python-peps/pep-0652.rst

PEP: 652
Title: Maintaining the Stable ABI
Author: Petr Viktorin <encukou@gmail.com>
Discussions-To: https://discuss.python.org/t/pre-pep-maintaining-the-stable-abi/6986/
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 09-Feb-2021
Python-Version: 3.10


Abstract
========

CPython's Limited C-API and Stable ABI, introduced in :pep:`384`,
will be formalized in a single definitive file, tested, and documented.


Motivation
==========

:pep:`384` defined a Limited API and Stable ABI, which allow extenders and
embedders of CPython to compile extension modules that are binary-compatible
with any subsequent version of 3.x.
In theory, this brings several advantages:

* A module can be built only once per platform and support multiple versions
  of Python, reducing time, power and maintainer attention needed for builds
  (in exchange for potentially worse performance).

* Binary wheels using the Stable ABI work with new versions of CPython
  throughout the pre-release period, and can be tested in environments where
  building from source is not practical.

* As a welcome side effect of the Limited API's hiding of implementation
  details, this API is becoming a viable target for alternate Python
  implementations that would be incompatible with the full C API.

However, in hindsight, PEP 384 and its implementation has several issues:

* It is ill-defined. According to PEP 384, functions are *opt-out*:
  all functions not specially marked are part of the Stable ABI.
  In practice, for Windows there's a list that's *opt-in*.
  For users there is a ``#define`` that should make only the Stable ABI
  available, but there is no process that ensures it is kept up-to date.
  Neither is there a process for updating the documentation.
* Until recently, the Stable ABI was not tested at all. It tends to break.
  For example, changing a function to a macro can break the Stable ABI as the
  function symbol is removed.
* There is no way to deprecate parts of the Limited API.
* It is incomplete. Some operations are not available in the Stable ABI,
  with little reason except "we forgot".
  (This last point is one the PEP will not help with, however.)

This PEP defines the Limited API more clearly and introducess process
designed to make the Stable ABI and Limited API more useful and robust.


Rationale
=========

This PEP contains a lot of clarifications and definitions, but just one big
technical change: the Stable ABI will be explicitly listed in
a human-maintained “manifest” file.

There have been efforts to collect such lists automatically, e.g. by scanning
the symbols exported from Python.
Such automation might seem easier to maintain than a handcrafted file,
but has major issues: for example, the set exported symbols has
platform-specific variations.
Also, the cost of updating an explicit manifest is small compared
to the overall work that should go into changing API that will need to
be suppported forever (or until Python 3 reaches end of life, if that
comes sooner).

This PEP proposes automatically generating things *from* the manifest:
initially documentation and DLL contents, with later possibilities
for also automating tests.


Stable ABI vs. Limited API
==========================

:pep:`384` and this document deal with the *Limited API* and the *Stable ABI*,
two related but distinct concepts. In short:

* The *Stable ABI* is a promise that certain extensions compiled with
  CPython 3.x will be binary compatible with all subsequent versions
  of CPython 3.x.
* The *Limited API* is a subset of CPython's C API that produces such extensions.

This section clarifies these terms and defines some of their semantics
(either pre-existing or newly proposed here).

The word “Extensions” is used as a shorthand for all code that uses the
Python API, e.g. extension modules or software that embeds Python.


Stable ABI
----------

The CPython *Stable ABI* is a promise that extensions built against
a specific Stable ABI version will be compatible with any newer
interpreter of the same major version.

The Stable ABI does not define a complete binary interface:
important details like the layout of structures in memory or function
calling conventions are determined by the platform and the compiler and
its settings.
The Stable ABI promise only applies if these lower-details are also stable.

For example, an extension built with the CPython 3.10 Stable ABI will be usable
with CPython 3.11, 3.12, etc.
It will not necessarily be compatible with CPython 4.0, nor with CPython 3.10
on a different platform.

The Stable ABI is not generally forward-compatible: an extension built and
tested with CPython 3.10 will not generally be compatible with CPython 3.9.

.. note::
   For example, starting in Python 3.10, the ``Py_tp_doc`` slot may be set to
   ``NULL``, while in older versions, a ``NULL`` value will likely crash the
   interpreter.

The Stable ABI trades performance for its stability.
For example, extensions built for a specific CPython version will automatically
use faster macros instead of functions in the Stable ABI.

Future Python versions may deprecate some members of the Stable ABI.
Deprecated members will still work, but may suffer from issues like reduced
performance or, in the most extreme cases, memory/resource leaks.

Limited API
-----------

The Stable ABI promise holds for extensions compiled from code that restricts
itself to the *Limited API* (application programming interface).
The Limited API is a subset of CPython's C API.

Extensions that target the Limited API should define the preprocessor macro
``Py_LIMITED_API`` to either ``3`` or the current ``PYTHON_API_VERSION``.
This will enable Stable ABI versions of several functions and limit definitions
to the Limited API.
(However, note that the macro is not perfect: due to technical issues or
oversights, some non-limited API might be exposed even with it defined.)

