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.

3 files changed, 14 insertions, 7 deletions

diff --git a/sql/item_cmpfunc.h b/sql/item_cmpfunc.h
index c81a1621f8a..2f98c68bb5f 100644
--- a/sql/item_cmpfunc.h
+++ b/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; }
diff --git a/sql/item_func.cc b/sql/item_func.cc
index 8529a1d08ca..1b83f86d19c 100644
--- a/sql/item_func.cc
+++ b/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
 
diff --git a/sql/item_windowfunc.h b/sql/item_windowfunc.h
index b4953c8a6ac..0dee60915f8 100644
--- a/sql/item_windowfunc.h
+++ b/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;