Serialize frozenset elements deterministically

Author: brandtbucher

Python/marshal.c

@@ -503,9 +503,42 @@ w_complex_object(PyObject *v, char flag, WFILE *p)
             W_TYPE(TYPE_SET, p);
         n = PySet_GET_SIZE(v);
         W_SIZE(n, p);
+        // bpo-37596: We need to write the elements in a deterministic order!
+        // Since we know that they all need marshal support anyways, this can
+        // be conveniently accomplished by using their marshal serializations
+        // as sort keys:
+        PyObject *pairs = PyList_New(0);
+        if (pairs == NULL) {
+            p->error = WFERR_NOMEMORY;
+            return;
+        }
+        PyObject *pair = NULL;
         while (_PySet_NextEntry(v, &pos, &value, &hash)) {
+            PyObject *dump = PyMarshal_WriteObjectToString(value, p->version);
+            if (dump == NULL) {
+                p->error = WFERR_UNMARSHALLABLE;
+                goto anyset_done;
+            }
+            pair = PyTuple_Pack(2, dump, value);
+            Py_DECREF(dump);
+            if (pair == NULL || PyList_Append(pairs, pair)) {
+                p->error = WFERR_NOMEMORY;
+                goto anyset_done;
+            }
+            Py_CLEAR(pair);
+        }
+        if (PyList_Sort(pairs)) {
+            p->error = WFERR_UNMARSHALLABLE;
+            goto anyset_done;
+        }
+        for (Py_ssize_t i = 0; i < n; i++) {
+            PyObject *pair = PyList_GET_ITEM(pairs, i);
+            PyObject *value = PyTuple_GET_ITEM(pair, 1);
             w_object(value, p);
         }
+    anyset_done:
+        Py_XDECREF(pairs);
+        Py_XDECREF(pair);
     }
     else if (PyCode_Check(v)) {
         PyCodeObject *co = (PyCodeObject *)v;