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bpo-43916: Add Py_TPFLAGS_DISALLOW_INSTANTIATION type flag (GH-25721) 

Add a new Py_TPFLAGS_DISALLOW_INSTANTIATION type flag to disallow
creating type instances: set tp_new to NULL and don't create the
"__new__" key in the type dictionary.

The flag is set automatically on static types if tp_base is NULL or
&PyBaseObject_Type and tp_new is NULL.

Use the flag on the following types:

* _curses.ncurses_version type
* _curses_panel.panel
* _tkinter.Tcl_Obj
* _tkinter.tkapp
* _tkinter.tktimertoken
* _xxsubinterpretersmodule.ChannelID
* sys.flags type
* sys.getwindowsversion() type
* sys.version_info type

Update MyStr example in the C API documentation to use
Py_TPFLAGS_DISALLOW_INSTANTIATION.

Add _PyStructSequence_InitType() function to create a structseq type
with the Py_TPFLAGS_DISALLOW_INSTANTIATION flag set.

type_new() calls _PyType_CheckConsistency() at exit.
This commit is contained in:
Victor Stinner 2021-04-30 12:46:15 +02:00 committed by GitHub
parent b73b5fb9ea
commit 3bb09947ec
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
13 changed files with 144 additions and 102 deletions
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28
Doc/c-api/typeobj.rst
View File

@ -1199,6 +1199,25 @@ and :c:type:`PyType_Type` effectively act as defaults.)
.. versionadded:: 3.10
.. data:: Py_TPFLAGS_DISALLOW_INSTANTIATION
Disallow creating instances of the type: set
:c:member:`~PyTypeObject.tp_new` to NULL and don't create the ``__new__``
key in the type dictionary.
The flag must be set before creating the type, not after. For example, it
must be set before :c:func:`PyType_Ready` is called on the type.
The flag is set automatically on :ref:`static types <static-types>` if
:c:member:`~PyTypeObject.tp_base` is NULL or ``&PyBaseObject_Type`` and
:c:member:`~PyTypeObject.tp_new` is NULL.
**Inheritance:**
This flag is not inherited.
.. versionadded:: 3.10
.. c:member:: const char* PyTypeObject.tp_doc
@ -1761,6 +1780,9 @@ and :c:type:`PyType_Type` effectively act as defaults.)
in :c:member:`~PyTypeObject.tp_new`, while for mutable types, most initialization should be
deferred to :c:member:`~PyTypeObject.tp_init`.
Set the :const:`Py_TPFLAGS_DISALLOW_INSTANTIATION` flag to disallow creating
instances of the type in Python.
**Inheritance:**
This field is inherited by subtypes, except it is not inherited by
@ -2596,7 +2618,8 @@ A type that supports weakrefs, instance dicts, and hashing::
};
A str subclass that cannot be subclassed and cannot be called
to create instances (e.g. uses a separate factory func)::
to create instances (e.g. uses a separate factory func) using
:c:data:`Py_TPFLAGS_DISALLOW_INSTANTIATION` flag::
typedef struct {
PyUnicodeObject raw;
@ -2609,8 +2632,7 @@ to create instances (e.g. uses a separate factory func)::
.tp_basicsize = sizeof(MyStr),
.tp_base = NULL, // set to &PyUnicode_Type in module init
.tp_doc = "my custom str",
.tp_flags = Py_TPFLAGS_DEFAULT,
.tp_new = NULL,
.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISALLOW_INSTANTIATION,
.tp_repr = (reprfunc)myobj_repr,
};

4
Doc/whatsnew/3.10.rst
View File

@ -1708,6 +1708,10 @@ New Features
These functions allow to activate, deactivate and query the state of the garbage collector from C code without
having to import the :mod:`gc` module.
* Add a new :c:data:`Py_TPFLAGS_DISALLOW_INSTANTIATION` type flag to disallow
creating type instances.
(Contributed by Victor Stinner in :issue:`43916`.)
Porting to Python 3.10
----------------------

4
Include/object.h
View File

@ -327,6 +327,10 @@ given type object has a specified feature.
#define Py_TPFLAGS_MAPPING (1 << 6)
#endif
/* Disallow creating instances of the type: set tp_new to NULL and don't create
* the "__new__" key in the type dictionary. */
#define Py_TPFLAGS_DISALLOW_INSTANTIATION (1UL << 7)
/* Set if the type object is immutable: type attributes cannot be set nor deleted */
#define Py_TPFLAGS_IMMUTABLETYPE (1UL << 8)

