MDEV-21887: federatedx crashes on SELECT ... INTO query in select_handler code

- Don't try to push down SELECTs that have a side effect

- In case the storage engine did support pushdown of SELECT with an INTO
  clause, write the rows we've got from it into select->join->result,
  and not thd->protocol.  This way, SELECT ... INTO ... FROM
  smart_engine_table will put the result into where instructed, and
  NOT send it to the client.

mysql-test/suite/federated/federatedx_create_handlers.result

@@ -301,7 +301,27 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	7	
 1	PRIMARY	<derived2>	ref	key0	key0	18	federated.t3.name	2	
 2	PUSHED DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL
-DROP TABLE federated.t1, federated.t2, federated.t3;
+#
+# MDEV-21887: federatedx crashes on SELECT ... INTO query in select_handler code
+#
+CREATE TABLE federated.t4 (
+id int(20) NOT NULL,
+name varchar(16) NOT NULL default ''
+) engine=myisam;
+insert into federated.t4 select * from federated.t1;
+select * from federated.t4;
+id	name
+1	zzz
+3	xxx
+4	xxx
+5	yyy
+7	yyy
+select name into @var from federated.t1 where id=3 limit 1 ;
+select @var;
+@var
+xxx
+select name into outfile 'tmp.txt' from federated.t1;
+DROP TABLE federated.t1, federated.t2, federated.t3, federated.t4;
 connection slave;
 DROP TABLE federated.t1, federated.t2;
 connection default;

mysql-test/suite/federated/federatedx_create_handlers.test

@@ -147,8 +147,27 @@ FROM federated.t3,
        SELECT * FROM federated.t1 WHERE id >= 5) t
 WHERE federated.t3.name=t.name;
 
+--echo #
+--echo # MDEV-21887: federatedx crashes on SELECT ... INTO query in select_handler code
+--echo #
 
-DROP TABLE federated.t1, federated.t2, federated.t3;
+CREATE TABLE federated.t4 (
+  id int(20) NOT NULL,
+  name varchar(16) NOT NULL default ''
+) engine=myisam;
+insert into federated.t4 select * from federated.t1;
+
+--sorted_result
+select * from federated.t4;
+
+select name into @var from federated.t1 where id=3 limit 1 ;
+select @var;
+select name into outfile 'tmp.txt' from federated.t1;
+
+let $path=`select concat(@@datadir, 'test/tmp.txt')`;
+remove_file $path;
+
+DROP TABLE federated.t1, federated.t2, federated.t3, federated.t4;
 
 connection slave;
 DROP TABLE federated.t1, federated.t2;

sql/select_handler.cc

@@ -77,18 +77,17 @@ bool Pushdown_select::init()
 
 bool Pushdown_select::send_result_set_metadata()
 {
-  THD *thd= handler->thd;
-  Protocol *protocol= thd->protocol;
   DBUG_ENTER("Pushdown_select::send_result_set_metadata");
 
 #ifdef WITH_WSREP
+  THD *thd= handler->thd;
   if (WSREP(thd) && thd->wsrep_retry_query)
   {
     WSREP_DEBUG("skipping select metadata");
     DBUG_RETURN(false);
   }
   #endif /* WITH_WSREP */
-  if (protocol->send_result_set_metadata(&result_columns,
+  if (select->join->result->send_result_set_metadata(result_columns,
                                          Protocol::SEND_NUM_ROWS |
                                          Protocol::SEND_EOF))
     DBUG_RETURN(true);
@@ -99,21 +98,10 @@ bool Pushdown_select::send_result_set_metadata()
 
 bool Pushdown_select::send_data()
 {
-  THD *thd= handler->thd;
-  Protocol *protocol= thd->protocol;
   DBUG_ENTER("Pushdown_select::send_data");
 
-  protocol->prepare_for_resend();
-  if (protocol->send_result_set_row(&result_columns))
-  {
-    protocol->remove_last_row();
+  if (select->join->result->send_data(result_columns))
     DBUG_RETURN(true);
-  }
-
-  thd->inc_sent_row_count(1);
-
-  if (thd->vio_ok())
-    DBUG_RETURN(protocol->write());
 
   DBUG_RETURN(false);
 }
@@ -121,16 +109,10 @@ bool Pushdown_select::send_data()
 
 bool Pushdown_select::send_eof()
 {
-  THD *thd= handler->thd;
   DBUG_ENTER("Pushdown_select::send_eof");
 
-  /*
-    Don't send EOF if we're in error condition (which implies we've already
-    sent or are sending an error)
-  */
-  if (thd->is_error())
+  if (select->join->result->send_eof())
     DBUG_RETURN(true);
-  ::my_eof(thd);
   DBUG_RETURN(false);
 }
 

storage/federatedx/federatedx_pushdown.cc

@@ -182,6 +182,16 @@ create_federatedx_select_handler(THD* thd, SELECT_LEX *sel)
       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 uses an "INTO @var" or
+    "INTO OUTFILE". It is also unlikely to work if the select has some
+    other kind of side effect.
+  */
+  if (sel->uncacheable & UNCACHEABLE_SIDEEFFECT)
+    return NULL;
+
   handler= new ha_federatedx_select_handler(thd, sel);
 
   return handler;
@@ -285,9 +295,10 @@ int ha_federatedx_select_handler::end_scan()
 
   DBUG_RETURN(0);
 }
-
-void ha_federatedx_select_handler::print_error(int, unsigned long)
+\
+void ha_federatedx_select_handler::print_error(int error, myf error_flag)
 {
+  select_handler::print_error(error, error_flag);
 }