gh-123271: Make builtin zip method safe under free-threading

Include/internal/pycore_object.h

@@ -159,6 +159,24 @@ static inline void _Py_RefcntAdd(PyObject* op, Py_ssize_t n)
 }
 #define _Py_RefcntAdd(op, n) _Py_RefcntAdd(_PyObject_CAST(op), n)
 
+static inline int _Py_Reuse_Immutable_Object(PyObject *ob) {
+    /* Function to check whether an immutable object such as a tuple can be
+     * modified inplace by the owning method
+     */
+#if !defined(Py_GIL_DISABLED)
+    return Py_REFCNT(ob) == 1;
+#else
+    uint32_t local = _Py_atomic_load_uint32_relaxed(&ob->ob_ref_local);
+    if (local != 1)
+        return 0;
+    Py_ssize_t shared = _Py_atomic_load_ssize_relaxed(&ob->ob_ref_shared);
+    if (shared != 0) {
+        return 0;
+    }
+    return ob->ob_tid == _Py_ThreadId();
+#endif
+}
+
 PyAPI_FUNC(void) _Py_SetImmortal(PyObject *op);
 PyAPI_FUNC(void) _Py_SetImmortalUntracked(PyObject *op);
 

Lib/test/test_free_threading/test_zip.py

@@ -0,0 +1,39 @@
+import unittest
+from threading import Thread
+
+from test.support import threading_helper
+
+
+class ZipThreading(unittest.TestCase):
+    @staticmethod
+    def work(enum):
+        while True:
+            try:
+                next(enum)
+            except StopIteration:
+                break
+
+    @threading_helper.reap_threads
+    @threading_helper.requires_working_threading()
+    def test_threading(self):
+        number_of_threads = 8
+        number_of_iterations = 40
+        n = 40_000
+        enum = zip(range(n), range(n))
+        for _ in range(number_of_iterations):
+            worker_threads = []
+            for ii in range(number_of_threads):
+                worker_threads.append(
+                    Thread(
+                        target=self.work,
+                        args=[
+                            enum,
+                        ],
+                    )
+                )
+            _ = [t.start() for t in worker_threads]
+            _ = [t.join() for t in worker_threads]
+
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2024-08-23-21-20-34.gh-issue-123271.xeVViR.rst

@@ -0,0 +1 @@
+Make concurrent iterations over the same :func:`zip` iterator safe under free-threading.

Python/bltinmodule.c

@@ -2971,7 +2971,8 @@ zip_next(zipobject *lz)
 
     if (tuplesize == 0)
         return NULL;
-    if (Py_REFCNT(result) == 1) {
+
+    if (_Py_Reuse_Immutable_Object(result)) {
         Py_INCREF(result);
         for (i=0 ; i < tuplesize ; i++) {
             it = PyTuple_GET_ITEM(lz->ittuple, i);