MDEV-12851: Case with window functions query crashes server

The "is null" function performs one operation which no other Item_func
does, which is to update used tables during fix_length_and_dec().

This however can not be performed before window functions have had a
chance to resolve their order by and partition by definitions, which
happens after the initial setup_fields call. Consequently, do not call
Item_func_isnull update_used_tables during fix_length_and_dec().

There was another issue detected once the crash was resolved.
Because window functions did not implement is_null() method, we would
end up returning bad results for "is null" and "is not null" functions.
Implemented is_null() method for Item_windowfunc.

mysql-test/r/win.result

@@ -3097,3 +3097,33 @@ a
 300
 300
 drop table t;
+#
+# MDEV-12851 case with window functions query crashes server
+#
+create table t1(dt datetime);
+insert into t1 values ('2017-05-17'), ('2017-05-18');
+select dt,
+case when (max(dt) over (order by dt rows between 1 following and 1 following) is null)
+then '9999-12-31 12:00:00'
+       else max(dt) over (order by dt rows between 1 following and 1 following)
+end x,
+case when (max(dt) over (order by dt rows between 1 following and 1 following) is not null)
+then '9999-12-31 12:00:00'
+       else max(dt) over (order by dt rows between 1 following and 1 following)
+end x
+from t1;
+dt	x	x
+2017-05-17 00:00:00	2017-05-18 00:00:00	9999-12-31 12:00:00
+2017-05-18 00:00:00	9999-12-31 12:00:00	NULL
+drop table t1;
+create table t1(i int);
+insert into t1 values (null),(1),(2);
+select max(i) over (order by i),
+max(i) over (order by i) is null,
+max(i) over (order by i) is not null
+from t1;
+max(i) over (order by i)	max(i) over (order by i) is null	max(i) over (order by i) is not null
+NULL	1	0
+1	0	1
+2	0	1
+drop table t1;

mysql-test/t/win.test

@@ -1887,3 +1887,30 @@ insert into t values (1, 10), (2, 20), (3, 30);
 prepare stmt from "SELECT (CASE WHEN sum(t.a) over (partition by t.b)=1 THEN 1000 ELSE 300 END) AS a FROM t";
 execute stmt;
 drop table t;
+
+--echo #
+--echo # MDEV-12851 case with window functions query crashes server
+--echo #
+
+create table t1(dt datetime);
+insert into t1 values ('2017-05-17'), ('2017-05-18');
+select dt,
+       case when (max(dt) over (order by dt rows between 1 following and 1 following) is null)
+       then '9999-12-31 12:00:00'
+       else max(dt) over (order by dt rows between 1 following and 1 following)
+       end x,
+       case when (max(dt) over (order by dt rows between 1 following and 1 following) is not null)
+       then '9999-12-31 12:00:00'
+       else max(dt) over (order by dt rows between 1 following and 1 following)
+       end x
+from t1;
+
+drop table t1;
+
+create table t1(i int);
+insert into t1 values (null),(1),(2);
+select max(i) over (order by i),
+       max(i) over (order by i) is null,
+       max(i) over (order by i) is not null
+from t1;
+drop table t1;

sql/item_cmpfunc.h

@@ -1770,11 +1770,6 @@ public:
   Item_func_isnull(THD *thd, Item *a): Item_func_null_predicate(thd, a) {}
   longlong val_int();
   enum Functype functype() const { return ISNULL_FUNC; }
-  void fix_length_and_dec()
-  {
-    Item_func_null_predicate::fix_length_and_dec();
-    update_used_tables();
-  }
   const char *func_name() const { return "isnull"; }
   void print(String *str, enum_query_type query_type);
   enum precedence precedence() const { return CMP_PRECEDENCE; }

sql/item_func.cc

@@ -153,8 +153,9 @@ void Item_func::sync_with_sum_func_and_with_field(List<Item> &list)
     is to allow all Item_field() objects to setup pointers to the table fields.
 
     Sets as a side effect the following class variables:
-      maybe_null	Set if any argument may return NULL
-      with_sum_func	Set if any of the arguments contains a sum function
+      maybe_null        Set if any argument may return NULL
+      with_sum_func     Set if any of the arguments contains a sum function
+      with_window_func  Set if any of the arguments contain a window function
       with_field        Set if any of the arguments contains or is a field
       used_tables_cache Set to union of the tables used by arguments
 

sql/item_windowfunc.h

@@ -848,6 +848,17 @@ public:
     read_value_from_result_field= true;
   }
 
+  bool is_null()
+  {
+    if (force_return_blank)
+      return false;
+
+    if (read_value_from_result_field)
+      return result_field->is_null();
+
+    return window_func()->is_null();
+  }
+
   double val_real() 
   {
     double res;