* drop the unreasonable list invariant that ob_item should never come back

to NULL during the lifetime of the object.

* listobject.c nevertheless did not conform to the other invariants,
  either; fixed.

* listobject.c now uses list_clear() as the obvious internal way to clear
  a list, instead of abusing list_ass_slice() for that.  It makes it easier
  to enforce the invariant about ob_item == NULL.

* listsort() sets allocated to -1 during sort; any mutation will set it
  to a value >= 0, so it is a safe way to detect mutation.  A negative
  value for allocated does not cause a problem elsewhere currently.
  test_sort.py has a new test for this fix.

* listsort() leak: if items were added to the list during the sort, AND if
  these items had a __del__ that puts still more stuff into the list,
  then this more stuff (and the PyObject** array to hold them) were
  overridden at the end of listsort() and never released.

Lib/test/test_sort.py

@@ -150,6 +150,23 @@ class TestBugs(unittest.TestCase):
         L.sort(None)
         self.assertEqual(L, range(50))
 
+    def test_undetected_mutation(self):
+        # Python 2.4a1 did not always detect mutation
+        memorywaster = []
+        for i in range(20):
+            def mutating_cmp(x, y):
+                L.append(3)
+                L.pop()
+                return cmp(x, y)
+            L = [1,2]
+            self.assertRaises(ValueError, L.sort, mutating_cmp)
+            def mutating_cmp(x, y):
+                L.append(3)
+                del L[:]
+                return cmp(x, y)
+            self.assertRaises(ValueError, L.sort, mutating_cmp)
+            memorywaster = [memorywaster]
+
 #==============================================================================
 
 class TestDecorateSortUndecorate(unittest.TestCase):