MDEV-5426: Assertion `toku_ft_needed_unlocked(src_h)' failed (errno=11) ...

- the problem was caused by EXPLAIN INSERT SELECT. For that statement, 
  the code would call select_insert::prepare2(), which would call 
  handler->ha_start_bulk_insert().  The corresponding handler->end_bulk_insert() 
  call is made from select_insert::send_eof or select_insert::abort_result_set 
  which are never called for EXPLAIN INSERT SELECT.
- Fixed by re-using approach of mysql-5.6: don't call ha_start_bulk_insert() if 
  we are in EXPLAIN.

sql/sql_insert.cc

@@ -3555,7 +3555,8 @@ int select_insert::prepare2(void)
 {
   DBUG_ENTER("select_insert::prepare2");
   if (thd->lex->current_select->options & OPTION_BUFFER_RESULT &&
-      thd->locked_tables_mode <= LTM_LOCK_TABLES)
+      thd->locked_tables_mode <= LTM_LOCK_TABLES &&
+      !thd->lex->describe)
     table->file->ha_start_bulk_insert((ha_rows) 0);
   DBUG_RETURN(0);
 }

storage/tokudb/mysql-test/tokudb_mariadb/r/mdev5426.result

+CREATE TABLE t1 (i INT) ENGINE=TokuDB;
+EXPLAIN INSERT INTO t1 SELECT * FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	1	Using temporary
+INSERT INTO t1 SELECT * FROM t1;
+DROP TABLE t1;

storage/tokudb/mysql-test/tokudb_mariadb/t/mdev5426.test

+
+CREATE TABLE t1 (i INT) ENGINE=TokuDB;
+EXPLAIN INSERT INTO t1 SELECT * FROM t1;
+
+--connect con1,localhost,root,,test
+INSERT INTO t1 SELECT * FROM t1;
+
+--connection default
+--disconnect con1
+DROP TABLE t1;