Apply Cody's suggestions

Co-authored-by: Cody Maloney <cmaloney@theoreticalchaos.com>

Author: StanFromIreland

Lib/test/test_bytes.py

@@ -3063,5 +3063,18 @@ def resize_stress(ba):
         with threading_helper.start_threads(threads):
             pass
 
+    @threading_helper.reap_threads
+    @threading_helper.requires_working_threading()
+    def test_free_threading_bytearray_resize_other_thread(self):
+        # Shrinking a bytearray whose buffer another thread owns must not
+        # adopt the immortal single-byte bytes object a the buffer.
+        ba = bytearray(b'abc')
+        thread = threading.Thread(target=ba.resize, args=(1,))
+        with threading_helper.start_threads([thread]):
+            pass
+        ba[0] = ord('X')
+        self.assertEqual(ba, bytearray(b'X'))
+        self.assertEqual(ord(b'a'), ord('a'))
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-09-05-17-32-11.gh-issue-156995.Tk3bYt.rst

@@ -1,3 +1,5 @@
-Fix :meth:`bytearray.take_bytes` leaving the bytearray with a shared buffer
-when exactly one byte remained, so that writing to the bytearray modified
-the single-byte :class:`bytes` object with the same value.
+Fix :class:`bytearray` sharing its buffer with the single-byte :class:`bytes`
+object of the same value, so that writing to the bytearray modified that
+:class:`bytes` object. This happened with :meth:`bytearray.take_bytes` when
+exactly one byte remained, and on the free-threaded build when a bytearray was
+shrunk to one byte from another thread.

Objects/bytearrayobject.c

@@ -46,6 +46,8 @@ _getbytevalue(PyObject* arg, int *value)
 static void
 bytearray_reinit_from_bytes(PyByteArrayObject *self, Py_ssize_t size,
                             Py_ssize_t alloc) {
+    /* Only the empty bytes may be immortal. */
+    assert((alloc == 0) == _Py_IsImmortal(self->ob_bytes_object));
     self->ob_bytes = self->ob_start = PyBytes_AS_STRING(self->ob_bytes_object);
     Py_SET_SIZE(self, size);
     FT_ATOMIC_STORE_SSIZE_RELAXED(self->ob_alloc, alloc);

Objects/bytesobject.c

@@ -3368,14 +3368,12 @@ _PyBytes_Resize(PyObject **pv, Py_ssize_t newsize)
         return 0;
     }
     if (!_PyObject_IsUniquelyReferenced(v)) {
-        if (oldsize < newsize) {
-            *pv = _PyBytes_FromSize(newsize, 0);
-            if (*pv) {
-                memcpy(PyBytes_AS_STRING(*pv), PyBytes_AS_STRING(v), oldsize);
-            }
-        }
-        else {
-            *pv = PyBytes_FromStringAndSize(PyBytes_AS_STRING(v), newsize);
+        // Allocate and then copy so we don't get a shared immortal
+        // one-character singleton!
+        *pv = _PyBytes_FromSize(newsize, 0);
+        if (*pv) {
+            memcpy(PyBytes_AS_STRING(*pv), PyBytes_AS_STRING(v),
+                   Py_MIN(oldsize, newsize));
         }
         Py_DECREF(v);
         return (*pv == NULL) ? -1 : 0;