gh-121464: Make concurrent iteration over enumerate safe under free-threading

Lib/test/test_free_threading/test_enumerate.py

@@ -0,0 +1,38 @@
+import unittest
+import sys
+from threading import Thread, Barrier
+
+from test.support import threading_helper
+
+threading_helper.requires_working_threading(module=True)
+
+class EnumerateThreading(unittest.TestCase):
+
+    @threading_helper.reap_threads
+    def test_threading(self):
+        number_of_threads = 10
+        number_of_iterations = 8
+        n = 100
+        start = sys.maxsize - 40
+        barrier = Barrier(number_of_threads)
+        def work(enum):
+            barrier.wait()
+            while True:
+                try:
+                    _ = next(enum)
+                except StopIteration:
+                    break
+
+        for it in range(number_of_iterations):
+            enum = enumerate(tuple(range(start, start + n)))
+            worker_threads = []
+            for ii in range(number_of_threads):
+                worker_threads.append(
+                    Thread(target=work, args=[enum]))
+            with threading_helper.start_threads(worker_threads):
+                pass
+
+            barrier.reset()
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2024-10-19-20-22-19.gh-issue-121464.IHwfpK.rst

@@ -0,0 +1 @@
+Make concurrent iterations over the same :func:`enumerate` iterator safe under free-threading. See `Strategy for Iterators in Free Threading <https://github.com/python/cpython/issues/124397>`_.

Objects/enumobject.c

@@ -171,32 +171,45 @@ enum_traverse(PyObject *op, visitproc visit, void *arg)
     return 0;
 }
 
+// increment en_longindex with lock held, return the next index to be used
+// or NULL on error
+static inline PyObject *
+increment_longindex_lock_held(enumobject *en)
+{
+    PyObject *next_index = en->en_longindex;
+    if (next_index == NULL) {
+        next_index = PyLong_FromSsize_t(PY_SSIZE_T_MAX);
+        if (next_index == NULL) {
+            return NULL;
+        }
+    }
+    assert(next_index != NULL);
+    PyObject *stepped_up = PyNumber_Add(next_index, en->one);
+    if (stepped_up == NULL) {
+        return NULL;
+    }
+    en->en_longindex = stepped_up;
+    return next_index;
+}
+
 static PyObject *
 enum_next_long(enumobject *en, PyObject* next_item)
 {
     PyObject *result = en->en_result;
     PyObject *next_index;
-    PyObject *stepped_up;
     PyObject *old_index;
     PyObject *old_item;
 
-    if (en->en_longindex == NULL) {
-        en->en_longindex = PyLong_FromSsize_t(PY_SSIZE_T_MAX);
-        if (en->en_longindex == NULL) {
-            Py_DECREF(next_item);
-            return NULL;
-        }
-    }
-    next_index = en->en_longindex;
-    assert(next_index != NULL);
-    stepped_up = PyNumber_Add(next_index, en->one);
-    if (stepped_up == NULL) {
+
+    Py_BEGIN_CRITICAL_SECTION(en);
+    next_index = increment_longindex_lock_held(en);
+    Py_END_CRITICAL_SECTION();
+    if (next_index == NULL) {
         Py_DECREF(next_item);
         return NULL;
     }
-    en->en_longindex = stepped_up;
 
-    if (Py_REFCNT(result) == 1) {
+    if (_PyObject_IsUniquelyReferenced(result)) {
         Py_INCREF(result);
         old_index = PyTuple_GET_ITEM(result, 0);
         old_item = PyTuple_GET_ITEM(result, 1);
@@ -237,17 +250,18 @@ enum_next(PyObject *op)
     if (next_item == NULL)
         return NULL;
 
-    if (en->en_index == PY_SSIZE_T_MAX)
+    Py_ssize_t en_index = FT_ATOMIC_LOAD_SSIZE_RELAXED(en->en_index);
+    if (en_index == PY_SSIZE_T_MAX)
         return enum_next_long(en, next_item);
 
-    next_index = PyLong_FromSsize_t(en->en_index);
+    next_index = PyLong_FromSsize_t(en_index);
     if (next_index == NULL) {
         Py_DECREF(next_item);
         return NULL;
     }
-    en->en_index++;
+    FT_ATOMIC_STORE_SSIZE_RELAXED(en->en_index, en_index + 1);
 
-    if (Py_REFCNT(result) == 1) {
+    if (_PyObject_IsUniquelyReferenced(result)) {
         Py_INCREF(result);
         old_index = PyTuple_GET_ITEM(result, 0);
         old_item = PyTuple_GET_ITEM(result, 1);
@@ -277,10 +291,14 @@ static PyObject *
 enum_reduce(PyObject *op, PyObject *Py_UNUSED(ignored))
 {
     enumobject *en = _enumobject_CAST(op);
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(en);
     if (en->en_longindex != NULL)
-        return Py_BuildValue("O(OO)", Py_TYPE(en), en->en_sit, en->en_longindex);
+        result = Py_BuildValue("O(OO)", Py_TYPE(en), en->en_sit, en->en_longindex);
     else
-        return Py_BuildValue("O(On)", Py_TYPE(en), en->en_sit, en->en_index);
+        result = Py_BuildValue("O(On)", Py_TYPE(en), en->en_sit, en->en_index);
+    Py_END_CRITICAL_SECTION();
+    return result;
 }
 
 PyDoc_STRVAR(reduce_doc, "Return state information for pickling.");