[3.15] gh-159184: Do not compare set elements when marshalling a set (GH-159196) (#159205)

Set elements are written in the order of their dumps.  They were sorted
as (dump, element) pairs, so elements with the same dump were compared.
That failed for a set of several complex NaNs, which the compiler can
produce by constant folding, making the module impossible to marshal.
On this branch the failure was reported as MemoryError.

Sort (dump, index, element) tuples, in which the indexes are all
different.  Elements with the same dump stay in the order of the set,
as float NaNs did before.

(cherry picked from commit 1967a87)

Author: gpshead

Lib/test/test_marshal.py

@@ -582,6 +582,44 @@ def test_unmarshallable(self):
                 with self.assertRaisesRegex(ValueError, "unmarshallable object"):
                     marshal.dumps((arg, memoryview(b'')))
 
+    def test_unorderable_set_items(self):
+        # Set items are sorted by their marshalled representation, they
+        # are not compared, even if NaNs which are distinct objects have
+        # the same marshalled representation.
+        nan = float('nan')
+        for kind in set, frozenset:
+            for version in range(marshal.version + 1):
+                with self.subTest(kind=kind, version=version):
+                    sample = kind([complex(nan, 0), complex(nan, 0), 1j,
+                                   (complex(nan, 0),), (complex(nan, 0),),
+                                   float('nan'), float('nan')])
+                    self.assertEqual(len(sample), 7)
+                    new = marshal.loads(marshal.dumps(sample, version))
+                    self.assertIs(type(new), kind)
+                    self.assertEqual(sorted(map(repr, new)),
+                                     sorted(map(repr, sample)))
+
+    def test_unorderable_set_items_in_code(self):
+        # A constant set containing NaNs which are distinct objects.
+        # Run in a subprocess: constants of code objects are immortal
+        # in the free-threaded build.
+        script = textwrap.dedent("""
+            import marshal
+            source = 'x in {(1e999-1e999)*1j, (1e999-1e999)*2j, 1.5}'
+            code = compile(source, '<marshal test>', 'eval')
+            def consts(code):
+                return [sorted(map(repr, x)) if isinstance(x, frozenset) else x
+                        for x in code.co_consts]
+            assert ['(nan+nanj)', '(nan+nanj)', '1.5'] in consts(code)
+            for version in range(marshal.version + 1):
+                new = marshal.loads(marshal.dumps(code, version))
+                assert new.co_code == code.co_code
+                assert consts(new) == consts(code)
+                assert eval(new, {'x': 1.5}) is True
+                assert eval(new, {'x': complex('nan')}) is False
+            """)
+        assert_python_ok('-c', script)
+
     def test_set_changed_size(self):
         # A set is modified while its items are being marshalled.
         class Item:

Misc/NEWS.d/next/Library/2026-10-11-08-41-00.gh-issue-159184.Nn4tOr.rst

@@ -0,0 +1,3 @@
+Fix :func:`marshal.dumps` raising :exc:`MemoryError` for a set or frozenset
+containing several complex NaNs, which also made it impossible to write a
+:file:`.pyc` file for source code containing such a constant.

Python/marshal.c

@@ -646,9 +646,13 @@ w_complex_object(PyObject *v, char flag, WFILE *p)
         // bpo-37596: To support reproducible builds, sets and frozensets need
         // to have their elements serialized in a consistent order (even when
         // they have been scrambled by hash randomization). To ensure this, we
-        // use an order equivalent to sorted(v, key=marshal.dumps):
-        PyObject *pairs = PyList_New(n);
-        if (pairs == NULL) {
+        // use an order equivalent to sorted(v, key=marshal.dumps).
+        // The list has a (dump, index, element) tuple for each element.
+        // The indexes are all different, so that sorting never compares
+        // two elements: elements with the same dump (NaNs which are
+        // distinct objects) may not be orderable.
+        PyObject *items = PyList_New(n);
+        if (items == NULL) {
             p->error = WFERR_NOMEMORY;
             return;
         }
@@ -672,34 +676,41 @@ w_complex_object(PyObject *v, char flag, WFILE *p)
                 Py_DECREF(value);
                 break;
             }
-            PyObject *pair = _PyTuple_FromPairSteal(dump, value);
-            if (pair == NULL) {
+            PyObject *index = PyLong_FromSsize_t(i);
+            PyObject *item = NULL;
+            if (index != NULL) {
+                item = PyTuple_Pack(3, dump, index, value);
+                Py_DECREF(index);
+            }
+            Py_DECREF(dump);
+            Py_DECREF(value);
+            if (item == NULL) {
                 p->error = WFERR_NOMEMORY;
                 break;
             }
-            PyList_SET_ITEM(pairs, i++, pair);
+            PyList_SET_ITEM(items, i++, item);
         }
         Py_END_CRITICAL_SECTION();
         if (p->error == WFERR_UNMARSHALLABLE || p->error == WFERR_NOMEMORY) {
-            Py_DECREF(pairs);
+            Py_DECREF(items);
             return;
         }
         if (changed || i != n) {
             p->error = WFERR_SET_CHANGED_SIZE;
-            Py_DECREF(pairs);
+            Py_DECREF(items);
             return;
         }
-        if (PyList_Sort(pairs)) {
+        if (PyList_Sort(items)) {
             p->error = WFERR_NOMEMORY;
-            Py_DECREF(pairs);
+            Py_DECREF(items);
             return;
         }
         for (Py_ssize_t i = 0; i < n; i++) {
-            PyObject *pair = PyList_GET_ITEM(pairs, i);
-            value = PyTuple_GET_ITEM(pair, 1);
+            PyObject *item = PyList_GET_ITEM(items, i);
+            value = PyTuple_GET_ITEM(item, 2);
             w_object(value, p);
         }
-        Py_DECREF(pairs);
+        Py_DECREF(items);
     }
     else if (PyCode_Check(v)) {
         if (!p->allow_code) {