gh-140939: Fix memory leak in _PyBytes_FormatEx error path

Lib/test/test_bytes.py

@@ -802,6 +802,13 @@ def __int__(self):
             with self.assertRaisesRegex(TypeError, msg):
                 operator.mod(format_bytes, value)
 
+    def test_memory_leak_gh_140939(self):
+        # gh-140939: MemoryError is raised without leaking
+        _testcapi = import_helper.import_module('_testcapi')
+        with self.assertRaises(MemoryError):
+            b = self.type2test(b'%*b')
+            b % (_testcapi.PY_SSIZE_T_MAX, b'abc')
+
     def test_imod(self):
         b = self.type2test(b'hello, %b!')
         orig = b

Misc/NEWS.d/next/Core_and_Builtins/2025-11-03-17-21-38.gh-issue-140939.FVboAw.rst

@@ -0,0 +1,2 @@
+Fix memory leak when :class:`bytearray` or :class:`bytes` is formated with the
+``%*b`` format with a large width that results in a :exc:`MemoryError`.

Objects/bytesobject.c

@@ -985,6 +985,7 @@ _PyBytes_FormatEx(const char *format, Py_ssize_t format_len,
             if (alloc > 2) {
                 res = PyBytesWriter_GrowAndUpdatePointer(writer, alloc - 2, res);
                 if (res == NULL) {
+                    Py_XDECREF(temp);
                     goto error;
                 }
             }