Fix for Bug#56138 "valgrind errors about overlapping memory when double-assigning same variable",

and related small fixes.

mysql-test/t/user_var.test:
  test for bug
sql/field_conv.cc:
  From the C standard, memcpy() has undefined behaviour if to->ptr==from->ptr
sql/item_func.cc:
  In the case of BUG#56138, entry->value==ptr in which case memcpy()
  has undefined results per the C standard.
sql/sql_select.cc:
  Work around a bug in old gcc

mysql-test/r/user_var.result

@@ -447,4 +447,7 @@ IF(
 count(*), 1)
 1
 DROP TABLE t1;
+select @v:=@v:=sum(1) from dual;
+@v:=@v:=sum(1)
+1
 End of 5.1 tests

mysql-test/t/user_var.test

@@ -346,4 +346,11 @@ FROM t1 GROUP BY a LIMIT 1;
 
 DROP TABLE t1;
 
+#
+# BUG#56138 "valgrind errors about overlapping memory when
+# double-assigning same variable"
+#
+
+select @v:=@v:=sum(1) from dual;
+
 --echo End of 5.1 tests

sql/field_conv.cc

@@ -786,11 +786,8 @@ int field_conv(Field *to,Field *from)
          ((Field_varstring*)from)->length_bytes ==
           ((Field_varstring*)to)->length_bytes))
     {						// Identical fields
-#ifdef HAVE_purify
-      /* This may happen if one does 'UPDATE ... SET x=x' */
-      if (to->ptr != from->ptr)
-#endif
-        memcpy(to->ptr,from->ptr,to->pack_length());
+      // to->ptr==from->ptr may happen if one does 'UPDATE ... SET x=x'
+      memmove(to->ptr, from->ptr, to->pack_length());
       return 0;
     }
   }

sql/item_func.cc

@@ -3920,7 +3920,7 @@ update_hash(user_var_entry *entry, bool set_null, void *ptr, uint length,
       length--;					// Fix length change above
       entry->value[length]= 0;			// Store end \0
     }
-    memcpy(entry->value,ptr,length);
+    memmove(entry->value, ptr, length);
     if (type == DECIMAL_RESULT)
       ((my_decimal*)entry->value)->fix_buffer_pointer();
     entry->length= length;

sql/sql_select.cc

@@ -4034,8 +4034,12 @@ update_ref_and_keys(THD *thd, DYNAMIC_ARRAY *keyuse,JOIN_TAB *join_tab,
 	  continue;
       }
 
-#ifdef HAVE_purify
-      /* Valgrind complains about overlapped memcpy when save_pos==use. */
+#if defined(__GNUC__) && !MY_GNUC_PREREQ(4,4)
+      /*
+        Old gcc used a memcpy(), which is undefined if save_pos==use:
+        http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19410
+        http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39480
+      */
       if (save_pos != use)
 #endif
         *save_pos= *use;