Change cPickleCache to use PER_TypeCheck

Instead of testing object sizes.

This matches what the pure Python implementation does and is a
stronger test that the object really is compatible with the cache.
Previously, an object could potentially include ``cPersistent_HEAD``
and *not* set ``tp_base`` to ``cPersistenceCAPI->pertype`` and still
be eligible for the pickle cache; that is no longer the case.

This resolves several compiler warnings:

persistent/cPickleCache.c:521:43: warning: comparison of integers of different signs: 'Py_ssize_t' (aka 'long') and 'unsigned long' [-Wsign-compare]
                (v->ob_type->tp_basicsize >= sizeof(cPersistentObject))
                 ~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~
persistent/cPickleCache.c:709:39: warning: comparison of integers of different signs: 'Py_ssize_t' (aka 'long') and 'unsigned long' [-Wsign-compare]
    else if (v->ob_type->tp_basicsize < sizeof(cPersistentObject))

Fixes #69.

CHANGES.rst

 ``persistent`` Changelog
 ========================
 
-4.2.5 (unreleased)
+4.3.0 (unreleased)
 ------------------
 
 - Fix the possibility of a rare crash in the C extension when
   deallocating items. See https://github.com/zopefoundation/persistent/issues/66
 
+- Change cPickleCache's comparison of object sizes to determine
+  whether an object can go in the cache to use ``PyObject_TypeCheck()``.
+  This matches what the pure Python implementation does and is a
+  stronger test that the object really is compatible with the cache.
+  Previously, an object could potentially include ``cPersistent_HEAD``
+  and *not* set ``tp_base`` to ``cPersistenceCAPI->pertype`` and still
+  be eligible for the pickle cache; that is no longer the case. See
+  `issue 69 <https://github.com/zopefoundation/persistent/issues/69>`_.
 
 4.2.4.2 (2017-04-23)
 --------------------

persistent/cPickleCache.c

@@ -99,11 +99,6 @@ static char cPickleCache_doc_string[] =
 #include <stddef.h>
 #undef Py_FindMethod
 
-/* Python 2.4 backward compat */
-#if PY_MAJOR_VERSION <= 2 && PY_MINOR_VERSION < 5
-#define Py_ssize_t int
-typedef Py_ssize_t (*lenfunc)(PyObject *);
-#endif
 
 /* Python string objects to speed lookups; set by module init. */
 static PyObject *py__p_changed;
@@ -111,7 +106,7 @@ static PyObject *py__p_deactivate;
 static PyObject *py__p_jar;
 static PyObject *py__p_oid;
 
-static cPersistenceCAPIstruct *capi;
+static cPersistenceCAPIstruct *cPersistenceCAPI;
 
 /* This object is the pickle cache.  The CACHE_HEAD macro guarantees
    that layout of this struct is the same as the start of
@@ -523,7 +518,7 @@ cc_debug_info(ccobject *self)
             v = Py_BuildValue("Oi", k, v->ob_refcnt);
 
         else if (! PyType_Check(v) &&
-                (v->ob_type->tp_basicsize >= sizeof(cPersistentObject))
+                PER_TypeCheck(v)
                 )
             v = Py_BuildValue("Oisi",
                             k, v->ob_refcnt, v->ob_type->tp_name,
@@ -711,13 +706,12 @@ cc_new_ghost(ccobject *self, PyObject *args)
     {
         /* Its a persistent class, such as a ZClass. Thats ok. */
     }
-    else if (v->ob_type->tp_basicsize < sizeof(cPersistentObject))
+    else if (! PER_TypeCheck(v))
     {
         /* If it's not an instance of a persistent class, (ie Python
             classes that derive from persistent.Persistent, BTrees,
             etc), report an error.
 
-            TODO:  checking sizeof() seems a poor test.
         */
         PyErr_SetString(PyExc_TypeError,
                         "Cache values must be persistent objects.");
@@ -1034,13 +1028,11 @@ cc_add_item(ccobject *self, PyObject *key, PyObject *v)
     {
         /* Its a persistent class, such as a ZClass. Thats ok. */
     }
-    else if (v->ob_type->tp_basicsize < sizeof(cPersistentObject))
+    else if (! PER_TypeCheck(v))
     {
         /* If it's not an instance of a persistent class, (ie Python
             classes that derive from persistent.Persistent, BTrees,
             etc), report an error.
-
-            TODO:  checking sizeof() seems a poor test.
         */
         PyErr_SetString(PyExc_TypeError,
                         "Cache values must be persistent objects.");
@@ -1347,14 +1339,14 @@ module_init(void)
 #endif
 
 #ifdef PY3K
-    capi = (cPersistenceCAPIstruct *)PyCapsule_Import(CAPI_CAPSULE_NAME, 0);
+    cPersistenceCAPI = (cPersistenceCAPIstruct *)PyCapsule_Import(CAPI_CAPSULE_NAME, 0);
 #else
-    capi = (cPersistenceCAPIstruct *)PyCObject_Import(
-                    "persistent.cPersistence", "CAPI");
+    cPersistenceCAPI = (cPersistenceCAPIstruct *)PyCObject_Import(
+                       "persistent.cPersistence", "CAPI");
 #endif
-    if (!capi)
+    if (!cPersistenceCAPI)
         return NULL;
-    capi->percachedel = (percachedelfunc)cc_oid_unreferenced;
+    cPersistenceCAPI->percachedel = (percachedelfunc)cc_oid_unreferenced;
 
     py__p_changed = INTERN("_p_changed");
     if (!py__p_changed)