6
Include/structseq.h
View File

@ -27,6 +27,12 @@ PyAPI_FUNC(void) PyStructSequence_InitType(PyTypeObject *type,
PyAPI_FUNC(int) PyStructSequence_InitType2(PyTypeObject *type,
PyStructSequence_Desc *desc);
#endif
#ifdef Py_BUILD_CORE
extern int _PyStructSequence_InitType(
PyTypeObject *type,
PyStructSequence_Desc *desc,
unsigned long tp_flags);
#endif
PyAPI_FUNC(PyTypeObject*) PyStructSequence_NewType(PyStructSequence_Desc *desc);
PyAPI_FUNC(PyObject *) PyStructSequence_New(PyTypeObject* type);

5
Lib/test/test_sys.py
View File

@ -605,11 +605,12 @@ class SysModuleTest(unittest.TestCase):
def assert_raise_on_new_sys_type(self, sys_attr):
# Users are intentionally prevented from creating new instances of
# sys.flags, sys.version_info, and sys.getwindowsversion.
arg = sys_attr
attr_type = type(sys_attr)
with self.assertRaises(TypeError):
attr_type()
attr_type(arg)
with self.assertRaises(TypeError):
attr_type.__new__(attr_type)
attr_type.__new__(attr_type, arg)
def test_sys_flags_no_instantiation(self):
self.assert_raise_on_new_sys_type(sys.flags)

2
Misc/NEWS.d/next/C API/2021-04-29-17-35-48.bpo-43916.wvWt23.rst Normal file
View File

@ -0,0 +1,2 @@
Add a new :c:data:`Py_TPFLAGS_DISALLOW_INSTANTIATION` type flag to disallow
creating type instances. Patch by Victor Stinner.

3
Modules/_curses_panel.c
View File

@ -520,7 +520,7 @@ static PyType_Slot PyCursesPanel_Type_slots[] = {
static PyType_Spec PyCursesPanel_Type_spec = {
.name = "_curses_panel.panel",
.basicsize = sizeof(PyCursesPanelObject),
.flags = Py_TPFLAGS_DEFAULT,
.flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISALLOW_INSTANTIATION,
.slots = PyCursesPanel_Type_slots
};
@ -656,7 +656,6 @@ _curses_panel_exec(PyObject *mod)
if (state->PyCursesPanel_Type == NULL) {
return -1;
}
((PyTypeObject *)state->PyCursesPanel_Type)->tp_new = NULL;
if (PyModule_AddType(mod, state->PyCursesPanel_Type) < 0) {
return -1;

15
Modules/_cursesmodule.c
View File

@ -4793,25 +4793,18 @@ PyInit__curses(void)
#ifdef NCURSES_VERSION
/* ncurses_version */
if (NcursesVersionType.tp_name == NULL) {
if (PyStructSequence_InitType2(&NcursesVersionType,
&ncurses_version_desc) < 0)
if (_PyStructSequence_InitType(&NcursesVersionType,
&ncurses_version_desc,
Py_TPFLAGS_DISALLOW_INSTANTIATION) < 0) {
return NULL;
}
}
v = make_ncurses_version();
if (v == NULL) {
return NULL;
}
PyDict_SetItemString(d, "ncurses_version", v);
Py_DECREF(v);
/* prevent user from creating new instances */
NcursesVersionType.tp_init = NULL;
NcursesVersionType.tp_new = NULL;
if (PyDict_DelItemString(NcursesVersionType.tp_dict, "__new__") < 0 &&
PyErr_ExceptionMatches(PyExc_KeyError))
{
PyErr_Clear();
}
#endif /* NCURSES_VERSION */
SetDictInt("ERR", ERR);

