +993,7 @@ static bool check_master_connection(sys_var *self, THD *thd, set_var *var)
 static Sys_var_session_lexstring Sys_default_master_connection(
        "default_master_connection",
        "Master connection to use for all slave variables and slave commands",
-       NO_SET_STMT SESSION_ONLY(default_master_connection),
+       SESSION_ONLY(default_master_connection),
        NO_CMD_LINE, IN_SYSTEM_CHARSET,
        DEFAULT(""), MAX_CONNECTION_NAME, ON_CHECK(check_master_connection));
 #endif