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Leaking the PyTypeObject structure in the C API indirectly causes ABI issue (especially for statically allocated types), cause practical issues when old fields are removed and new fields are added (ex: tp_vectorcall addition and tp_print removal caused a lot of troubles with C code generated by Cython: see bpo-37250), prevents us to add feature and experiment optimization.
I don't expect that we will be able to make PyTypeObject opaque soon. The purpose of this issue is to track the work done towards this goal.
I propose to slowly prepare the Python code base, the C API and third party code (especially Cython) to make PyTypeObject structure opaque.
We have to identify most common code patterns which access directly PyTypeObject fields, provide helper functions, and ease the migration to solutions which don't access directly PyTypeObject.
See also bpo-39573: "Make PyObject an opaque structure in the limited C API" and bpo-39947 "Make the PyThreadState structure opaque (move it to the internal C API)".
Longer rationale about making structures of the C API opaque:
* https://pythoncapi.readthedocs.io/
* https://pythoncapi.readthedocs.io/bad_api.html
* https://pythoncapi.readthedocs.io/optimization_ideas.html
--
Multiple practical issues are preventing us to make PyTypeObject opaque right now.
(*) Many C extension modules are still using statically allocated types: there is an on-going effort in bpo-40077 to convert C extension modules one by one to PyType_FromSpec().
(*) Py_TYPE(obj)->tp_name is commonly accessed to format an error message. Example:
 PyErr_Format(PyExc_TypeError, "exec() globals must be a dict, not %.100s",
 Py_TYPE(globals)->tp_name);
I worked on bpo-34595 and started a discussion on python-dev to propose to add a new %T formatter to PyUnicode_FromFormatV() and so indirectly to PyUnicode_FromFormat() and PyErr_Format():
https://mail.python.org/archives/list/python-dev@python.org/thread/HKYUMTVHNBVB5LJNRMZ7TPUQKGKAERCJ/#3UAMHYG6UF4MPLXBZORHO4JVKUBRUZ53
Sadly, we failed to reach a consensus and I gave up on this idea. We should reconsider this idea.
We need to agree on how types should be formatted:
* just the name without any dot "type_name",
* qualified name "something.type_name",
* fully qualified name "module.something.type_name"
There is also the question of breaking applications which rely on the current exact error message. And the question of removing legacy "%.100s" which was used before Python was able to allocate a buffer large enough to arbitrary string length. When an error is formatted in pure Python, names are never truncated.
(*) Call the function of the parent type when a method is overriden in a subclass. Example with PyTypeObject.tp_free called in a deallocator:
static void
abc_data_dealloc(_abc_data *self)
{
 PyTypeObject *tp = Py_TYPE(self);
 ...
 tp->tp_free(self);
 Py_DECREF(tp);
}
(*) The PEP 384 provides the most generic PyType_GetSlot() but it's not convenient to use: need to handle error (NULL), need to cast the void* into the expected type (error prone cast), etc.
We should slowly add more and more helper functions for most common use cases. We can try to convert a few C extension modules of the Python stdlib to see which use cases are the most common.
Hopefully, many use cases are already abstracted by widely used functions like PyNumber_Add(), PySequence_Size(), etc.
(*) Likely other issues that I forgot.

Macros and static inline functions of the public C API which access directly PyTypeObject fields. There may be more.
#define _PyObject_SIZE(typeobj) ( (typeobj)->tp_basicsize )
static inline vectorcallfunc
PyVectorcall_Function(PyObject *callable)
{
 ...
 tp = Py_TYPE(callable);
 offset = tp->tp_vectorcall_offset;
 ...
}
#define PyObject_CheckBuffer(obj) \
 ((Py_TYPE(obj)->tp_as_buffer != NULL) && \
 (Py_TYPE(obj)->tp_as_buffer->bf_getbuffer != NULL))
#define PyIndex_Check(obj) \
 (Py_TYPE(obj)->tp_as_number != NULL && \
 Py_TYPE(obj)->tp_as_number->nb_index != NULL)
#define PyObject_GET_WEAKREFS_LISTPTR(o) \
 ((PyObject **) (((char *) (o)) + Py_TYPE(o)->tp_weaklistoffset))
static inline int
PyType_HasFeature(PyTypeObject *type, unsigned long feature) {
#ifdef Py_LIMITED_API
 return ((PyType_GetFlags(type) & feature) != 0);
#else
 return ((type->tp_flags & feature) != 0);
#endif
}
#define _PyObject_SIZE(typeobj) ( (typeobj)->tp_basicsize )