9
Modules/_tkinter.c
View File

@ -1002,7 +1002,7 @@ static PyType_Spec PyTclObject_Type_spec = {
"_tkinter.Tcl_Obj",
sizeof(PyTclObject),
0,
Py_TPFLAGS_DEFAULT,
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISALLOW_INSTANTIATION,
PyTclObject_Type_slots,
};
@ -3294,7 +3294,7 @@ static PyType_Spec Tktt_Type_spec = {
"_tkinter.tktimertoken",
sizeof(TkttObject),
0,
Py_TPFLAGS_DEFAULT,
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISALLOW_INSTANTIATION,
Tktt_Type_slots,
};
@ -3349,7 +3349,7 @@ static PyType_Spec Tkapp_Type_spec = {
"_tkinter.tkapp",
sizeof(TkappObject),
0,
Py_TPFLAGS_DEFAULT,
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISALLOW_INSTANTIATION,
Tkapp_Type_slots,
};
@ -3537,7 +3537,6 @@ PyInit__tkinter(void)
Py_DECREF(m);
return NULL;
}
((PyTypeObject *)o)->tp_new = NULL;
if (PyModule_AddObject(m, "TkappType", o)) {
Py_DECREF(o);
Py_DECREF(m);
@ -3550,7 +3549,6 @@ PyInit__tkinter(void)
Py_DECREF(m);
return NULL;
}
((PyTypeObject *)o)->tp_new = NULL;
if (PyModule_AddObject(m, "TkttType", o)) {
Py_DECREF(o);
Py_DECREF(m);
@ -3563,7 +3561,6 @@ PyInit__tkinter(void)
Py_DECREF(m);
return NULL;
}
((PyTypeObject *)o)->tp_new = NULL;
if (PyModule_AddObject(m, "Tcl_Obj", o)) {
Py_DECREF(o);
Py_DECREF(m);

20
Modules/_xxsubinterpretersmodule.c
View File

@ -1780,7 +1780,12 @@ static PyTypeObject ChannelIDtype = {
0, /* tp_getattro */
0, /* tp_setattro */
0, /* tp_as_buffer */
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, /* tp_flags */
// Use Py_TPFLAGS_DISALLOW_INSTANTIATION so the type cannot be instantiated
// from Python code. We do this because there is a strong relationship
// between channel IDs and the channel lifecycle, so this limitation avoids
// related complications. Use the _channel_id() function instead.
Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
| Py_TPFLAGS_DISALLOW_INSTANTIATION, /* tp_flags */
channelid_doc, /* tp_doc */
0, /* tp_traverse */
0, /* tp_clear */
@ -1791,19 +1796,6 @@ static PyTypeObject ChannelIDtype = {
0, /* tp_methods */
0, /* tp_members */
channelid_getsets, /* tp_getset */
0, /* tp_base */
0, /* tp_dict */
0, /* tp_descr_get */
0, /* tp_descr_set */
0, /* tp_dictoffset */
0, /* tp_init */
0, /* tp_alloc */
// Note that we do not set tp_new to channelid_new. Instead we
// set it to NULL, meaning it cannot be instantiated from Python
// code. We do this because there is a strong relationship between
// channel IDs and the channel lifecycle, so this limitation avoids
// related complications.
NULL, /* tp_new */
};

12
Objects/structseq.c
View File

@ -459,8 +459,10 @@ initialize_members(PyStructSequence_Desc *desc, PyMemberDef* members,
members[k].name = NULL;
}
int
PyStructSequence_InitType2(PyTypeObject *type, PyStructSequence_Desc *desc)
_PyStructSequence_InitType(PyTypeObject *type, PyStructSequence_Desc *desc,
unsigned long tp_flags)
{
PyMemberDef *members;
Py_ssize_t n_members, n_unnamed_members;
@ -488,7 +490,7 @@ PyStructSequence_InitType2(PyTypeObject *type, PyStructSequence_Desc *desc)
type->tp_base = &PyTuple_Type;
type->tp_methods = structseq_methods;
type->tp_new = structseq_new;
type->tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC;
type->tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | tp_flags;
type->tp_traverse = (traverseproc) structseq_traverse;
n_members = count_members(desc, &n_unnamed_members);
@ -516,6 +518,12 @@ PyStructSequence_InitType2(PyTypeObject *type, PyStructSequence_Desc *desc)
return 0;
}
int
PyStructSequence_InitType2(PyTypeObject *type, PyStructSequence_Desc *desc)
{
return _PyStructSequence_InitType(type, desc, 0);
}
void
PyStructSequence_InitType(PyTypeObject *type, PyStructSequence_Desc *desc)
{

