Fix for bug bug#23651 "Server crashes when trigger which uses

stored function invoked from different connections".

Invocation of trigger which was using stored function from different
connections caused server crashes (for non-debug server this happened
in highly concurrent environment, but debug server failed on assertion
in relatively simple scenario).

Item_func_sp was not safe to use in triggers (in other words for
re-execution from different threads) as artificial TABLE object
pointed by Item_func_sp::dummy_table referenced incorrect THD
object. To fix the problem we force re-initialization of this
object for each re-execution of statement.

mysql-test/r/trigger.result

@@ -1241,4 +1241,19 @@ i	j
 2	2
 13	13
 drop table t1;
+drop table if exists t1;
+drop function if exists f1;
+create table t1 (i int);
+create function f1() returns int return 10;
+create trigger t1_bi before insert on t1 for each row set @a:= f1() + 10;
+insert into t1 values ();
+select @a;
+@a
+20
+insert into t1 values ();
+select @a;
+@a
+20
+drop table t1;
+drop function f1;
 End of 5.0 tests

mysql-test/t/trigger.test

@@ -1499,4 +1499,24 @@ select * from t1;
 drop table t1;
 
 
+#
+# Bug #23651 "Server crashes when trigger which uses stored function
+#             invoked from different connections".
+#
+--disable_warnings
+drop table if exists t1;
+drop function if exists f1;
+--enable_warnings
+create table t1 (i int);
+create function f1() returns int return 10;
+create trigger t1_bi before insert on t1 for each row set @a:= f1() + 10;
+insert into t1 values ();
+select @a;
+connection addconroot1;
+insert into t1 values ();
+select @a;
+connection default;
+drop table t1;
+drop function f1;
+
 --echo End of 5.0 tests

sql/item_func.cc

@@ -4845,6 +4845,7 @@ Item_func_sp::cleanup()
     result_field= NULL;
   }
   m_sp= NULL;
+  dummy_table->s= NULL;
   Item_func::cleanup();
 }