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

Backport to 10.4:

- 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.

1 files changed, 12 insertions, 1 deletions

diff --git a/storage/federatedx/federatedx_pushdown.cc b/storage/federatedx/federatedx_pushdown.cc
index 2bcee943308..15b0b0d3d4e 100644
--- a/storage/federatedx/federatedx_pushdown.cc
+++ b/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;
@@ -286,8 +296,9 @@ 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);
 }