gdb: new setters and getters for __dict__, and attributes

GDB is currently using the Python unlimited API. Migrating the codebase
to the Python limited API would have for benefit to make a GDB build
artifacts compatible with older and newer versions of Python that they
were built with.

This patch prepares the ground for migrating the existing C extension
types from static types to heap-allocated ones, by removing the
dependency on tp_dictoffset, which is unavailable when using the limited
API.

One of the most common incompatibilities in the current static type
declarations is the tp_dictoffset slot, which specifies the dictionary
offset within the instance structure. Historically, the unlimited
API has provided two approaches to supply a dictionary for __dict__:

 * A managed dictionary.
   Setting Py_TPFLAGS_MANAGED_DICT in tp_flags indicates that the
   instances of the type have a __dict__ attribute, and that the
   dictionary is managed by Python.
   According to the Python documentation, this is the recommended approach.
   However, this flag was introduced in 3.12, together with
   PyObject_VisitManagedDict() and PyObject_ClearManagedDict(), neither
   of which is part of the limited API (at least for now). As a result,
   this recommended approach is not viable in the context of the limited
   API.

 * An instance dictionary, for which the offset in the instance is
   provided via tp_dictoffset.
   According to the Python documentation, this "tp slot" is on the
   deprecation path, and Py_TPFLAGS_MANAGED_DICT should be used instead.
   Given the age of the GDB codebase and the requirement to support older
   Python versions (>= 3.4), no need to argue that today, the implementation
   of __dict__ relies on tp_dictoffset. However, in the context of the
   limited API, PyType_Slot does not provide a Py_tp_dictoffset member, so
   another approach is needed to provide __dict__ to instances of C extension
   types.

Given the constraints of the limited API, the proposed solution consists
in providing a dictionary through a common base class, gdbpy__dict__wrapper.
This helper class owns a dictionary member corresponding to __dict__, and
any C extension type requiring a __dict__ must inherit from it. Since
extension object must also be convertible to PyObject, this wrapper class
publicly inherits from PyObject as well.
Access to the dictionary is provided via a custom getter defined in a
PyGetSetDef, similarily to what was previously done with gdb_py_generic_dict().
Because __dict__ participates in attribute look-up, and since this dictionary
is neither managed by Python nor exposed via tp_dictoffset, custom
implementations of tp_getattro and tp_setattro are required to correctly
redirect attribute look-ups to the dictionary. These custom implementations
— equivalent to PyObject_GenericGetAttr() and PyObject_GenericSetAttr() —
must be installed via tp_getattro / tp_setattro for static types, or
Py_tp_getattro / Py_tp_setattro for heap-allocated types.

- gdbpy__dict__wrapper: a base class for C extension objects that own a
  __dict__.
- gdb_py_generic_dict_getter: a __dict__ getter for extension types
  derived from gdbpy__dict__wrapper.
- gdb_py_generic_getattro: equivalent of PyObject_GenericGetAttr, but
  fixes the look-up of __dict__.
- gdb_py_generic_setattro: equivalent of PyObject_GenericSetAttr, but
  fixes the look-up of __dict__.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23830
Approved-By: Tom Tromey <tom@tromey.com>

1 files changed, 5 insertions, 5 deletions

diff --git a/gdb/python/py-event.c b/gdb/python/py-event.c
index c985159a6f7..8fb21b7fdad 100644
--- a/gdb/python/py-event.c
+++ b/gdb/python/py-event.c
@@ -101,8 +101,8 @@ GDBPY_INITIALIZE_FILE (gdbpy_initialize_event);
 
 static gdb_PyGetSetDef event_object_getset[] =
 {
-  { "__dict__", gdb_py_generic_dict, NULL,
-    "The __dict__ for this event.", &event_object_type },
+  { "__dict__", gdb_py_generic_dict_getter, NULL,
+    "The __dict__ for this event.", NULL },
   { NULL }
 };
 
@@ -124,8 +124,8 @@ PyTypeObject event_object_type =
   0,                                          /* tp_hash  */
   0,                                          /* tp_call */
   0,                                          /* tp_str */
-  0,                                          /* tp_getattro */
-  0,                                          /* tp_setattro */
+  gdb_py_generic_getattro,                    /* tp_getattro */
+  gdb_py_generic_setattro,                    /* tp_setattro */
   0,                                          /* tp_as_buffer */
   Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,   /* tp_flags */
   "GDB event object",                         /* tp_doc */
@@ -142,7 +142,7 @@ PyTypeObject event_object_type =
   0,                                          /* tp_dict */
   0,                                          /* tp_descr_get */
   0,                                          /* tp_descr_set */
-  offsetof (event_object, dict),              /* tp_dictoffset */
+  0,                                          /* tp_dictoffset */
   0,                                          /* tp_init */
   0                                           /* tp_alloc */
 };