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.

1 files changed, 67 insertions, 32 deletions

diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 1f8e2572a2..0f7f280bca 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -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;
 }