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https://sourceware.org/git/binutils-gdb.git
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60cd57ff6e
All of our custom Python types are created as structs, like this:
struct some_new_type : public PyObject
{
... various fields ...
};
Then instances of this struct are created by calling PyObject_New,
either directly within GDB's C++ code, or within Python when a user's
Python script creates an instance of that class.
The problem is that Python is written in C, and PyObject_New doesn't
call any constructors for `some_new_type`, nor for any of the fields
within `some_new_type`.
If `some_new_type` is Plain Old Data (POD), then this is fine. Or, to
be more C++ specific, if `some_new_type` is trivially default
constructable, then we're fine.
But if a field within `some_new_type` has a non-trivial constructor,
then we're in trouble as that constructor will never be run.
An example of a problematic field type is frame_info_ptr. The
constructor for this type registers the new object with a central
management object, recording the `this` pointer, using this type within
`some_new_type` will not work as expected; frame invalidation will not
show up within the frame_info_ptr as you might expect.
And so, this type trait exists. Whenever a struct is created to define
a new Python type we should add a line like:
static_assert (gdb::is_python_allocatable_v<some_new_type>);
This will fail if any field of `some_new_type` are unsuitable for this
use.
We don't actually check is_trivially_default_constructible here. Some
types, e.g. ui_file_style::color, have non-trivial (or no default)
constructors, but are still safe to use within `some_new_type` because
their constructors just initialise data fields; there's nothing
"special" that the constructor does that cannot be achieved by
assigning the fields after creation with PyObject_New.
What actually matters is that the type is trivially destructible
(Python won't call C++ destructors, so destructors with side effects,
like deregistering from a list, would be skipped) and trivially
copyable (Python may copy objects with memcpy). Types like
frame_info_ptr, whose constructors and destructors have side effects
such as registering with a central management object, will be caught
because they are neither trivially destructible nor trivially copyable.
Simple POD types like ui_file_style are trivially destructible and
copyable, so pass this trait.
This commit adds the new type trait, and makes use of it in all cases
but one, pending_frame_object in python/py-unwind.c, has a field of
type frame_info_ptr, which is currently broken. This will be fixed,
and the static_assert added, in the next commit.
Approved-By: Tom Tromey <tom@tromey.com>
108 lines
3.5 KiB
C++
108 lines
3.5 KiB
C++
/* Python reference-holding class
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Copyright (C) 2016-2026 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#ifndef GDB_PYTHON_PY_REF_H
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#define GDB_PYTHON_PY_REF_H
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#include "gdbsupport/gdb_ref_ptr.h"
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#include "python-traits.h"
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/* A policy class for gdb::ref_ptr for Python reference counting. */
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struct gdbpy_ref_policy
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{
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static void incref (PyObject *ptr)
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{
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Py_INCREF (ptr);
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}
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static void decref (PyObject *ptr)
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{
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Py_DECREF (ptr);
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}
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};
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/* A gdb::ref_ptr that has been specialized for Python objects or
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their "subclasses". */
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template<typename T = PyObject> using gdbpy_ref
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= gdb::ref_ptr<T, gdbpy_ref_policy>;
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/* A wrapper class for Python extension objects that have a __dict__ attribute.
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Any Python C object extension needing __dict__ should inherit from this
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class. Given that the C extension object must also be convertible to
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PyObject, this wrapper class publicly inherits from PyObject as well.
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Access to the dict requires a custom getter defined via PyGetSetDef.
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gdb_PyGetSetDef my_object_getset[] =
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{
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gdbpy_dict_wrapper_cfg_dict_getter ("object"),
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...
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{ nullptr }
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};
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It is also important to note that __dict__ is used during the attribute
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look-up. Since this dictionary is not managed by Python and is not exposed
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via tp_dictoffset, custom attribute getter (tp_getattro) and setter
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(tp_setattro) are required to correctly redirect attribute access to the
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dictionary:
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- gdb_py_generic_getattro (), assigned to tp_getattro for static types,
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or Py_tp_getattro for heap-allocated types.
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- gdb_py_generic_setattro (), assigned to tp_setattro for static types,
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or Py_tp_setattro for heap-allocated types. */
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struct gdbpy_dict_wrapper : public PyObject
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{
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/* Dictionary holding user-added attributes.
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This is the __dict__ attribute of the object. */
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PyObject *dict;
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/* Compute the address of the __dict__ attribute for the given PyObject. */
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static PyObject **compute_addr (PyObject *self)
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{
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auto *wrapper = reinterpret_cast<gdbpy_dict_wrapper *> (self);
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return &wrapper->dict;
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}
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#define gdbpy_dict_wrapper_cfg_dict_getter(object_name) \
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{ \
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"__dict__", /* name */ \
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(getter) gdb_py_generic_dict_getter, \
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(setter) nullptr, \
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"The __dict__ for this " object_name ".", /* doc */ \
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nullptr, /* closure */ \
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}
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#define gdbpy_dict_wrapper_getsetattro \
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/*tp_getattro*/ \
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gdb_py_generic_getattro, \
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/*tp_setattro*/ \
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gdb_py_generic_setattro
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/* Allocate the dictionary pointed by 'dict'.
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Note: this method should be called once the object was allocated,
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when setting its attributes. */
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bool allocate_dict ()
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{
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dict = PyDict_New ();
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return dict != nullptr;
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}
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};
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static_assert (gdb::is_python_allocatable_v<gdbpy_dict_wrapper>);
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#endif /* GDB_PYTHON_PY_REF_H */
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