[3.15] gh-159430: Add _PyObject_LookupSpecial error checks in bytes/weakref (GH-159432) (GH-159474)

gh-159430: Add _PyObject_LookupSpecial error checks in bytes/weakref (GH-159432)

_PyObject_LookupSpecial needs a PyErr_Occurred() check to handle
failures while during the lookup, or else it can fail with a "returned
a result with an exception set" error.

Add some missing ones in bytes and weakref/
(cherry picked from commit 3646ea5)

Co-authored-by: Michael J. Sullivan <sully@msully.net>

Author: miss-islington

Lib/test/test_bytes.py

@@ -901,6 +901,19 @@ def test_memory_leak_gh_140939(self):
             b = self.type2test(b'%*b')
             b % (_testcapi.PY_SSIZE_T_MAX, b'abc')
 
+    def test_mod_bytes_lookup_error(self):
+        # An error raised while looking up __bytes__ propagates.
+        class BadBytes:
+            @property
+            def __bytes__(self):
+                raise ZeroDivisionError
+            def __buffer__(self, flags):
+                return memoryview(b'abc')
+
+        b = self.type2test(b'%b')
+        with self.assertRaises(ZeroDivisionError):
+            b % BadBytes()
+
     def test_imod(self):
         b = self.type2test(b'hello, %b!')
         orig = b

Lib/test/test_weakref.py

@@ -162,6 +162,17 @@ def __getattr__(self, x):
         self.assertIn('MyConfig', repr(obj_weakref))
         self.assertIn('MyConfig', str(obj_weakref))
 
+    def test_repr_name_lookup_error(self):
+        # An error raised while looking up __name__ propagates.
+        class BadName:
+            @property
+            def __name__(self):
+                raise ZeroDivisionError
+
+        obj = BadName()
+        with self.assertRaises(ZeroDivisionError):
+            repr(weakref.ref(obj))
+
     def test_basic_callback(self):
         self.check_basic_callback(C)
         self.check_basic_callback(create_function)

Misc/NEWS.d/next/Core_and_Builtins/2026-10-14-11-40-06.gh-issue-159430.OWpyPm.rst

@@ -0,0 +1,2 @@
+Report the correct exception if looking up :meth:`~object.__bytes__` raises
+an exception while performing %-formatting of :class:`bytes`.

Misc/NEWS.d/next/Library/2026-10-14-11-41-00.gh-issue-159430.ZEW-mx.rst

@@ -0,0 +1,2 @@
+Report the correct exception if looking up ``__name__`` raises an exception
+when calling :func:`repr` on a :class:`weakref.ref`.

Objects/bytesobject.c

@@ -592,6 +592,9 @@ format_obj(PyObject *v, Py_ssize_t argidx, PyObject *key,
         *plen = PyBytes_GET_SIZE(result);
         return result;
     }
+    else if (PyErr_Occurred()) {
+        return NULL;
+    }
     /* does it support buffer protocol? */
     if (PyObject_CheckBuffer(v)) {
         /* maybe we can avoid making a copy of the buffer object here? */

Objects/weakrefobject.c

@@ -221,6 +221,10 @@ weakref_repr(PyObject *self)
     }
 
     PyObject *name = _PyObject_LookupSpecial(obj, &_Py_ID(__name__));
+    if (name == NULL && PyErr_Occurred()) {
+        Py_DECREF(obj);
+        return NULL;
+    }
     PyObject *repr;
     if (name == NULL || !PyUnicode_Check(name)) {
         repr = PyUnicode_FromFormat(