Add gc header size to returned sizeof information

Currently, sys.getsizeof only returns the objects size without taking the
GC_HEAD into account.
This patch addresses this issue.

[Martin v. Löwis]

http://codereview.appspot.com/2414/diff/24/6
File Doc/library/sys.rst (right):

http://codereview.appspot.com/2414/diff/24/6#newcode419
Line 419: func:`getsizeof` calls the object's __sizeof__ method and add's an
additional
add's -> adds

http://codereview.appspot.com/2414/diff/24/7
File Lib/test/test_sys.py (right):

http://codereview.appspot.com/2414/diff/24/7#newcode436
Line 436: if type(o).__flags__ & SizeofTest.TPFLAGS_HAVE_GC:
alternatively: self.TPFLAGS_HAVE_GC

http://codereview.appspot.com/2414/diff/24/8
File Modules/_testcapimodule.c (right):

http://codereview.appspot.com/2414/diff/24/8#newcode970
Line 970: PyModule_AddObject(m, "GC_HEADSIZE",
PyInt_FromSsize_t(sizeof(PyGC_Head)));
Notice that the constants above are upper-case because they expose the
likewise-spelled C macros, so this doesn't *have* to be upper-case. PEP8 is
silent on this specific issue. 

If this is meant to follow the autoconf convention, it should be
SIZEOF_PYGC_HEAD (i.e. SIZEOF_T).

http://codereview.appspot.com/2414/diff/24/5
File Python/sysmodule.c (right):

http://codereview.appspot.com/2414/diff/24/5#newcode660
Line 660: Py_DECREF(res);
res is still NULL here, so you can't DECREF it.

http://codereview.appspot.com/2414/diff/24/5#newcode689
Line 689: Py_DECREF(gc_head_size);
You meant to preserve gc_head_size across this call, so you shouldn't decref it.
Instead, you should decref the old value of res (which you don't keep in a
variable).

Alternative to PyNumber_Add, you can also try to do the computation in
Py_ssize_t (i.e. using asssize/+/fromssize)

[Martin v. Löwis]

http://codereview.appspot.com/2414/diff/12/14
File Doc/library/sys.rst (right):

http://codereview.appspot.com/2414/diff/12/14#newcode412
Line 412: .. function:: getsizeof(object[, default][, gc_head=True])
I would like to reconsider the addition of the gc_head parameter. I still can't
understand why anybody would like to omit this memory usage from the size of the
object. If there is a real need for such a parameter, it can always be added
later; taking it back if not needed is more difficult.

http://codereview.appspot.com/2414/diff/12/13
File Python/sysmodule.c (right):

http://codereview.appspot.com/2414/diff/12/13#newcode688
Line 688: /* failed. is default value is given? */
"Has a default value been given" or "Was a default value requested"?