[3.14] gh-156995: Fix _PyBytes_Resize() in Free Threading (#156996)

Lib/test/test_bytes.py

@@ -1507,7 +1507,6 @@ def test_resize(self):
         self.assertRaises(MemoryError, bytearray().resize, sys.maxsize)
         self.assertRaises(MemoryError, bytearray(1000).resize, sys.maxsize)
 
-
     def test_setitem(self):
         def setitem_as_mapping(b, i, val):
             b[i] = val
@@ -2841,5 +2840,18 @@ 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()

Lib/test/test_capi/test_bytes.py

@@ -1,3 +1,4 @@
+import sys
 import unittest
 from test.support import import_helper
 
@@ -231,26 +232,41 @@ def test_decodeescape(self):
 
     def test_resize(self):
         """Test _PyBytes_Resize()"""
-        resize = _testcapi.bytes_resize
+        _resize = _testcapi.bytes_resize
+
+        def resize(obj, size, new):
+            result = _resize(obj, size, new)
+            if 1 <= len(result):
+                if new or size != len(obj):
+                    # gh-156995: Make sure that the result is a fresh object.
+                    # Previously, _PyBytes_Resize(&obj, 1) returned a singleton
+                    # if _PyObject_IsUniquelyReferenced() is false.
+                    self.assertEqual(sys.getrefcount(result), 1)
+                    self.assertFalse(sys._is_immortal(result))
+            else:
+                # check that the result is the empty bytes string singleton
+                self.assertTrue(sys._is_immortal(result))
+            return result
 
         for new in True, False:
-            self.assertEqual(resize(b'abc', 0, new), b'')
-            self.assertEqual(resize(b'abc', 1, new), b'a')
-            self.assertEqual(resize(b'abc', 2, new), b'ab')
-            self.assertEqual(resize(b'abc', 3, new), b'abc')
-            b = resize(b'abc', 4, new)
-            self.assertEqual(len(b), 4)
-            self.assertEqual(b[:3], b'abc')
-
-            self.assertEqual(resize(b'a', 0, new), b'')
-            self.assertEqual(resize(b'a', 1, new), b'a')
-            b = resize(b'a', 2, new)
-            self.assertEqual(len(b), 2)
-            self.assertEqual(b[:1], b'a')
-
-            self.assertEqual(resize(b'', 0, new), b'')
-            self.assertEqual(len(resize(b'', 1, new)), 1)
-            self.assertEqual(len(resize(b'', 2, new)), 2)
+            with self.subTest(new=new):
+                self.assertEqual(resize(b'abc', 0, new), b'')
+                self.assertEqual(resize(b'abc', 1, new), b'a')
+                self.assertEqual(resize(b'abc', 2, new), b'ab')
+                self.assertEqual(resize(b'abc', 3, new), b'abc')
+                b = resize(b'abc', 4, new)
+                self.assertEqual(len(b), 4)
+                self.assertEqual(b[:3], b'abc')
+
+                self.assertEqual(resize(b'a', 0, new), b'')
+                self.assertEqual(resize(b'a', 1, new), b'a')
+                b = resize(b'a', 2, new)
+                self.assertEqual(len(b), 2)
+                self.assertEqual(b[:1], b'a')
+
+                self.assertEqual(resize(b'', 0, new), b'')
+                self.assertEqual(len(resize(b'', 1, new)), 1)
+                self.assertEqual(len(resize(b'', 2, new)), 2)
 
         self.assertRaises(SystemError, resize, b'abc', -1, False)
         self.assertRaises(SystemError, resize, bytearray(b'abc'), 3, False)

Misc/NEWS.d/next/C_API/2026-09-12-21-00-42.gh-issue-156995.UVjqQe.rst

@@ -0,0 +1,3 @@
+On Free Threading, ``_PyBytes_Resize(&obj, 1)`` no longer returns a single byte
+singleton is the current thread is different than the thread which created the
+object. Patch by Stan Ulbrych.

Objects/bytesobject.c

@@ -3243,14 +3243,12 @@ _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;