gh-123271: Make builtin zip method safe under free-threading (#123272)

The `zip_next` function uses a common optimization technique for methods
that generate tuples. The iterator maintains an internal reference to
the returned tuple. When the method is called again, it checks if the
internal tuple's reference count is 1. If so, the tuple can be reused.
However, this approach is not safe under the free-threading build:
after checking the reference count, another thread may perform the same
check and also reuse the tuple. This can result in a double decref on
the items of the replaced tuple and a double incref (memory leak) on
the items of the tuple being set.

This adds a function, `_PyObject_IsUniquelyReferenced` that
encapsulates the stricter logic necessary for the free-threaded build:
the internal tuple must be owned by the current thread, have a local
refcount of one, and a shared refcount of zero.

Include/internal/pycore_object.h

@@ -159,6 +159,23 @@ static inline void _Py_RefcntAdd(PyObject* op, Py_ssize_t n)
 }
 #define _Py_RefcntAdd(op, n) _Py_RefcntAdd(_PyObject_CAST(op), n)
 
+// Checks if an object has a single, unique reference. If the caller holds a
+// unique reference, it may be able to safely modify the object in-place.
+static inline int
+_PyObject_IsUniquelyReferenced(PyObject *ob)
+{
+#if !defined(Py_GIL_DISABLED)
+    return Py_REFCNT(ob) == 1;
+#else
+    // NOTE: the entire ob_ref_shared field must be zero, including flags, to
+    // ensure that other threads cannot concurrently create new references to
+    // this object.
+    return (_Py_IsOwnedByCurrentThread(ob) &&
+            _Py_atomic_load_uint32_relaxed(&ob->ob_ref_local) == 1 &&
+            _Py_atomic_load_ssize_relaxed(&ob->ob_ref_shared) == 0);
+#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 +1,41 @@
+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 = 8
+        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,
+                        ],
+                    )
+                )
+            for t in worker_threads:
+                t.start()
+            for t in worker_threads:
+                t.join()
+
+
+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 (_PyObject_IsUniquelyReferenced(result)) {
         Py_INCREF(result);
         for (i=0 ; i < tuplesize ; i++) {
             it = PyTuple_GET_ITEM(lz->ittuple, i);