99
Objects/typeobject.c
View File

@ -158,6 +158,11 @@ _PyType_CheckConsistency(PyTypeObject *type)
CHECK(!(type->tp_flags & Py_TPFLAGS_READYING));
CHECK(type->tp_dict != NULL);
if (type->tp_flags & Py_TPFLAGS_DISALLOW_INSTANTIATION) {
CHECK(type->tp_new == NULL);
CHECK(_PyDict_ContainsId(type->tp_dict, &PyId___new__) == 0);
}
return 1;
#undef CHECK
}
@ -1111,8 +1116,7 @@ type_call(PyTypeObject *type, PyObject *args, PyObject *kwds)
if (type->tp_new == NULL) {
_PyErr_Format(tstate, PyExc_TypeError,
"cannot create '%.100s' instances",
type->tp_name);
"cannot create '%s' instances", type->tp_name);
return NULL;
}
@ -3185,6 +3189,8 @@ type_new_impl(type_new_ctx *ctx)
if (type_new_init_subclass(type, ctx->kwds) < 0) {
goto error;
}
assert(_PyType_CheckConsistency(type));
return (PyObject *)type;
error:
@ -5651,7 +5657,6 @@ type_add_getset(PyTypeObject *type)
static void
inherit_special(PyTypeObject *type, PyTypeObject *base)
{
/* Copying tp_traverse and tp_clear is connected to the GC flags */
if (!(type->tp_flags & Py_TPFLAGS_HAVE_GC) &&
(base->tp_flags & Py_TPFLAGS_HAVE_GC) &&
@ -5662,23 +5667,7 @@ inherit_special(PyTypeObject *type, PyTypeObject *base)
if (type->tp_clear == NULL)
type->tp_clear = base->tp_clear;
}
{
/* The condition below could use some explanation.
It appears that tp_new is not inherited for static types
whose base class is 'object'; this seems to be a precaution
so that old extension types don't suddenly become
callable (object.__new__ wouldn't insure the invariants
that the extension type's own factory function ensures).
Heap types, of course, are under our control, so they do
inherit tp_new; static extension types that specify some
other built-in type as the default also
inherit object.__new__. */
if (base != &PyBaseObject_Type ||
(type->tp_flags & Py_TPFLAGS_HEAPTYPE)) {
if (type->tp_new == NULL)
type->tp_new = base->tp_new;
}
}
if (type->tp_basicsize == 0)
type->tp_basicsize = base->tp_basicsize;
@ -5941,6 +5930,7 @@ inherit_slots(PyTypeObject *type, PyTypeObject *base)
}
static int add_operators(PyTypeObject *);
static int add_tp_new_wrapper(PyTypeObject *type);
static int
@ -5991,6 +5981,7 @@ type_ready_set_bases(PyTypeObject *type)
type->tp_base = base;
}
}
assert(type->tp_base != NULL || type == &PyBaseObject_Type);
/* Now the only way base can still be NULL is if type is
* &PyBaseObject_Type. */
@ -6249,6 +6240,50 @@ type_ready_add_subclasses(PyTypeObject *type)
}
// Set tp_new and the "__new__" key in the type dictionary.
// Use the Py_TPFLAGS_DISALLOW_INSTANTIATION flag.
static int
type_ready_set_new(PyTypeObject *type)
{
PyTypeObject *base = type->tp_base;
/* The condition below could use some explanation.
It appears that tp_new is not inherited for static types whose base
class is 'object'; this seems to be a precaution so that old extension
types don't suddenly become callable (object.__new__ wouldn't insure the
invariants that the extension type's own factory function ensures).
Heap types, of course, are under our control, so they do inherit tp_new;
static extension types that specify some other built-in type as the
default also inherit object.__new__. */
if (type->tp_new == NULL
&& base == &PyBaseObject_Type
&& !(type->tp_flags & Py_TPFLAGS_HEAPTYPE))
{
type->tp_flags |= Py_TPFLAGS_DISALLOW_INSTANTIATION;
}
if (!(type->tp_flags & Py_TPFLAGS_DISALLOW_INSTANTIATION)) {
if (type->tp_new != NULL) {
// If "__new__" key does not exists in the type dictionary,
// set it to tp_new_wrapper().
if (add_tp_new_wrapper(type) < 0) {
return -1;
}
}
else {
// tp_new is NULL: inherit tp_new from base
type->tp_new = base->tp_new;
}
}
else {
// Py_TPFLAGS_DISALLOW_INSTANTIATION sets tp_new to NULL
type->tp_new = NULL;
}
return 0;
}
static int
type_ready(PyTypeObject *type)
{
@ -6275,6 +6310,9 @@ type_ready(PyTypeObject *type)
if (type_ready_mro(type) < 0) {
return -1;
}
if (type_ready_set_new(type) < 0) {
return -1;
}
if (type_ready_fill_dict(type) < 0) {
return -1;
}
@ -6898,8 +6936,8 @@ tp_new_wrapper(PyObject *self, PyObject *args, PyObject *kwds)
"__new__() called with non-type 'self'");
return NULL;
}
type = (PyTypeObject *)self;
if (!PyTuple_Check(args) || PyTuple_GET_SIZE(args) < 1) {
PyErr_Format(PyExc_TypeError,
"%s.__new__(): not enough arguments",
@ -6961,16 +6999,18 @@ static struct PyMethodDef tp_new_methoddef[] = {
static int
add_tp_new_wrapper(PyTypeObject *type)
{
PyObject *func;
int r = _PyDict_ContainsId(type->tp_dict, &PyId___new__);
if (r > 0)
if (r > 0) {
return 0;
if (r < 0)
}
if (r < 0) {
return -1;
func = PyCFunction_NewEx(tp_new_methoddef, (PyObject *)type, NULL);
if (func == NULL)
}
PyObject *func = PyCFunction_NewEx(tp_new_methoddef, (PyObject *)type, NULL);
if (func == NULL) {
return -1;
}
r = _PyDict_SetItemId(type->tp_dict, &PyId___new__, func);
Py_DECREF(func);
return r;
@ -8558,11 +8598,6 @@ add_operators(PyTypeObject *type)
Py_DECREF(descr);
}
}
if (type->tp_new != NULL) {
if (add_tp_new_wrapper(type) < 0) {
return -1;
}
}
return 0;
}