New changeset 38aefc585f60a77d66f4fbe5a37594a488b53474 by Victor Stinner in branch 'master':
bpo-40170: PyObject_GET_WEAKREFS_LISTPTR() becomes a function (GH-19377)
https://github.com/python/cpython/commit/38aefc585f60a77d66f4fbe5a37594a488b53474

Just manually verified that PR19377, when compiled against xlc - crashes during make:
	rm -f libpython3.9d.a
	ar rcs libpython3.9d.a Modules/getbuildinfo.o Parser/acceler.o Parser/grammar1.o Parser/listnode.o Parser/node.o Parser/parser.o Parser/token.o Parser/myreadline.o Parser/parsetok.o Parser/tokenizer.o Objects/abstract.o Objects/accu.o Objects/boolobject.o Objects/bytes_methods.o Objects/bytearrayobject.o Objects/bytesobject.o Objects/call.o Objects/capsule.o Objects/cellobject.o Objects/classobject.o Objects/codeobject.o Objects/complexobject.o Objects/descrobject.o Objects/enumobject.o Objects/exceptions.o Objects/genobject.o Objects/fileobject.o Objects/floatobject.o Objects/frameobject.o Objects/funcobject.o Objects/interpreteridobject.o Objects/iterobject.o Objects/listobject.o Objects/longobject.o Objects/dictobject.o Objects/odictobject.o Objects/memoryobject.o Objects/methodobject.o Objects/moduleobject.o Objects/namespaceobject.o Objects/object.o Objects/obmalloc.o Objects/picklebufobject.o Objects/rangeobject.o Objects/setobject.o Objects/sliceobject.o Objects/structseq.o Objects/tupleobject.o Objects/typeobject.o Objects/unicodeobject.o Objects/unicodectype.o Objects/weakrefobject.o Python/_warnings.o Python/Python-ast.o Python/asdl.o Python/ast.o Python/ast_opt.o Python/ast_unparse.o Python/bltinmodule.o Python/ceval.o Python/codecs.o Python/compile.o Python/context.o Python/dynamic_annotations.o Python/errors.o Python/frozenmain.o Python/future.o Python/getargs.o Python/getcompiler.o Python/getcopyright.o Python/getplatform.o Python/getversion.o Python/graminit.o Python/hamt.o Python/import.o Python/importdl.o Python/initconfig.o Python/marshal.o Python/modsupport.o Python/mysnprintf.o Python/mystrtoul.o Python/pathconfig.o Python/peephole.o Python/preconfig.o Python/pyarena.o Python/pyctype.o Python/pyfpe.o Python/pyhash.o Python/pylifecycle.o Python/pymath.o Python/pystate.o Python/pythonrun.o Python/pytime.o Python/bootstrap_hash.o Python/structmember.o Python/symtable.o Python/sysmodule.o Python/thread.o Python/traceback.o Python/getopt.o Python/pystrcmp.o Python/pystrtod.o Python/pystrhex.o Python/dtoa.o Python/formatter_unicode.o Python/fileutils.o Python/dynload_shlib.o Modules/config.o Modules/getpath.o Modules/main.o Modules/gcmodule.o Modules/posixmodule.o Modules/errnomodule.o Modules/pwdmodule.o Modules/_sre.o Modules/_codecsmodule.o Modules/_weakref.o Modules/_functoolsmodule.o Modules/_operator.o Modules/_collectionsmodule.o Modules/_abc.o Modules/itertoolsmodule.o Modules/atexitmodule.o Modules/signalmodule.o Modules/_stat.o Modules/timemodule.o Modules/_threadmodule.o Modules/_localemodule.o Modules/_iomodule.o Modules/iobase.o Modules/fileio.o Modules/bytesio.o Modules/bufferedio.o Modules/textio.o Modules/stringio.o Modules/faulthandler.o Modules/_tracemalloc.o Modules/hashtable.o Modules/symtablemodule.o Modules/xxsubtype.o Python/frozen.o
	./Modules/makexp_aix Modules/python.exp . libpython3.9d.a; xlc_r -Wl,-bE:Modules/python.exp -lld -o python Programs/python.o libpython3.9d.a -lintl -ldl -lm -lm 
	 ./python -E -S -m sysconfig --generate-posix-vars ; if test $? -ne 0 ; then echo "generate-posix-vars failed" ; rm -f ./pybuilddir.txt ; exit 1 ; fi
Objects/genobject.c:127: _PyObject_GC_TRACK: Assertion "!(((PyGC_Head *)(op)-1)->_gc_next != 0)" failed: object already tracked by the garbage collector
Enable tracemalloc to get the memory block allocation traceback
object address : 30084150
object refcount : 0
object type : 20013aa8
object type name: generator
object repr : <refcnt 0 at 30084150>
Fatal Python error: _PyObject_AssertFailed: _PyObject_AssertFailed
Python runtime state: core initialized
Current thread 0x00000001 (most recent call first):
 File "<frozen importlib._bootstrap_external>", line 1593 in _setup
 File "<frozen importlib._bootstrap_external>", line 1634 in _install
 File "<frozen importlib._bootstrap>", line 1189 in _install_external_importers
/bin/sh: 24117648 IOT/Abort trap(coredump)
make: 1254-004 The error code from the last command is 134.
Stop.
FYI: about two hours ago I verified that xlc and 08050e959e6c40839cd2c9e5f6a4fd1513e3d605 : bpo-40147: Fix a compiler warning on Windows in Python/compile.c (GH-19389)
all was green.

