Frame the NEWS as a concurrency issue and add the grow-during-__buffer__ test

Author: tonghuaroot

Lib/test/test_bytes.py

@@ -1818,6 +1818,10 @@ def __buffer__(self, flags):
         b = bytearray(b'x' * 100)
         b.extend(Evil(lambda: b.__delitem__(slice(30, None))))
         self.assertEqual(b, b'x' * 30 + b'ABCDEFGH')
+        # grow during __buffer__: the data lands at the original end.
+        b = bytearray(b'x' * 10)
+        b.extend(Evil(lambda: b.extend(b'y' * 100)))
+        self.assertEqual(b, b'x' * 10 + b'ABCDEFGH' + b'y' * 100)
 
     def test_iconcat(self):
         b = bytearray(b"abc")

Misc/NEWS.d/next/Core_and_Builtins/2026-07-11-15-45-00.gh-issue-153578.Qm4Zt9.rst

@@ -1,2 +1,3 @@
-Fix a crash in :meth:`bytearray.extend` when the argument's
-:meth:`~object.__buffer__` method resizes the bytearray. Patch by tonghuaroot.
+Fix an out-of-bounds write in :meth:`bytearray.extend` when the bytearray is
+resized while the argument's :meth:`~object.__buffer__` is being acquired, for
+example by another thread. Patch by tonghuaroot.