39
Python/sysmodule.c
View File

@ -2810,56 +2810,35 @@ _PySys_InitCore(PyThreadState *tstate, PyObject *sysdict)
/* version_info */
if (VersionInfoType.tp_name == NULL) {
if (PyStructSequence_InitType2(&VersionInfoType,
&version_info_desc) < 0) {
if (_PyStructSequence_InitType(&VersionInfoType,
&version_info_desc,
Py_TPFLAGS_DISALLOW_INSTANTIATION) < 0) {
goto type_init_failed;
}
}
version_info = make_version_info(tstate);
SET_SYS("version_info", version_info);
/* prevent user from creating new instances */
VersionInfoType.tp_init = NULL;
VersionInfoType.tp_new = NULL;
res = PyDict_DelItemString(VersionInfoType.tp_dict, "__new__");
if (res < 0 && _PyErr_ExceptionMatches(tstate, PyExc_KeyError)) {
_PyErr_Clear(tstate);
}
/* implementation */
SET_SYS("implementation", make_impl_info(version_info));
// sys.flags: updated in-place later by _PySys_UpdateConfig()
if (FlagsType.tp_name == 0) {
if (PyStructSequence_InitType2(&FlagsType, &flags_desc) < 0) {
if (_PyStructSequence_InitType(&FlagsType, &flags_desc,
Py_TPFLAGS_DISALLOW_INSTANTIATION) < 0) {
goto type_init_failed;
}
}
SET_SYS("flags", make_flags(tstate->interp));
/* prevent user from creating new instances */
FlagsType.tp_init = NULL;
FlagsType.tp_new = NULL;
res = PyDict_DelItemString(FlagsType.tp_dict, "__new__");
if (res < 0) {
if (!_PyErr_ExceptionMatches(tstate, PyExc_KeyError)) {
goto err_occurred;
}
_PyErr_Clear(tstate);
}
#if defined(MS_WINDOWS)
/* getwindowsversion */
if (WindowsVersionType.tp_name == 0)
if (PyStructSequence_InitType2(&WindowsVersionType,
&windows_version_desc) < 0) {
if (WindowsVersionType.tp_name == 0) {
if (_PyStructSequence_InitType(&WindowsVersionType,
&windows_version_desc,
Py_TPFLAGS_DISALLOW_INSTANTIATION) < 0) {
goto type_init_failed;
}
/* prevent user from creating new instances */
WindowsVersionType.tp_init = NULL;
WindowsVersionType.tp_new = NULL;
assert(!_PyErr_Occurred(tstate));
res = PyDict_DelItemString(WindowsVersionType.tp_dict, "__new__");
if (res < 0 && _PyErr_ExceptionMatches(tstate, PyExc_KeyError)) {
_PyErr_Clear(tstate);
}
#endif