gh-140650: fix SystemError in io.BufferedWriter.close when closed errors

Lib/test/test_io/test_bufferedio.py

@@ -962,6 +962,14 @@ def test_args_error(self):
         with self.assertRaisesRegex(TypeError, "BufferedWriter"):
             self.tp(self.BytesIO(), 1024, 1024, 1024)
 
+    def test_close_without_closed(self):
+        # gh-140650: check TypeError is raised
+        class MockRawIOWithoutClosed(self.MockRawIO):
+            closed = NotImplemented
+
+        bufio = self.tp(MockRawIOWithoutClosed())
+        self.assertRaises(TypeError, bufio.close)
+
 
 class PyBufferedWriterTest(BufferedWriterTest, PyTestCase):
     tp = pyio.BufferedWriter

Misc/NEWS.d/next/Library/2025-10-27-00-40-49.gh-issue-140650.DYJPJ9.rst

@@ -0,0 +1,2 @@
+Fix an issue where closing :class:`io.BufferedWriter` could crash
+if the closed attribute had the wrong type.

Modules/_io/bufferedio.c

@@ -555,10 +555,9 @@ _io__Buffered_close_impl(buffered *self)
     }
     /* gh-138720: Use IS_CLOSED to match flush CHECK_CLOSED. */
     r = IS_CLOSED(self);
-    if (r < 0)
-        goto end;
     if (r > 0) {
-        res = Py_NewRef(Py_None);
+        if (!PyErr_Occurred())
+            res = Py_NewRef(Py_None);
         goto end;
     }