Support cyclic garbage collection.

experimental/cslots/.gitignore

@@ -1,4 +1,5 @@
 *.c
 *.so
 *.egg-info
+*.html
 build

experimental/cslots/cslots.pyx

@@ -10,8 +10,8 @@ Value unions, followed by a series of byes that give the value union correspondi
 """
 
 from libc.stdlib cimport calloc, free
-from cpython.object cimport PyObject
-from cpython.ref cimport Py_XINCREF, Py_XDECREF
+from cpython.object cimport PyObject, PyTypeObject, traverseproc, visitproc, Py_TPFLAGS_HAVE_GC
+from cpython.ref cimport Py_XINCREF, Py_XDECREF, Py_CLEAR
 
 
 from sys import intern
@@ -34,6 +34,9 @@ cdef class CObject:
     # This points to value_count PyObject *s, followed by index_count bytes
     cdef PyObject **values
 
+    # Note: Adding any non-slot objects to this object will require tp_traverse and tp_clear to be
+    # changed.
+
     def __init__(self):
 
         cdef int i
@@ -60,6 +63,42 @@ cdef class CObject:
         free(self.values)
 
 
+cdef int cobject_tp_traverse(PyObject *raw, visitproc visit, void *arg) except -1:
+    """
+    Supports cyclic garbage collection by visiting objects in slots.
+    """
+
+
+    cdef CObject self = <CObject> raw
+
+    for i in range(self.value_count):
+        if self.values[i] is not NULL:
+            visit(<PyObject *>self.values[i], arg)
+
+    return 0
+
+
+cdef int cobject_tp_clear(object raw) except -1:
+    """
+    Supports cyclic garbage collection by clearing slots to break a cycle.
+    """
+
+    cdef CObject self = raw
+
+    for i in range(self.value_count):
+        if self.values[i] is not NULL:
+            Py_CLEAR(self.values[i])
+
+    return 0
+
+
+# Assign the functions to the tp_traverse and tp_clear slots of CObject.
+cdef PyTypeObject *cobject = <PyTypeObject *> CObject
+cobject.tp_traverse = cobject_tp_traverse
+cobject.tp_clear = cobject_tp_clear
+cobject.tp_flags = cobject.tp_flags | Py_TPFLAGS_HAVE_GC
+
+
 cdef class Slot:
 
     # The index of this slot inside the indexes contained by CObject.

experimental/cslots/test.sh

@@ -2,6 +2,6 @@
 
 set -e
 
-cython cslots.pyx
+cython -a cslots.pyx
 pip install -e .
 python3 -m pytest