gh-122527: Fix a crash on deallocation of `PyStructSequence` (GH-122577)

The `PyStructSequence` destructor would crash if it was deallocated after
its type's dictionary was cleared by the GC, because it couldn't compute
the "real size" of the instance. This could occur with relatively
straightforward code in the free-threaded build or with a reference
cycle involving the type in the default build, due to differing orders
in which `tp_clear()` was called.

Account for the non-sequence fields in `tp_basicsize` and use that,
along with `Py_SIZE()`, to compute the "real" size of a
`PyStructSequence` in the dealloc function. This avoids the accesses to
the type's dictionary during dealloc, which were unsafe.

4 files changed, 41 insertions, 7 deletions

diff --git a/Lib/test/test_structseq.py b/Lib/test/test_structseq.py
index 6aec63e..d0bc0bd 100644
--- a/Lib/test/test_structseq.py
+++ b/Lib/test/test_structseq.py
@@ -2,8 +2,10 @@ import copy
 import os
 import pickle
 import re
+import textwrap
 import time
 import unittest
+from test.support import script_helper
 
 
 class StructSeqTest(unittest.TestCase):
@@ -342,6 +344,17 @@ class StructSeqTest(unittest.TestCase):
         with self.assertRaisesRegex(TypeError, error_message):
             copy.replace(r, st_mode=1, error=2)
 
+    def test_reference_cycle(self):
+        # gh-122527: Check that a structseq that's part of a reference cycle
+        # with its own type doesn't crash. Previously, if the type's dictionary
+        # was cleared first, the structseq instance would crash in the
+        # destructor.
+        script_helper.assert_python_ok("-c", textwrap.dedent(r"""
+            import time
+            t = time.gmtime()
+            type(t).refcyle = t
+        """))
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index 7e4bc98..709355e 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -1823,7 +1823,8 @@ class SizeofTest(unittest.TestCase):
         # symtable entry
         # XXX
         # sys.flags
-        check(sys.flags, vsize('') + self.P * len(sys.flags))
+        # FIXME: The +1 will not be necessary once gh-122575 is fixed
+        check(sys.flags, vsize('') + self.P * (1 + len(sys.flags)))
 
     def test_asyncgen_hooks(self):
         old = sys.get_asyncgen_hooks()
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2024-08-01-19-13-58.gh-issue-122527.eztso6.rst b/Misc/NEWS.d/next/Core_and_Builtins/2024-08-01-19-13-58.gh-issue-122527.eztso6.rst
new file mode 100644
index 0000000..f697ed9
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2024-08-01-19-13-58.gh-issue-122527.eztso6.rst
@@ -0,0 +1,4 @@
+Fix a crash that occurred when a ``PyStructSequence`` was deallocated after
+its type's dictionary was cleared by the GC.  The type's
+:c:member:`~PyTypeObject.tp_basicsize` now accounts for non-sequence fields
+that aren't included in the :c:macro:`Py_SIZE` of the sequence.
diff --git a/Objects/structseq.c b/Objects/structseq.c
index d8289f2..94f09b3 100644
--- a/Objects/structseq.c
+++ b/Objects/structseq.c
@@ -41,12 +41,20 @@ get_type_attr_as_size(PyTypeObject *tp, PyObject *name)
     get_type_attr_as_size(tp, &_Py_ID(n_sequence_fields))
 #define REAL_SIZE_TP(tp) \
     get_type_attr_as_size(tp, &_Py_ID(n_fields))
-#define REAL_SIZE(op) REAL_SIZE_TP(Py_TYPE(op))
+#define REAL_SIZE(op) get_real_size((PyObject *)op)
 
 #define UNNAMED_FIELDS_TP(tp) \
     get_type_attr_as_size(tp, &_Py_ID(n_unnamed_fields))
 #define UNNAMED_FIELDS(op) UNNAMED_FIELDS_TP(Py_TYPE(op))
 
+static Py_ssize_t
+get_real_size(PyObject *op)
+{
+    // Compute the real size from the visible size (i.e., Py_SIZE()) and the
+    // number of non-sequence fields accounted for in tp_basicsize.
+    Py_ssize_t hidden = Py_TYPE(op)->tp_basicsize - offsetof(PyStructSequence, ob_item);
+    return Py_SIZE(op) + hidden / sizeof(PyObject *);
+}
 
 PyObject *
 PyStructSequence_New(PyTypeObject *type)
@@ -120,6 +128,9 @@ structseq_dealloc(PyStructSequence *obj)
     PyObject_GC_UnTrack(obj);
 
     PyTypeObject *tp = Py_TYPE(obj);
+    // gh-122527: We can't use REAL_SIZE_TP() or any macros that access the
+    // type's dictionary here, because the dictionary may have already been
+    // cleared by the garbage collector.
     size = REAL_SIZE(obj);
     for (i = 0; i < size; ++i) {
         Py_XDECREF(obj->ob_item[i]);
@@ -565,10 +576,14 @@ initialize_members(PyStructSequence_Desc *desc,
 
 static void
 initialize_static_fields(PyTypeObject *type, PyStructSequence_Desc *desc,
-                         PyMemberDef *tp_members, unsigned long tp_flags)
+                         PyMemberDef *tp_members, Py_ssize_t n_members,
+                         unsigned long tp_flags)
 {
     type->tp_name = desc->name;
-    type->tp_basicsize = sizeof(PyStructSequence) - sizeof(PyObject *);
+    // Account for hidden members in tp_basicsize because they are not
+    // included in the variable size.
+    Py_ssize_t n_hidden = n_members - desc->n_in_sequence;
+    type->tp_basicsize = sizeof(PyStructSequence) + (n_hidden - 1) * sizeof(PyObject *);
     type->tp_itemsize = sizeof(PyObject *);
     type->tp_dealloc = (destructor)structseq_dealloc;
     type->tp_repr = (reprfunc)structseq_repr;
@@ -621,7 +636,7 @@ _PyStructSequence_InitBuiltinWithFlags(PyInterpreterState *interp,
         if (members == NULL) {
             goto error;
         }
-        initialize_static_fields(type, desc, members, tp_flags);
+        initialize_static_fields(type, desc, members, n_members, tp_flags);
 
         _Py_SetImmortal((PyObject *)type);
     }
@@ -684,7 +699,7 @@ PyStructSequence_InitType2(PyTypeObject *type, PyStructSequence_Desc *desc)
     if (members == NULL) {
         return -1;
     }
-    initialize_static_fields(type, desc, members, 0);
+    initialize_static_fields(type, desc, members, n_members, 0);
     if (initialize_static_type(type, desc, n_members, n_unnamed_members) < 0) {
         PyMem_Free(members);
         return -1;
@@ -760,7 +775,8 @@ _PyStructSequence_NewType(PyStructSequence_Desc *desc, unsigned long tp_flags)
     /* The name in this PyType_Spec is statically allocated so it is */
     /* expected that it'll outlive the PyType_Spec */
     spec.name = desc->name;
-    spec.basicsize = sizeof(PyStructSequence) - sizeof(PyObject *);
+    Py_ssize_t hidden = n_members - desc->n_in_sequence;
+    spec.basicsize = (int)(sizeof(PyStructSequence) + (hidden - 1) * sizeof(PyObject *));
     spec.itemsize = sizeof(PyObject *);
     spec.flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | tp_flags;
     spec.slots = slots;