gh-135557: Use atomic writes on heapq operations

Lib/test/test_free_threading/test_heapq.py

@@ -3,7 +3,7 @@
 import heapq
 
 from enum import Enum
-from threading import Thread, Barrier
+from threading import Thread, Barrier, Lock
 from random import shuffle, randint
 
 from test.support import threading_helper
@@ -178,6 +178,33 @@ def heapreplace_max_func(max_heap, replace_items):
         self.assertEqual(len(max_heap), OBJECT_COUNT)
         self.test_heapq.check_max_invariant(max_heap)
 
+    def test_lock_free_list_read(self):
+        n, n_threads = 1_000, 10
+        l = []
+        barrier = Barrier(n_threads * 2)
+
+        count = 0
+        lock = Lock()
+
+        def worker():
+            with lock:
+                nonlocal count
+                x = count
+                count += 1
+
+            barrier.wait()
+            for i in range(n):
+                if x % 2:
+                    heapq.heappush(l, 1)
+                    heapq.heappop(l)
+                else:
+                    try:
+                        l[0]
+                    except IndexError:
+                        pass
+
+        self.run_concurrently(worker, (), n_threads * 2)
+
     @staticmethod
     def is_sorted_ascending(lst):
         """

Misc/NEWS.d/next/Library/2025-06-17-23-13-56.gh-issue-135557.Bfcy4v.rst

@@ -0,0 +1,2 @@
+Fix races on :mod:`heapq` updates and :class:`list` reads on the :term:`free threaded <free threading>`
+build.

Modules/_heapqmodule.c

@@ -12,6 +12,7 @@ annotated by François Pinard, and converted to C by Raymond Hettinger.
 
 #include "Python.h"
 #include "pycore_list.h"          // _PyList_ITEMS(), _PyList_AppendTakeRef()
+#include "pycore_pyatomic_ft_wrappers.h"
 
 #include "clinic/_heapqmodule.c.h"
 
@@ -59,8 +60,8 @@ 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;
+        FT_ATOMIC_STORE_PTR_RELAXED(arr[parentpos], newitem);
+        FT_ATOMIC_STORE_PTR_RELAXED(arr[pos], parent);
         pos = parentpos;
     }
     return 0;
@@ -108,8 +109,8 @@ siftup(PyListObject *heap, Py_ssize_t pos)
         /* Move the smaller child up. */
         tmp1 = arr[childpos];
         tmp2 = arr[pos];
-        arr[childpos] = tmp2;
-        arr[pos] = tmp1;
+        FT_ATOMIC_STORE_PTR_RELAXED(arr[childpos], tmp2);
+        FT_ATOMIC_STORE_PTR_RELAXED(arr[pos], tmp1);
         pos = childpos;
     }
     /* Bubble it up to its final resting place (by sifting its parents down). */
@@ -172,8 +173,9 @@ 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)) {
+    PyListObject *list = _PyList_CAST(heap);
+    FT_ATOMIC_STORE_PTR_RELAXED(list->ob_item[0], lastelt);
+    if (siftup_func(list, 0)) {
         Py_DECREF(returnitem);
         return NULL;
     }
@@ -208,8 +210,9 @@ heapreplace_internal(PyObject *heap, PyObject *item, int siftup_func(PyListObjec
     }
 
     returnitem = PyList_GET_ITEM(heap, 0);
-    PyList_SET_ITEM(heap, 0, Py_NewRef(item));
-    if (siftup_func((PyListObject *)heap, 0)) {
+    PyListObject *list = _PyList_CAST(heap);
+    FT_ATOMIC_STORE_PTR_RELAXED(list->ob_item[0], Py_NewRef(item));
+    if (siftup_func(list, 0)) {
         Py_DECREF(returnitem);
         return NULL;
     }
@@ -284,8 +287,9 @@ _heapq_heappushpop_impl(PyObject *module, PyObject *heap, PyObject *item)
     }
 
     returnitem = PyList_GET_ITEM(heap, 0);
-    PyList_SET_ITEM(heap, 0, Py_NewRef(item));
-    if (siftup((PyListObject *)heap, 0)) {
+    PyListObject *list = _PyList_CAST(heap);
+    FT_ATOMIC_STORE_PTR_RELAXED(list->ob_item[0], Py_NewRef(item));
+    if (siftup(list, 0)) {
         Py_DECREF(returnitem);
         return NULL;
     }
@@ -437,8 +441,8 @@ siftdown_max(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;
+        FT_ATOMIC_STORE_PTR_RELAXED(arr[parentpos], newitem);
+        FT_ATOMIC_STORE_PTR_RELAXED(arr[pos], parent);
         pos = parentpos;
     }
     return 0;
@@ -486,8 +490,8 @@ siftup_max(PyListObject *heap, Py_ssize_t pos)
         /* Move the smaller child up. */
         tmp1 = arr[childpos];
         tmp2 = arr[pos];
-        arr[childpos] = tmp2;
-        arr[pos] = tmp1;
+        FT_ATOMIC_STORE_PTR_RELAXED(arr[childpos], tmp2);
+        FT_ATOMIC_STORE_PTR_RELAXED(arr[pos], tmp1);
         pos = childpos;
     }
     /* Bubble it up to its final resting place (by sifting its parents down). */
@@ -621,8 +625,9 @@ _heapq_heappushpop_max_impl(PyObject *module, PyObject *heap, PyObject *item)
     }
 
     returnitem = PyList_GET_ITEM(heap, 0);
-    PyList_SET_ITEM(heap, 0, Py_NewRef(item));
-    if (siftup_max((PyListObject *)heap, 0) < 0) {
+    PyListObject *list = _PyList_CAST(heap);
+    FT_ATOMIC_STORE_PTR_RELAXED(list->ob_item[0], Py_NewRef(item));
+    if (siftup_max(list, 0) < 0) {
         Py_DECREF(returnitem);
         return NULL;
     }