CLX-77: INSERT ... SELECT returns rows to the client instead of inserting

Make Pushdown_select write output rows into select->join->result, instead of
thd->protocol.

This makes
- SELECT ... INTO @var
- SELECT ... INTO OUTFILE
- INSERT INTO myisam_table SELECT ... FROM clustrix_table
work as intended.

Also fixed the federatedx select pushdown handler:
- Do not fail an assert if the backend no resultset. Produce an error.
- For the SELECT .. INTO syntax, refuse to use Select Handler, because
the impelementation doesn't support this.

mysql-test/suite/xpand/basics.result

@@ -45,5 +45,27 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	10000	
 2	DERIVED	intandtext	ALL	NULL	NULL	NULL	NULL	10000	
 DROP TABLE intandtext;
+set 
+optimizer_switch=default,
+xpand_derived_handler= default,
+xpand_select_handler=default;
+#
+# CLX-77: INSERT ... SELECT returns rows to the client instead of inserting
+#
+drop table if exists t1,t2;
+create table t1 (a int) engine=xpand;
+insert into t1 values (1);
+select a into @var from t1;
+select @var;
+@var
+1
+# This must not emit output to the client:
+select a into outfile 'tmpfile1' from t1;
+create table t2 (a int) engine=myisam;
+insert into t2 select * from t1;
+select * from t2;
+a
+1
+drop table t1,t2;
 USE test;
 DROP DATABASE xpd;

mysql-test/suite/xpand/basics.test

@@ -34,5 +34,34 @@ EXPLAIN SELECT i,t FROM (SELECT i,t FROM intandtext) t;
 
 DROP TABLE intandtext;
 
+set 
+  optimizer_switch=default,
+  xpand_derived_handler= default,
+  xpand_select_handler=default;
+
+--echo #
+--echo # CLX-77: INSERT ... SELECT returns rows to the client instead of inserting
+--echo #
+--disable_warnings
+drop table if exists t1,t2;
+--enable_warnings
+
+create table t1 (a int) engine=xpand;
+insert into t1 values (1);
+
+select a into @var from t1;
+select @var;
+
+--echo # This must not emit output to the client:
+select a into outfile 'tmpfile1' from t1;
+let $file=`select concat(@@datadir,'/clx/tmpfile1')`;
+
+--remove_file $file
+
+create table t2 (a int) engine=myisam;
+insert into t2 select * from t1;
+select * from t2;
+
+drop table t1,t2;
 USE test;
 DROP DATABASE xpd;

storage/federatedx/federatedx_pushdown.cc

@@ -181,6 +181,15 @@ create_federatedx_select_handler(THD* thd, SELECT_LEX *sel)
     else if (ht != tbl->table->file->partition_ht())
       return 0;
   }
+  
+  /*
+    Currently, ha_federatedx_select_handler::init_scan just takes the 
+    thd->query and sends it to the backend.
+    This obviously won't work if the SELECT has an INTO part.
+    Refuse to work in this case.
+  */
+  if (thd->lex->result)
+    return NULL;
 
   /*
     Currently, ha_federatedx_select_handler::init_scan just takes the