Add gc header size to returned sizeof information

Lib/test/test_sys.py

Index: Lib/test/test_sys.py
===================================================================
--- Lib/test/test_sys.py	(revision 64518)
+++ Lib/test/test_sys.py	(working copy)
@@ -408,6 +408,8 @@
 
 class SizeofTest(unittest.TestCase):
 
+    TPFLAGS_HAVE_GC = 1<<14
+
     def setUp(self):
         self.c = len(struct.pack('c', ' '))
         self.H = len(struct.pack('H', 0))
@@ -419,6 +421,8 @@
         self.header = 'lP'
         if hasattr(sys, "gettotalrefcount"):
             self.header += '2P'
+        import _testcapi
+        self.gc_headsize = _testcapi.SIZEOF_PYGC_HEAD
         self.file = open(test.test_support.TESTFN, 'wb')
 
     def tearDown(self):
@@ -428,6 +432,11 @@
     def check_sizeof(self, o, size, size2=None):
         """Check size of o. Possible are size and optionally size2)."""
         result = sys.getsizeof(o)
+        # add GC header size
+        if type(o).__flags__ & self.TPFLAGS_HAVE_GC:
+            size += self.gc_headsize
+            if size2 != None:
+                size2 += self.gc_headsize
         msg = 'wrong size for %s: got %d, expected ' % (type(o), result)
         if (size2 != None) and (result != size):
             self.assertEqual(result, size2, msg + str(size2))
@@ -443,6 +452,25 @@
         """
         return struct.calcsize(fmt + '0P')
 
+    def test_gc_head_size(self):
+        # Check that the gc header size is added to objects tracked by the gc.
+        h = self.header
+        size = self.calcsize
+        gc_header_size = self.gc_headsize
+        # bool objects are not gc tracked
+        self.assertEqual(sys.getsizeof(True), size(h + 'l'))
+        self.assertEqual(sys.getsizeof(True, gc_head=True), size(h + 'l'))
+        self.assertEqual(sys.getsizeof(True, gc_head=False), size(h + 'l'))
+        # but lists are
+        self.assertEqual(sys.getsizeof([]), size(h + 'P PP') + gc_header_size)
+        self.assertEqual(sys.getsizeof([], gc_head=False), size(h + 'P PP'))
+
+    def test_default(self):
+        h = self.header
+        size = self.calcsize
+        self.assertEqual(sys.getsizeof(True, -1), size(h + 'l'))
+        self.assertEqual(sys.getsizeof(True, -1, gc_head=True), size(h + 'l'))
+        
     def test_standardtypes(self):
         h = self.header
         size = self.calcsize