Add support for Python 3.9.

Guard use of the internal functions _Py_*Reference behind correct ifdefs.

Also fix use of deprecated function PyEval_CallObject to be PyObject_CallObject.

Fixes #124

CHANGES.rst

@@ -19,6 +19,10 @@
   ``PersistentMapping.popitem`` to no longer mark the object as
   changed if it was empty.
 
+- Add support for Python 3.9a3+ by updating references to deprecated
+  and moved functions. See `issue 124 <https://github.com/zopefoundation/persistent/issues/124>`_.
+
+
 4.5.1 (2019-11-06)
 ------------------
 

persistent/cPersistence.c

@@ -290,7 +290,7 @@ changed(cPersistentObject *self)
         }
         Py_INCREF(self);
         PyTuple_SET_ITEM(arg, 0, (PyObject *)self);
-        result = PyEval_CallObject(meth, arg);
+        result = PyObject_CallObject(meth, arg);
         Py_DECREF(arg);
         Py_DECREF(meth);
         if (result == NULL)

persistent/cPickleCache.c

@@ -614,54 +614,61 @@ cc_oid_unreferenced(ccobject *self, PyObject *oid)
         not release the global interpreter lock until this is
         complete. */
 
-    PyObject *v;
+    PyObject *dead_pers_obj;
 
     /* If the cache has been cleared by GC, data will be NULL. */
     if (!self->data)
         return;
 
-    v = PyDict_GetItem(self->data, oid);
-    assert(v);
-    assert(v->ob_refcnt == 0);
+    dead_pers_obj = PyDict_GetItem(self->data, oid);
+    assert(dead_pers_obj);
+    assert(dead_pers_obj->ob_refcnt == 0);
     /* Need to be very hairy here because a dictionary is about
         to decref an already deleted object.
     */
 
-#ifdef Py_TRACE_REFS
+#if defined(Py_TRACE_REFS) && !defined(Py_LIMITED_API)
     /* This is called from the deallocation function after the
         interpreter has untracked the reference.  Track it again.
+	Starting in 3.9, these functions aren't available with the
+	stable (limited) API.
+
+	XXX: Why? Why not simply INCREF it? The CPython documentation
+	explicitly states these functions are for internal use only.
     */
-    _Py_NewReference(v);
+    _Py_NewReference(dead_pers_obj);
     /* Don't increment total refcount as a result of the
         shenanigans played in this function.  The _Py_NewReference()
         call above creates artificial references to v.
     */
     _Py_RefTotal--;
-    assert(v->ob_type);
+    assert(dead_pers_obj->ob_type);
 #else
-    Py_INCREF(v);
+    Py_INCREF(dead_pers_obj);
 #endif
-    assert(v->ob_refcnt == 1);
+    assert(dead_pers_obj->ob_refcnt == 1);
     /* Incremement the refcount again, because delitem is going to
-        DECREF it.  If it's refcount reached zero again, we'd call back to
+        DECREF it.  If its refcount reached zero again, we'd call back to
         the dealloc function that called us.
     */
-    Py_INCREF(v);
+    Py_INCREF(dead_pers_obj);
 
     /* TODO:  Should we call _Py_ForgetReference() on error exit? */
     if (PyDict_DelItem(self->data, oid) < 0)
         return;
-    Py_DECREF((ccobject *)((cPersistentObject *)v)->cache);
-    ((cPersistentObject *)v)->cache = NULL;
+    Py_DECREF((ccobject *)((cPersistentObject *)dead_pers_obj)->cache);
+    ((cPersistentObject *)dead_pers_obj)->cache = NULL;
 
-    assert(v->ob_refcnt == 1);
+    assert(dead_pers_obj->ob_refcnt == 1);
 
-    /* Undo the temporary resurrection.
-        Don't DECREF the object, because this function is called from
+    /* Don't DECREF the object, because this function is called from
         the object's dealloc function. If the refcnt reaches zero, it
         will all be invoked recursively.
     */
-    _Py_ForgetReference(v);
+#if defined(Py_TRACE_REFS) && !defined(Py_LIMITED_API)
+    /* But we need to undo the temporary resurrection. */
+    _Py_ForgetReference(dead_pers_obj);
+#endif
 }
 
 static PyObject *

setup.py

@@ -44,6 +44,12 @@ def _read_file(filename):
     ext_modules = []
     headers = []
 else:
+    define_macros = (
+        # We currently use macros like PyBytes_AS_STRING
+        # and internal functions like _PyObject_GetDictPtr
+        # that make it impossible to use the stable (limited) API.
+        # ('Py_LIMITED_API', '0x03050000'),
+    )
     ext_modules = [
         Extension(
             name='persistent.cPersistence',
@@ -55,7 +61,8 @@ def _read_file(filename):
                 'persistent/cPersistence.h',
                 'persistent/ring.h',
                 'persistent/ring.c',
-            ]
+            ],
+            define_macros=list(define_macros),
         ),
         Extension(
             name='persistent.cPickleCache',
@@ -67,13 +74,15 @@ def _read_file(filename):
                 'persistent/cPersistence.h',
                 'persistent/ring.h',
                 'persistent/ring.c',
-            ]
+            ],
+            define_macros=list(define_macros),
         ),
         Extension(
             name='persistent._timestamp',
             sources=[
                 'persistent/_timestamp.c',
             ],
+            define_macros=list(define_macros),
         ),
     ]
     headers = [