bpo-34068: _io__IOBase_close_impl could call _PyObject_SetAttrId with an exception set (GH-8282)

Lib/test/test_io.py

@@ -968,6 +968,16 @@ class IOTest(unittest.TestCase):
                 self.assertSequenceEqual(buffer[result:], unused)
                 self.assertEqual(len(reader.avail), avail - result)
 
+    def test_close_assert(self):
+        class R(self.IOBase):
+            def __setattr__(self, name, value):
+                pass
+            def flush(self):
+                raise OSError()
+        f = R()
+        # This would cause an assertion failure.
+        self.assertRaises(OSError, f.close)
+
 
 class CIOTest(IOTest):