[3.9] bpo-42083: Allow NULL doc in PyStructSequence_NewType

Misc/NEWS.d/next/C API/2021-05-04-15-13-31.bpo-42083.ekXnbR.rst

@@ -0,0 +1,2 @@
+Fix crash in :c:func:`PyStructSequence_NewType` when passed ``NULL`` in the
+documentation string slot.

Modules/_testcapimodule.c

@@ -3731,6 +3731,25 @@ test_structseq_newtype_doesnt_leak(PyObject *Py_UNUSED(self),
     Py_RETURN_NONE;
 }
 
+static PyObject *
+test_structseq_newtype_null_descr_doc(PyObject *Py_UNUSED(self),
+                              PyObject *Py_UNUSED(args))
+{
+    PyStructSequence_Field descr_fields[1] = {
+        (PyStructSequence_Field){NULL, NULL}
+    };
+    // Test specifically for NULL .doc field.
+    PyStructSequence_Desc descr = {"_testcapi.test_descr", NULL, &descr_fields[0], 0};
+
+    PyTypeObject* structseq_type = PyStructSequence_NewType(&descr);
+    assert(structseq_type != NULL);
+    assert(PyType_Check(structseq_type));
+    assert(PyType_FastSubclass(structseq_type, Py_TPFLAGS_TUPLE_SUBCLASS));
+    Py_DECREF(structseq_type);
+
+    Py_RETURN_NONE;
+}
+
 static PyObject *
 test_incref_decref_API(PyObject *ob, PyObject *Py_UNUSED(ignored))
 {
@@ -5335,6 +5354,8 @@ static PyMethodDef TestMethods[] = {
     {"test_decref_doesnt_leak", test_decref_doesnt_leak,         METH_NOARGS},
     {"test_structseq_newtype_doesnt_leak",
         test_structseq_newtype_doesnt_leak, METH_NOARGS},
+    {"test_structseq_newtype_null_descr_doc",
+        test_structseq_newtype_null_descr_doc, METH_NOARGS},
     {"test_incref_decref_API",  test_incref_decref_API,          METH_NOARGS},
     {"test_long_and_overflow",  test_long_and_overflow,          METH_NOARGS},
     {"test_long_as_double",     test_long_as_double,             METH_NOARGS},

Objects/structseq.c

@@ -467,12 +467,17 @@ PyStructSequence_NewType(PyStructSequence_Desc *desc)
     /* Initialize Slots */
     slots[0] = (PyType_Slot){Py_tp_dealloc, (destructor)structseq_dealloc};
     slots[1] = (PyType_Slot){Py_tp_repr, (reprfunc)structseq_repr};
-    slots[2] = (PyType_Slot){Py_tp_doc, (void *)desc->doc};
-    slots[3] = (PyType_Slot){Py_tp_methods, structseq_methods};
-    slots[4] = (PyType_Slot){Py_tp_new, structseq_new};
-    slots[5] = (PyType_Slot){Py_tp_members, members};
-    slots[6] = (PyType_Slot){Py_tp_traverse, (traverseproc)structseq_traverse};
-    slots[7] = (PyType_Slot){0, 0};
+    slots[2] = (PyType_Slot){Py_tp_methods, structseq_methods};
+    slots[3] = (PyType_Slot){Py_tp_new, structseq_new};
+    slots[4] = (PyType_Slot){Py_tp_members, members};
+    slots[5] = (PyType_Slot){Py_tp_traverse, (traverseproc)structseq_traverse};
+    if (desc->doc) {
+        slots[6] = (PyType_Slot){Py_tp_doc, (void *)desc->doc};
+        slots[7] = (PyType_Slot){0, 0};
+    }
+    else {
+        slots[6] = (PyType_Slot){0, 0};
+    }
 
     /* Initialize Spec */
     /* The name in this PyType_Spec is statically allocated so it is */