Closes #23247: Fix a crash in the StreamWriter.reset() of CJK codecs

Lib/test/multibytecodec_support.py

@@ -270,6 +270,13 @@ class TestBase:
 
                 self.assertEqual(ostream.getvalue(), self.tstring[0])
 
+    def test_streamwriter_reset_no_pending(self):
+        # Issue #23247: Calling reset() on a fresh StreamWriter instance
+        # (without pending data) must not crash
+        stream = BytesIO()
+        writer = self.writer(stream)
+        writer.reset()
+
 
 class TestBase_Mapping(unittest.TestCase):
     pass_enctest = []

Misc/NEWS

@@ -66,6 +66,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #23247: Fix a crash in the StreamWriter.reset() of CJK codecs.
+
 - Issue #18622: unittest.mock.mock_open().reset_mock would recurse infinitely.
   Patch from Nicola Palumbo and Laurent De Buyst.
 

Modules/cjkcodecs/multibytecodec.c

@@ -1629,6 +1629,9 @@ mbstreamwriter_reset(MultibyteStreamWriterObject *self)
 {
     PyObject *pwrt;
 
+    if (!self->pending)
+        Py_RETURN_NONE;
+
     pwrt = multibytecodec_encode(self->codec, &self->state,
                     self->pending, NULL, self->errors,
                     MBENC_FLUSH | MBENC_RESET);