gh-101828: Fix jisx0213 codecs removing null characters

Lib/test/multibytecodec_support.py

@@ -282,6 +282,22 @@ def test_incrementalencoder_del_segfault(self):
         with self.assertRaises(AttributeError):
             del e.errors
 
+    def test_null_terminator(self):
+        # see gh-101828
+        if any(enc in self.encoding for enc in ('shift', 'euc_jis')):
+            text = "バルーンフルーツ"
+        else:
+            text = "Spam"
+        encode_w_null = (text + "\0").encode(self.encoding)
+        encode_plus_null = text.encode(self.encoding) + "\0".encode(self.encoding)
+        self.assertTrue(encode_w_null.endswith(b'\x00'))
+        self.assertEqual(encode_w_null, encode_plus_null)
+
+        encode_w_null_2 = (text + "\0" + text + "\0").encode(self.encoding)
+        encode_plus_null_2 = encode_plus_null + encode_plus_null
+        self.assertEqual(encode_w_null_2.count(b'\x00'), 2)
+        self.assertEqual(encode_w_null_2, encode_plus_null_2)
+
 
 class TestBase_Mapping(unittest.TestCase):
     pass_enctest = []

Misc/NEWS.d/next/Library/2025-09-25-20-16-10.gh-issue-101828.yTxJlJ.rst

@@ -0,0 +1,3 @@
+Fix ``'shift_jisx0213'``, ``'shift_jis_2004'``, ``'euc_jisx0213'`` and
+``'euc_jis_2004'`` codecs truncating null chars
+as they were treated as part of multi-character sequences.

Modules/cjkcodecs/_codecs_jp.c

@@ -192,8 +192,11 @@ ENCODER(euc_jis_2004)
                                 JISX0213_ENCPAIRS);
                             if (code == DBCINV)
                                 return 1;
-                        } else
+                        }
+                        else if (c2 != 0) {
+                            /* Don't consume null char as part of pair */
                             insize = 2;
+                        }
                     }
                 }
             }
@@ -611,8 +614,10 @@ ENCODER(shift_jis_2004)
                             if (code == DBCINV)
                                 return 1;
                             }
-                            else
+                            else if (ch2 != 0) {
+                                /* Don't consume null char as part of pair */
                                 insize = 2;
+                            }
                         }
                     }
                 }