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, 25 insertions, 9 deletions

diff --git a/gdb/python/python-internal.h b/gdb/python/python-internal.h
index 95619bf775e..f6915a62b7a 100644
--- a/gdb/python/python-internal.h
+++ b/gdb/python/python-internal.h
@@ -107,6 +107,15 @@ typedef unsigned long gdb_py_ulongest;
 
 #endif /* HAVE_LONG_LONG */
 
+#if PY_VERSION_HEX < 0x030a0000
+static inline PyObject *
+Py_NewRef (PyObject *obj)
+{
+  Py_INCREF (obj);
+  return obj;
+}
+#endif
+
 /* A template variable holding the format character (as for
    Py_BuildValue) for a given type.  */
 template<typename T>
@@ -384,22 +393,27 @@ struct gdbpy_breakpoint_object
 extern gdbpy_breakpoint_object *bppy_pending_object;
 
 
-struct thread_object
+struct thread_object : public gdbpy_dict_wrapper
 {
-  PyObject_HEAD
-
   /* The thread we represent.  */
   struct thread_info *thread;
 
   /* The Inferior object to which this thread belongs.  */
   PyObject *inf_obj;
-
-  /* Dictionary holding user-added attributes.  This is the __dict__
-     attribute of the object.  */
-  PyObject *dict;
 };
 
-struct inferior_object;
+using thread_map_t
+  = gdb::unordered_map<thread_info *, gdbpy_ref<thread_object>>;
+
+struct inferior_object : public gdbpy_dict_wrapper
+{
+  /* The inferior we represent.  */
+  struct inferior *inferior;
+
+  /* thread_object instances under this inferior.  This owns a
+     reference to each object it contains.  */
+  thread_map_t *threads;
+};
 
 extern struct cmd_list_element *set_python_list;
 extern struct cmd_list_element *show_python_list;
@@ -989,7 +1003,9 @@ gdbpy_ref<> gdb_py_object_from_longest (LONGEST l);
 gdbpy_ref<> gdb_py_object_from_ulongest (ULONGEST l);
 int gdb_py_int_as_long (PyObject *, long *);
 
-PyObject *gdb_py_generic_dict (PyObject *self, void *closure);
+PyObject *gdb_py_generic_dict_getter (PyObject *self, void *closure);
+PyObject *gdb_py_generic_getattro (PyObject *self, PyObject *attr);
+int gdb_py_generic_setattro (PyObject *self, PyObject *attr, PyObject *value);
 
 int gdb_pymodule_addobject (PyObject *mod, const char *name,
 			    PyObject *object);