fix lock-free list access on heapq

Author: xuantengh

Modules/_heapqmodule.c

@@ -59,8 +59,8 @@ siftdown(PyListObject *heap, Py_ssize_t startpos, Py_ssize_t pos)
         arr = _PyList_ITEMS(heap);
         parent = arr[parentpos];
         newitem = arr[pos];
-        arr[parentpos] = newitem;
-        arr[pos] = parent;
+        _Py_atomic_store_ptr(&arr[parentpos], newitem);
+        _Py_atomic_store_ptr(&arr[pos], parent);
         pos = parentpos;
     }
     return 0;
@@ -108,8 +108,8 @@ siftup(PyListObject *heap, Py_ssize_t pos)
         /* Move the smaller child up. */
         tmp1 = arr[childpos];
         tmp2 = arr[pos];
-        arr[childpos] = tmp2;
-        arr[pos] = tmp1;
+        _Py_atomic_store_ptr(&arr[childpos], tmp2);
+        _Py_atomic_store_ptr(&arr[pos], tmp1);
         pos = childpos;
     }
     /* Bubble it up to its final resting place (by sifting its parents down). */
@@ -172,8 +172,10 @@ heappop_internal(PyObject *heap, int siftup_func(PyListObject *, Py_ssize_t))
     if (!n)
         return lastelt;
     returnitem = PyList_GET_ITEM(heap, 0);
-    PyList_SET_ITEM(heap, 0, lastelt);
-    if (siftup_func((PyListObject *)heap, 0)) {
+    // We're in the critical section now
+    PyListObject *list = _PyList_CAST(heap);
+    _Py_atomic_store_ptr(&list->ob_item[0], lastelt);
+    if (siftup_func(list, 0)) {
         Py_DECREF(returnitem);
         return NULL;
     }