From 0ba3386b8dfb9f22bdf64c3eb258070768b10bf3 Mon Sep 17 00:00:00 2001
From: "Gregory P. Smith" <68491+gpshead@users.noreply.github.com>
Date: Sat, 31 Oct 2026 05:57:17 -0700
Subject: [PATCH] gh-160518: Fix hz encoder dropping half of the ~~ escape
 (GH-160519)

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.
(cherry picked from commit 1259f1602f870185a4980c80c525e110c4b9c614)

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>
---
 Lib/test/test_codecencodings_cn.py                     | 10 ++++++++++
 .../2026-10-22-08-15-57.gh-issue-160518.hZt1Ld.rst     |  3 +++
 2 files changed, 13 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-10-22-08-15-57.gh-issue-160518.hZt1Ld.rst

diff --git a/Lib/test/test_codecencodings_cn.py b/Lib/test/test_codecencodings_cn.py
index 2a450718d5aae57..35a2d7fb1ebf297 100644
--- a/Lib/test/test_codecencodings_cn.py
+++ b/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()
diff --git a/Misc/NEWS.d/next/Library/2026-10-22-08-15-57.gh-issue-160518.hZt1Ld.rst b/Misc/NEWS.d/next/Library/2026-10-22-08-15-57.gh-issue-160518.hZt1Ld.rst
new file mode 100644
index 000000000000000..3a3c6805ec82aea
--- /dev/null
+++ b/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.