Just checked - seems to be SPECIFIC to xlc-v16 as neither xlv-v11 nor xlc-v13 have any issues building.

Py_TRASHCAN_BEGIN() access directly PyTypeObject.tp_dealloc:
#define Py_TRASHCAN_BEGIN(op, dealloc) \
 Py_TRASHCAN_BEGIN_CONDITION(op, \
 Py_TYPE(op)->tp_dealloc == (destructor)(dealloc))
It should use PyType_GetSlot() or a new getter function (to read PyTypeObject.tp_dealloc) should be added.

> It should use PyType_GetSlot()
Oh. It seems like currently, PyType_GetSlot() can only be used on a heap allocated types :-( The function starts with:
 if (!PyType_HasFeature(type, Py_TPFLAGS_HEAPTYPE) || slot < 0) {
 PyErr_BadInternalCall();
 return NULL;
 }

> Just checked - seems to be SPECIFIC to xlc-v16 as neither xlv-v11 nor xlc-v13 have any issues building.
That sounds like an AIX specific issue. Please open a separated issue.

New changeset 9205520d8c43488696d66cbdd9aefbb21871c508 by Victor Stinner in branch 'master':
bpo-40170: PyObject_NEW() becomes an alias to PyObject_New() (GH-19379)
https://github.com/python/cpython/commit/9205520d8c43488696d66cbdd9aefbb21871c508

New changeset ef5c615f5ae72c4f6979159c94da46afefbfab9a by Victor Stinner in branch 'master':
bpo-40170: Convert PyObject_CheckBuffer() macro to a function (GH-19376)
https://github.com/python/cpython/commit/ef5c615f5ae72c4f6979159c94da46afefbfab9a

New changeset 45ec5b99aefa54552947049086e87ec01bc2fc9a by Victor Stinner in branch 'master':
bpo-40170: PyType_HasFeature() now always calls PyType_GetFlags() (GH-19378)
https://github.com/python/cpython/commit/45ec5b99aefa54552947049086e87ec01bc2fc9a

New changeset a15e260b708a98edaba86a2aa663c3f6b2abc964 by Victor Stinner in branch 'master':
bpo-40170: Add _PyIndex_Check() internal function (GH-19426)
https://github.com/python/cpython/commit/a15e260b708a98edaba86a2aa663c3f6b2abc964

Hum, once most changes will land, maybe it would be worth it to document them at:
https://docs.python.org/dev/whatsnew/3.9.html#build-and-c-api-changes 

New changeset 307b9d0144e719b016a47fcc43397c070615e01e by Victor Stinner in branch 'master':
bpo-40170: Remove PyIndex_Check() macro (GH-19428)
https://github.com/python/cpython/commit/307b9d0144e719b016a47fcc43397c070615e01e

/* Test if an object has a GC head */
#define PyObject_IS_GC(o) \
 (PyType_IS_GC(Py_TYPE(o)) \
 && (Py_TYPE(o)->tp_is_gc == NULL || Py_TYPE(o)->tp_is_gc(o)))
This macro should be converted to an opaque function.

> This macro should be converted to an opaque function
Can I try it?

