Bug#42733: Type-punning warnings when compiling MySQL --

           strict aliasing violations.

Another rather noisy violation of strict aliasing rules
is the spatial code which makes use of stack-based memory
(of type Geometry_buffer) to provide placement for Geometry
objects. Although a placement new is allowed to dynamically
change the type of a object, the object returned by the
new placement was being ignored and the original stack-based
object was being casted to the new type, thus violating strict
aliasing rules.

The solution is to reorganize the code so that the object
returned by the new placement is used instead of casting the
original object. Also, to ensure that the stack-based object
is properly aligned with respect to the objects it provides
placement for, a set of compiler-dependent macros and types
are introduced so that the alignment of objects can be inquired
and specified.

include/Makefile.am:
  Add new header.
include/my_compiler.h:
  Add new header.
include/my_global.h:
  Remove now-unnecessary macros.
sql/spatial.cc:
  Make object creation functions return the object whose type
  was dynamically changed by the new placement.
  
  Move static method from the header in order to avoid having
  to access a forward declaration.
sql/spatial.h:
  Object creation callbacks now take a array of chars as the
  storage area.
  
  Move create_by_typeid to a source file as to not access the
  forward declaration of Geometry_buffer.
  
  Ensure that Geometry_buffer is properly aligned.
sql/sql_show.cc:
  Use newly added aligned storage helper.

1 files changed, 11 insertions, 17 deletions

diff --git a/sql/spatial.h b/sql/spatial.h
index 86c2ed8c197..67edc077e04 100644
--- a/sql/spatial.h
+++ b/sql/spatial.h
@@ -16,6 +16,8 @@
 #ifndef _spatial_h
 #define _spatial_h
 
+#include <my_compiler.h>
+
 #ifdef HAVE_SPATIAL
 
 const uint SRID_SIZE= 4;
@@ -225,15 +227,18 @@ public:
   {
     wkb_xdr= 0,    /* Big Endian */
     wkb_ndr= 1     /* Little Endian */
-  };                                    
+  };
+
+  /** Callback which creates Geometry objects on top of a given placement. */
+  typedef Geometry *(*create_geom_t)(char *);
 
   class Class_info
   {
   public:
     LEX_STRING m_name;
     int m_type_id;
-    void (*m_create_func)(void *);
-    Class_info(const char *name, int type_id, void(*create_func)(void *));
+    create_geom_t m_create_func;
+    Class_info(const char *name, int type_id, create_geom_t create_func);
   };
 
   virtual const Class_info *get_class_info() const=0;
@@ -263,15 +268,7 @@ public:
   virtual int geometry_n(uint32 num, String *result) const { return -1; }
 
 public:
-  static Geometry *create_by_typeid(Geometry_buffer *buffer, int type_id)
-  {
-    Class_info *ci;
-    if (!(ci= find_class((int) type_id)))
-      return NULL;
-    (*ci->m_create_func)((void *)buffer);
-    return my_reinterpret_cast(Geometry *)(buffer);
-  }
-
+  static Geometry *create_by_typeid(Geometry_buffer *buffer, int type_id);
   static Geometry *construct(Geometry_buffer *buffer,
                              const char *data, uint32 data_len);
   static Geometry *create_from_wkt(Geometry_buffer *buffer,
@@ -528,11 +525,8 @@ public:
   const Class_info *get_class_info() const;
 };
 
-const int geometry_buffer_size= sizeof(Gis_point);
-struct Geometry_buffer
-{
-  void *arr[(geometry_buffer_size - 1)/sizeof(void *) + 1];
-};
+struct Geometry_buffer : public
+  my_aligned_storage<sizeof(Gis_point), MY_ALIGNOF(Gis_point)> {};
 
 #endif /*HAVE_SPATAIAL*/
 #endif