gh-128509: Use bytes singleton in marshal

Replace soft deprecated PyBytes_FromStringAndSize(NULL, n) with
PyBytes_FromStringAndSize(str, n) so marshal can get bytes singleton.

Author: vstinner

Lib/test/test_marshal.py

@@ -116,6 +116,13 @@ def test_bytes(self):
         for s in [b"", b"Andr\xe8 Previn", b"abc", b" "*10000]:
             self.helper(s)
 
+    @support.cpython_only
+    def test_bytes_singleton(self):
+        for sample in [b"", b"x"]:
+            new = marshal.loads(marshal.dumps(sample))
+            self.assertIs(new, sample)
+
+
 class ExceptionTestCase(unittest.TestCase):
     def test_exceptions(self):
         new = marshal.loads(marshal.dumps(StopIteration))

Python/marshal.c

@@ -1388,16 +1388,12 @@ r_object(RFILE *p)
                 }
                 break;
             }
-            v = PyBytes_FromStringAndSize((char *)NULL, n);
-            if (v == NULL)
-                break;
             ptr = r_string(n, p);
             if (ptr == NULL) {
-                Py_DECREF(v);
                 break;
             }
-            memcpy(PyBytes_AS_STRING(v), ptr, n);
-            retval = v;
+            // Get a singleton for 1-byte string
+            retval = PyBytes_FromStringAndSize(ptr, n);  // can be NULL
             R_REF(retval);
             break;
         }