New changeset 675d9a3d7afc767a2818c84da7ba4bf4181dcf26 by Hai Shi in branch 'master':
bpo-40170: Convert PyObject_IS_GC() macro to a function (GH-19464)
https://github.com/python/cpython/commit/675d9a3d7afc767a2818c84da7ba4bf4181dcf26

On python-dev, Ronald Oussoren asked to add support for the buffer protocol in PyType_FromSpec() and PyTypeSpec API:
Ronald:
> BTW. This will require growing the PyTypeSpec ABI a little, there are features you cannot implement using that API for example the buffer protocol.
https://mail.python.org/archives/list/python-dev@python.org/message/PGKRW7S2IUOWVRX6F7RT6VAWD3ZPUDYS/
See also PyType_FromSpec() issue with opaque PyObject:
https://bugs.python.org/issue39573#msg366473 

Something else that probably needs attention with the TypeSpec API is subclassing type in an extension when that subclass adds fields to the type object.
I use this in PyObjC to (dynamically) create types that have some additional data. I could probably work around this issue by adding a level of indirection (basically storing the extra data in a WeakKeyDictionary), but haven't looked into this yet.

> Py_TRASHCAN_BEGIN() access directly PyTypeObject.tp_dealloc
Looks like this macro not recorded in docs. Do we need using function to replace this macro?

> Looks like this macro not recorded in docs.
It never prevented anyone to use a function of the C API :-)

> It never prevented anyone to use a function of the C API :-)
Got it. If possible someone uses it, I will try to add a function to repalce it(MAYBE udpate this docs too;) )

> Py_TRASHCAN_BEGIN() access directly PyTypeObject.tp_dealloc (...) currently, PyType_GetSlot() can only be used on a heap allocated types
I created bpo-41073: [C API] PyType_GetSlot() should accept static types.

PyIter_Check() and PySequence_ITEM() macros access directly PyTypeObject members and must be converted to opaque functions:
#define PyIter_Check(obj) \
 (Py_TYPE(obj)->tp_iternext != NULL && \
 Py_TYPE(obj)->tp_iternext != &_PyObject_NextNotImplemented)
#define PySequence_ITEM(o, i)\
 ( Py_TYPE(o)->tp_as_sequence->sq_item(o, i) )

> PyIter_Check() and PySequence_ITEM() macros access directly PyTypeObject members and must be converted to opaque functions: (...)
PyIter_Check() and PySequence_ITEM() are declared as functions in the limited C API, but overriden with macros in the CPython C API.
I suggest to simply remove the macros to always declare them as functions.
See bpo-33738 "PyIndex_Check conflicts with PEP 384" which added the functions.
See also Tools/scripts/pep384_macrocheck.py script.

New changeset b26a0db8ea2de3a8a8e4b40e69fc8642c7d7cb68 by Victor Stinner in branch 'master':
Revert "bpo-40170: PyType_HasFeature() now always calls PyType_GetFlags() (GH-19378)" (GH-21390)
https://github.com/python/cpython/commit/b26a0db8ea2de3a8a8e4b40e69fc8642c7d7cb68

> New changeset 45ec5b99aefa54552947049086e87ec01bc2fc9a by Victor Stinner in branch 'master':
> bpo-40170: PyType_HasFeature() now always calls PyType_GetFlags() (GH-19378)
This change causes performance issues on macOS, see discussion starting at:
https://bugs.python.org/issue39542#msg372962
So I reverted the change. I will wait until my PEP 620 is accepted before considering to reapply it.
If it's reapplied, we have to make sure that Python internals currently using PyTuple_Check() still access directly PyTypeObject.tp_flags member. For example, a new _PyTuple_Check() function could be added and uses the internal _PyType_HasFeature() function.

New changeset a0a6f1167834c87f12e2eca11dd77143103e7691 by Miss Islington (bot) in branch '3.9':
Revert "bpo-40170: PyType_HasFeature() now always calls PyType_GetFlags() (GH-19378)" (GH-21390)
https://github.com/python/cpython/commit/a0a6f1167834c87f12e2eca11dd77143103e7691

Thanks for doing this. I can confirm the performance regression is fixed and that clean code is being generated for PyTuple_Check().
BTW, I support your efforts — just wanted to make sure we didn't unintentionally take a step backwards.

