gh-143007: fix crash .TextIOWrapper.seek with a custom __int__ method detaches

Lib/test/test_io/test_textio.py

@@ -1560,6 +1560,23 @@ def closed(self):
         wrapper = self.TextIOWrapper(raw)
         wrapper.close()  # should not crash
 
+    def test_issue143007(self):
+        # gh-143007: Null pointer dereference in TextIOWrapper.seek
+        # via re-entrant __int__
+        wrapper = self.TextIOWrapper(self.BytesIO(b"x"))
+
+        class Cookie(int):
+            def __new__(cls, wrapper):
+                obj = int.__new__(cls, 0)
+                obj.wrapper = wrapper
+                return obj
+            def __int__(self):
+                self.wrapper.detach()
+                return 0
+
+        with self.assertRaises(ValueError):
+            wrapper.seek(Cookie(wrapper), 0)  # should not crash
+
 
 class PyTextIOWrapperTest(TextIOWrapperTest, PyTestCase):
     shutdown_error = "LookupError: unknown encoding: ascii"

Misc/NEWS.d/next/Library/2025-12-21-21-30-48.gh-issue-143007.EtzUva.rst

@@ -0,0 +1,2 @@
+Fix crash in :meth:`io.TextIOWrapper.seek` when a custom cookie's
+``__int__`` method detaches the underlying buffer.

Modules/_io/textio.c

@@ -2644,6 +2644,10 @@ _io_TextIOWrapper_seek_impl(textio *self, PyObject *cookieObj, int whence)
     posobj = PyLong_FromOff_t(cookie.start_pos);
     if (posobj == NULL)
         goto fail;
+
+    // gh-143007 PyNumber_Long can call arbitrary code through __int__ which may detach the underlying buffer.
+    // So we need to re-check that the TextIOWrapper is still attached.
+    CHECK_ATTACHED(self);
     res = PyObject_CallMethodOneArg(self->buffer, &_Py_ID(seek), posobj);
     Py_DECREF(posobj);
     if (res == NULL)