Switch back to linked frame traversal

Traverse from the top (newest) of stack to oldest, as a linked
reference, instead of linking every frame against rootstate. This
behavior is expected by the tests.

Also fix f_back processing because Py 3.11 only tracks f_back
when FRAME_OWNED_BY_FRAME_OBJECT, i.e. the frame finished executing.
We don't always have this luxary.

For #41

src/heapy/rootstate.c

@@ -435,27 +435,17 @@ rootstate_traverse(NyHeapTraverse *ta)
         ts = is->tstate_head;
 #endif
         for (; ts; ts = ts->next) {
-#if PY_MAJOR_VERSION >= 3 && PY_MINOR_VERSION >= 11
-            PyFrameObject *frame;
             if (ts == bts && hv->limitframe) {
-                frame = (PyFrameObject *)Py_NewRef(hv->limitframe);
+                Py_VISIT(hv->limitframe);
             } else if (!hv->limitframe) {
-                frame = PyThreadState_GetFrame(ts);
-            }
-
-            while (frame) {
-                PyFrameObject *next_frame = PyFrame_GetBack(frame);
+#if PY_MAJOR_VERSION >= 3 && PY_MINOR_VERSION >= 11
+                PyFrameObject *frame = PyThreadState_GetFrame(ts);
                 Py_VISIT(frame);
-                Py_DECREF(frame);
-                frame = next_frame;
-            }
+                Py_XDECREF(frame);
 #else
-            if (ts == bts && hv->limitframe) {
-                Py_VISIT(hv->limitframe);
-            } else if (!hv->limitframe) {
                 Py_VISIT(ts->frame);
-            }
 #endif
+            }
             Py_VISIT(ts->c_profileobj);
             Py_VISIT(ts->c_traceobj);
 

src/heapy/stdtypes.c

@@ -217,6 +217,16 @@ frame_relate(NyHeapRelate *r)
     Py_ssize_t ncells = PyTuple_GET_SIZE(co->co_cellvars);
     Py_ssize_t nlocals = co->co_nlocals;;
     Py_ssize_t nfreevars = PyTuple_GET_SIZE(co->co_freevars);
+#endif
+#if PY_MAJOR_VERSION >= 3 && PY_MINOR_VERSION >= 11
+    // Py 3.11 only holds f_back when FRAME_OWNED_BY_FRAME_OBJECT
+    PyFrameObject *next_frame = PyFrame_GetBack(v);
+    if ((PyObject *)next_frame == r->tgt && r->visit(
+            NYHR_ATTRIBUTE, PyUnicode_FromString("f_back"), r)) {
+        Py_XDECREF(next_frame);
+        return 1;
+    }
+    Py_XDECREF(next_frame);
 #endif
     ATTR(f_back)
 #if PY_MAJOR_VERSION >= 3 && PY_MINOR_VERSION >= 11
@@ -335,7 +345,10 @@ frame_traverse(NyHeapTraverse *ta) {
 #if PY_MAJOR_VERSION >= 3 && PY_MINOR_VERSION >= 11
     /* _PyFrame_Traverse is not exposed and CPython's frame_traverse only
       calls it when FRAME_OWNED_BY_FRAME_OBJECT :( */
-    Py_VISIT(v->f_back);
+    PyFrameObject *next_frame = PyFrame_GetBack(v);
+    Py_VISIT(next_frame);
+    Py_XDECREF(next_frame);
+
     Py_VISIT(v->f_trace);
     Py_VISIT(iv->f_func);
     Py_VISIT(iv->f_code);