> Thanks for doing this. I can confirm the performance regression is fixed and that clean code is being generated for PyTuple_Check().
Thanks for checking!
> BTW, I support your efforts — just wanted to make sure we didn't unintentionally take a step backwards.
I tried to avoid changes which could affect performances.
I wrote PEP 620 for such changes.

New changeset 97d15ae1d8411b49b1fcdc0c67c51849dccce9c9 by Victor Stinner in branch 'master':
bpo-40170: Use inline _PyType_HasFeature() function (GH-22375)
https://github.com/python/cpython/commit/97d15ae1d8411b49b1fcdc0c67c51849dccce9c9

New changeset ba2958ed40d284228836735cbed4a155190e0998 by Victor Stinner in branch 'master':
bpo-40170: Fix PyType_Ready() refleak on static type (GH-23236)
https://github.com/python/cpython/commit/ba2958ed40d284228836735cbed4a155190e0998

New changeset ed1a5a5baca8f61e9a99c5be3adc16b1801514fe by Hai Shi in branch 'master':
bpo-40170: Hide impl detail of Py_TRASHCAN_BEGIN macro (GH-23235)
https://github.com/python/cpython/commit/ed1a5a5baca8f61e9a99c5be3adc16b1801514fe

See also bpo-41618: [C API] How many slots of static types should be exposed in PyType_GetSlot().

After GH-24533 and GH-24535, there's (AFAICS) only a handful of cases left.
$ grep -r "\->tp_" Include/:
Include/internal/pycore_abstract.h: PyNumberMethods *tp_as_number = Py_TYPE(obj)->tp_as_number;
Include/internal/pycore_interp.h: unsigned int version; // initialized from type->tp_version_tag
Include/internal/pycore_object.h: return ((type->tp_flags & feature) != 0);
Include/internal/pycore_object.h: Py_ssize_t offset = Py_TYPE(op)->tp_weaklistoffset;
Include/internal/pycore_object.h: && (Py_TYPE(obj)->tp_is_gc == NULL
Include/internal/pycore_object.h: || Py_TYPE(obj)->tp_is_gc(obj)));
Include/cpython/pyerrors.h:#define PyExceptionClass_Name(x) (((PyTypeObject*)(x))->tp_name)
Include/cpython/abstract.h: offset = tp->tp_vectorcall_offset;
Include/cpython/abstract.h: ( Py_TYPE(o)->tp_as_sequence->sq_item(o, i) )
Include/cpython/object.h: ((PyMemberDef *)(((char *)etype) + Py_TYPE(etype)->tp_basicsize))
Include/cpython/objimpl.h:#define _PyObject_SIZE(typeobj) ( (typeobj)->tp_basicsize )
Include/cpython/objimpl.h: _Py_SIZE_ROUND_UP((typeobj)->tp_basicsize + \
Include/cpython/objimpl.h: (nitems)*(typeobj)->tp_itemsize, \
Include/cpython/objimpl.h:#define PyType_SUPPORTS_WEAKREFS(t) ((t)->tp_weaklistoffset > 0)
Include/object.h: flags = type->tp_flags;
Should we strive to fix the cases in Include/internal as well?

> Should we strive to fix the cases in Include/internal as well?
No. The internal C API access directly to structure members on purpose, for best performances.

New changeset 871eb4237b9be95263ca13ba8856e78344eb9eba by Erlend Egeberg Aasland in branch 'master':
bpo-40170: Convert PyDescr_IsData() to static inline function (GH-24535)
https://github.com/python/cpython/commit/871eb4237b9be95263ca13ba8856e78344eb9eba

New changeset cc54001c2eb3b14320c1667b22602d69c90d5865 by Erlend Egeberg Aasland in branch 'master':
bpo-40170: Always define PyIter_Check() as a function (GH-24548)
https://github.com/python/cpython/commit/cc54001c2eb3b14320c1667b22602d69c90d5865

> New changeset cc54001c2eb3b14320c1667b22602d69c90d5865 by Erlend Egeberg Aasland in branch 'master':
> bpo-40170: Always define PyIter_Check() as a function (GH-24548)
For macOS which doesn't use LTO compiler optimization, we added private static inline functions of some "Check" functions. But I don't think that it's worth it here (I don't think that the function is commonly called in "hot code").

For PyExceptionClass_Name: Is it ok to just remove the macro version (like with GH-24548)?

> For PyExceptionClass_Name: Is it ok to just remove the macro version (like with GH-24548)?
Yes, I think so.

