Make list_ass_slice to be thread-safe(?)

Objects/listobject.c

@@ -506,13 +506,13 @@ list_item(PyObject *aa, Py_ssize_t i)
 }
 
 static PyObject *
-list_slice_lock_held(PyListObject *a, Py_ssize_t start, Py_ssize_t len)
+list_slice_lock_held(PyListObject *a, Py_ssize_t ilow, Py_ssize_t len)
 {
     PyListObject *np = (PyListObject *) list_new_prealloc(len);
     if (np == NULL) {
         return NULL;
     }
-    PyObject **src = a->ob_item + start;
+    PyObject **src = a->ob_item + ilow;
     PyObject **dest = np->ob_item;
     for (Py_ssize_t i = 0; i < len; i++) {
         PyObject *v = src[i];
@@ -525,25 +525,11 @@ list_slice_lock_held(PyListObject *a, Py_ssize_t start, Py_ssize_t len)
 static PyObject *
 list_slice(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh)
 {
-    PyListObject *np;
-    PyObject **src, **dest;
-    Py_ssize_t i, len;
-    len = ihigh - ilow;
+    Py_ssize_t len = ihigh - ilow;
     if (len <= 0) {
         return PyList_New(0);
     }
-    np = (PyListObject *) list_new_prealloc(len);
-    if (np == NULL)
-        return NULL;
-
-    src = a->ob_item + ilow;
-    dest = np->ob_item;
-    for (i = 0; i < len; i++) {
-        PyObject *v = src[i];
-        dest[i] = Py_NewRef(v);
-    }
-    Py_SET_SIZE(np, len);
-    return (PyObject *)np;
+    return list_slice_lock_held(a, ilow, len);
 }
 
 PyObject *
@@ -712,7 +698,7 @@ list_clear_slot(PyObject *self)
  * guaranteed the call cannot fail.
  */
 static int
-list_ass_slice(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
+list_ass_slice_impl(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
 {
     /* Because [X]DECREF can recursively invoke list operations on
        this list, we must postpone all [X]DECREF activity until
@@ -737,10 +723,10 @@ list_ass_slice(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
     else {
         if (a == b) {
             /* Special case "a[i:j] = a" -- copy b first */
-            v = list_slice(b, 0, Py_SIZE(b));
+            v = list_slice_lock_held(b, 0, Py_SIZE(b));
             if (v == NULL)
                 return result;
-            result = list_ass_slice(a, ilow, ihigh, v);
+            result = list_ass_slice_impl(a, ilow, ihigh, v);
             Py_DECREF(v);
             return result;
         }
@@ -817,6 +803,23 @@ list_ass_slice(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
 #undef b
 }
 
+static int
+list_ass_slice(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
+{
+    int ret;
+    if (v != NULL && PyList_CheckExact(v)) {
+        Py_BEGIN_CRITICAL_SECTION2(a, v);
+        ret = list_ass_slice_impl(a, ilow, ihigh, v);
+        Py_END_CRITICAL_SECTION2();
+    }
+    else {
+        Py_BEGIN_CRITICAL_SECTION(a);
+        ret = list_ass_slice_impl(a, ilow, ihigh, v);
+        Py_END_CRITICAL_SECTION();
+    }
+    return ret;
+}
+
 int
 PyList_SetSlice(PyObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
 {
@@ -2878,8 +2881,7 @@ list_remove_impl(PyListObject *self, PyObject *value)
         int cmp = PyObject_RichCompareBool(obj, value, Py_EQ);
         Py_DECREF(obj);
         if (cmp > 0) {
-            if (list_ass_slice(self, i, i+1,
-                               (PyObject *)NULL) == 0)
+            if (list_ass_slice_impl(self, i, i+1, NULL) == 0)
                 Py_RETURN_NONE;
             return NULL;
         }