Bug#52168 decimal casting catastrophes: crashes and valgrind errors on simple casts

The problem is that if a NULL is stored in an Item_cache_decimal object,
the associated my_decimal object is not initialized.  However, it is still
accessed when val_int() is called. The fix is to check for null_value
within val_int(), and return without accessing the my_decimal object when
the cached value is NULL.

Bug#52122 reports the same issue for val_real(), and this patch also includes
fixes for val_real() and val_str() and corresponding test cases from that
bug report.  

Also, NULL is returned from val_decimal() when value is null. This will
avoid that callers access an uninitialized my_decimal object.

Made similar changes to all other Item_cache classes.  Now all val_*
methods should return a well defined value when actual value is NULL.

1 files changed, 9 insertions, 0 deletions

diff --git a/sql/item.h b/sql/item.h
index 5f4f96f97d3..8360fa61498 100644
--- a/sql/item.h
+++ b/sql/item.h
@@ -3191,6 +3191,15 @@ public:
   {
     return this == item;
   }
+  /**
+     Check if saved item has a non-NULL value.
+     Will cache value of saved item if not already done. 
+     @return TRUE if cached value is non-NULL.
+   */
+  bool has_value()
+  {
+    return (value_cached || cache_value()) && !null_value;
+  }
   virtual void store(Item *item);
   virtual bool cache_value()= 0;
   bool basic_const_item() const