Thanks, Victor.
For PySequence_ITEM, I guess adding a private C version (for example _PySequence_Item) and redirecting the macro to the C version would be acceptable.
Ditto for PyHeapType_GET_MEMBERS and PyType_SUPPORTS_WEAKREFS.

New changeset cd80f430daa7dfe7feeb431ed34f88db5f64aa30 by Erlend Egeberg Aasland in branch 'master':
bpo-40170: Always define PyExceptionClass_Name() as a function (GH-24553)
https://github.com/python/cpython/commit/cd80f430daa7dfe7feeb431ed34f88db5f64aa30

> For PySequence_ITEM, I guess adding a private C version (for example _PySequence_Item) and redirecting the macro to the C version would be acceptable.
This can introduce a performance slowdown and so should wait until the PEP 620 is accepted.

> This can introduce a performance slowdown and so should wait until the PEP 620 is accepted.
Noted.

New changeset 630264a152115f9671d6b793455ef5c2cea09a97 by Erlend Egeberg Aasland in branch 'master':
 bpo-40170: Move 3 NEWS entries to the C API section (GH-24555)
https://github.com/python/cpython/commit/630264a152115f9671d6b793455ef5c2cea09a97

New changeset 99c7e9853fa13af414168f179213e3d2fae03a45 by Victor Stinner in branch 'main':
bpo-40170: Update What's New in Python 3.9 (GH-29470)
https://github.com/python/cpython/commit/99c7e9853fa13af414168f179213e3d2fae03a45

New changeset 69b3de65adc691cc5ad66e70e5c7caf61c202c79 by Miss Islington (bot) in branch '3.10':
bpo-40170: Update What's New in Python 3.9 (GH-29470)
https://github.com/python/cpython/commit/69b3de65adc691cc5ad66e70e5c7caf61c202c79

New changeset 80580f5ab85e3c45c8c5e8999963a891598d10bf by Miss Islington (bot) in branch '3.9':
bpo-40170: Update What's New in Python 3.9 (GH-29470) (GH-29472)
https://github.com/python/cpython/commit/80580f5ab85e3c45c8c5e8999963a891598d10bf

TODO:
* Macros still accessing directly PyTypeObject members:
 * PyHeapType_GET_MEMBERS()
 * PySequence_ITEM()
 * _PyObject_SIZE()
 * _PyObject_VAR_SIZE()
 * PyType_SUPPORTS_WEAKREFS()
* Try again to apply "bpo-40170: PyType_HasFeature() now always calls PyType_GetFlags() (GH-19378)" which has been reverted?
* Py_TYPE(obj)->tp_name is still commonly used to format error messages: see rejected bpo-34595
Other TODO tasks which can be addressed in follow-up issues:
* Add buffer protocol to PyType_FromSpec()
* Stdlib C extensions still define static types: see bpo-40077
* 3rd party C extensions still define static types: PEP 630 and others propose heap types

I searched for "_PyGC_FINALIZED" in top 5000 PyPI projects. It seems like only Cython is impacted.
ddtrace and guppy3 use directly the internal C API.
== Cython 0.29.26 ==
* Cython/Compiler/ModuleNode.py: finalised_check = '!_PyGC_FINALIZED(o)'
* Cython/Compiler/ModuleNode.py: '(!PyType_IS_GC(Py_TYPE(o)) || !_PyGC_FINALIZED(o))')
== ddtrace 0.57.3 ==
In ddtrace/profiling/collector/stack.pyx:
 IF PY_MINOR_VERSION >= 9:
 # Needed for accessing _PyGC_FINALIZED when we build with -DPy_BUILD_CORE
 cdef extern from "<internal/pycore_gc.h>":
 pass
== guppy3-3.1.2 ==
In src/heapy/hv.c:
#if PY_MAJOR_VERSION >= 3 && PY_MINOR_VERSION >= 9
# define Py_BUILD_CORE
/* PyGC_Head */
# undef _PyGC_FINALIZED
# include <internal/pycore_gc.h>
# undef Py_BUILD_CORE
#endif

I searched for "_PyObject_DebugMallocStats" in top 5000 PyPI projects. There is a single project using it: guppy3.
Extract of guppy3 src/heapy/xmemstats.c:
...
 dlptr__PyObject_DebugMallocStats = addr_of_symbol("_PyObject_DebugMallocStats");
...
static PyObject *
hp_xmemstats(PyObject *self, PyObject *args)
{
 if (dlptr__PyObject_DebugMallocStats) {
 fprintf(stderr, "======================================================================\n");
 fprintf(stderr, "Output from _PyObject_DebugMallocStats()\n\n");
 dlptr__PyObject_DebugMallocStats(stderr);
 }
 ...
}
addr_of_symbol() is implemented with dlsym() or GetModuleHandle(NULL)+GetProcAddress(): it searchs for the symbol in the current process.

