gh-160518: Fix hz encoder dropping half of the ~~ escape

Write both bytes of the escape before consuming the input character, so
that a retry after the output buffer is enlarged starts at the tilde.

Author: gpshead

Lib/test/test_codecencodings_cn.py

@@ -92,5 +92,15 @@ class Test_HZ(multibytecodec_support.TestBase, unittest.TestCase):
         (b'~{Dc~\n:C~}', 'strict', None), # ~\n only in ASCII mode
     )
 
+    def test_escape_at_buffer_boundary(self):
+        # '~~' must not be cut in two when the output buffer is enlarged.
+        for pairs in range(40):
+            for letters in range(3):
+                text = 'a\u4e2d' * pairs + 'x' * letters + '~{'
+                with self.subTest(pairs=pairs, letters=letters):
+                    encoded = text.encode('hz')
+                    self.assertEndsWith(encoded, b'~~{')
+                    self.assertEqual(encoded.decode('hz'), text)
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2026-10-22-08-15-57.gh-issue-160518.hZt1Ld.rst

@@ -0,0 +1,3 @@
+Fix the ``hz`` codec sometimes encoding ``~`` as a single ``~`` instead of
+``~~`` in text that mixes ASCII and Chinese characters, which produced data
+that did not decode back to the original text.

Modules/cjkcodecs/_codecs_cn.c

@@ -361,11 +361,13 @@ ENCODER(hz)
                 NEXT_OUT(2);
                 state->c[CN_STATE_OFFSET] = 0;
             }
-            WRITEBYTE1((unsigned char)c);
-            NEXT(1, 1);
             if (c == '~') {
-                WRITEBYTE1('~');
-                NEXT_OUT(1);
+                WRITEBYTE2('~', '~');
+                NEXT(1, 2);
+            }
+            else {
+                WRITEBYTE1((unsigned char)c);
+                NEXT(1, 1);
             }
             continue;
         }