In the top 5000 PyPI projects, the _PyObject_SIZE() and _PyObject_VAR_SIZE() functions are used by 7 projects:
* Cython-0.29.26
* frozendict-2.2.0: implement "sizeof" function, found in copies of Objects/dictobject.c file
* JPype1-1.3.0
* numpy-1.22.1: gentype_alloc() in numpy/core/src/multiarray/scalartypes.c.src, used as type tp_alloc functions
* pickle5-0.0.12
* pyobjc-core-8.2
* recordclass-0.17.1

New changeset af32b3ef1fbad3c2242627a14398320960a0cb45 by Victor Stinner in branch 'main':
bpo-40170: PyType_SUPPORTS_WEAKREFS() becomes a regular function (GH-30938)
https://github.com/python/cpython/commit/af32b3ef1fbad3c2242627a14398320960a0cb45

New changeset 6b491b9dc0b0fdfd1f07ea4e2151236186d8e7e6 by Victor Stinner in branch 'main':
bpo-40170: Remove _Py_GetAllocatedBlocks() function (GH-30940)
https://github.com/python/cpython/commit/6b491b9dc0b0fdfd1f07ea4e2151236186d8e7e6

New changeset 0575551f69ba9c999835bfb176a543d468083c03 by Victor Stinner in branch 'main':
bpo-40170: Move _Py_GetAllocatedBlocks() to pycore_pymem.h (GH-30943)
https://github.com/python/cpython/commit/0575551f69ba9c999835bfb176a543d468083c03

New changeset 18ea973c21ee4a6adc26be41027881043fa498eb by Victor Stinner in branch 'main':
bpo-40170: Remove PyHeapType_GET_MEMBERS() macro (GH-30942)
https://github.com/python/cpython/commit/18ea973c21ee4a6adc26be41027881043fa498eb

Changes already done:
* Macros converted to regular functions:
 
 * PyObject_GET_WEAKREFS_LISTPTR(); add internal inline _PyObject_GET_WEAKREFS_LISTPTR()
 * PyType_SUPPORTS_WEAKREFS(); add internal inline _PyType_SUPPORTS_WEAKREFS()
 * PyObject_CheckBuffer(); no fast internal API
 * PyObject_IS_GC(); no fast internal API
 * PyDescr_IsData(); no fast internal API
* Always implemented as a function, remove macro optimization in the non-limited API:
 * PyIter_Check(); no fast internal API
 * PyIndex_Check(); add internal inline _PyIndex_Check()
 * PyExceptionClass_Name(); no fast internal API
* Py_TRASHCAN_BEGIN() macro now calls _PyTrash_cond() function: no longer read directly tp_dealloc member
* PyObject_NEW() macro becomes an alias to PyObject_New()
* Remove PyHeapType_GET_MEMBERS() (move it to the internal C API)
PyType_HasFeature() is left unchanged since the change caused performance issue on macOS, whereas Python is not built with LTO.

I close the issue. While this issue is not fully fixed, it's a milestone of my "stable ABI" goal. I prefer to address remaining issues in new separted issues.
This issues is not fully fixed, there are are still a 4 macros which access directly PyTypeObject members:
* PySequence_ITEM()
* _PyObject_SIZE()
* _PyObject_VAR_SIZE()
* PyType_HasFeature() (if Py_LIMITED_API is not defined)
PySequence_ITEM() and PyType_HasFeature() are important for performance, I prefer to have a PEP before changing these two functions.
_PyObject_SIZE() and _PyObject_VAR_SIZE() should be made public with a better name like PyObject_SizeOf() and PyVarObject_SizeOf(). But I prefer to do that in a separated issue.
IMO it would be interesting to merge the PyHeapTypeObject structure into the PyTypeObject structure:
https://bugs.python.org/issue46433#msg411167
And make the PyTypeObject opaque: deprecate static types and promote the usage of heap types in C extensions. That's a big project which may be splitted into multiple issues and the final change may need its own PEP.