The Limited API is not guaranteed to be *stable*.
In the future, parts of the Limited API may be deprecated.
They may even be removed, as long as the *Stable ABI* is kept
stable and Python's general backwards compatibility policy, :pep:`387`,
is followed.

.. note::

   For example, a function declaration might be removed from public header
   files but kept in the library.
   This is currently a possibility for the future; this PEP does not to propose
   a concrete process for deprecations and removals.

The goal for the Limited API is to cover everything needed to interact
with the interpreter.
The main reason to not include a public API in the Limited subset
should be that it needs implementation details that change between CPython
versions (like struct memory layouts) – usually for performance reasons.

The Limited API is not limited to CPython. Other implementations are
encouraged to implement it and help drive its design.


Specification
=============

To make the Stable ABI more useful and robust, the following changes
are proposed.


Stable ABI Manifest
-------------------

All members of the Stable ABI – functions, typedefs, structs, data, macros,
and constants – will be explicitly listed in a single "manifest" file,
``Misc/stable_abi.txt``.

For structs, any fields that users of the Stable ABI are allowed to access
will be listed explicitly.

The manifest will also serve as the definitive list of the Limited API.
Members that are not part of the Limited API, but are part of the Stable ABI
(e.g. ``PyObject.ob_type``, which is accessible by the ``Py_TYPE`` macro),
will be annotated as such.

For items that are only available on some systems, the manifest will record the
feature macro that determines their presence (such as ``MS_WINDOWS`` or
``HAVE_FORK``).
To make the implementation (and usage from non-C languages) easier,
all such macros will be simple names.
If a future item needs a “negative” macro or complex expression (such as a
hypothetical ``#ifndef MACOSX`` or ``#if defined(POSIX) && !defined(LINUX)``),
a new feature macro will be derived.

The format of the manifest will be subject to change whenever needed.
It should be consumed only by scripts in the CPython repository.
If a stable list is needed, a script can be added to generate it.

The following wil be generated from the ABI manifest:

* Source for the Windows shared library, ``PC/python3dll.c``.
* Input for documentation (see below).
* Test case that checks the runtime availablility of symbols (see below).

The following will be checked against the Stable ABI manifest as part of
continuous integration:

* The reference count summary, ``Doc/data/refcounts.txt``, includes all
  function in the Stable ABI (among others).
* The functions/structs declared and constants/macros defined
  when ``Python.h`` is included with ``Py_LIMITED_API`` set.
  (Initially Linux only; checks on other systems may be added in the future.)

After the initial implementation, details such as function arguments will be
added and the manifest will be checked for internal consistency (e.g. all
types used in function signatures are part of the API).


Contents of the Stable ABI
--------------------------

The initial Stable ABI manifest will include:

* The Stable ABI specified in :pep:`384`.
* Everything listed in ``PC/python3dll.c``.
* All structs (struct typedefs) which these functions return or take as
  arguments. (Fields of such structs will not necessarily be added.)
* New type slots, such as ``Py_am_aiter``.
* The type flags  ``Py_TPFLAGS_DEFAULT``, ``Py_TPFLAGS_BASETYPE``,
  ``Py_TPFLAGS_HAVE_GC``, ``Py_TPFLAGS_METHOD_DESCRIPTOR``.
* The calling conventions ``METH_*`` (except deprecated ones).
* All API needed by macros is the Stable ABI (annotated as not being part of
  the Limited API).

Items that are no longer in CPython when this PEP is accepted will be removed
from the list.

Additional items may be aded to the initial manifest according to
the checklist below.


Documenting the Limited API
---------------------------

Notes saying “Part of the Limited API” will be added to Python's documentation
automatically, in a way similar to the notes on functions that return borrowed
references.

A complete list of all members of the Limited API will also be added to
the documentation.


Testing the Stable ABI
----------------------

An automatically generated test module will be added to ensure that all symbols
included in the Stable ABI are available at compile time.


Changing the Limited API
------------------------

A checklist for changing the Limited API, including adding new items to it
and removing existing ones, will be added to the `Devguide`_.
The checklist will 1) mention best practices and common pitfalls in Python
C API design and 2) guide the developer around the files that need changing and
scripts that need running when the Limited API is changed.

Below is the initial proposal for the checklist.
(After the PEP is accepted, see the Devguide for the current version.)

Note that the checklist applies to new changes; several items
in the *existing* Limited API are grandfathered and couldn't be added today.

Design considerations:

* Make sure the change does not break the Stable ABI of any version of Python
  since 3.5.
* Make sure no exposed names are private (i.e. begin with an underscore).
* Make sure the new API is well documented.
* Make sure the types of all parameters and return values of the added
  function(s) and all fields of the added struct(s) are be part of the
  Limited API (or standard C).

* Make sure the new API and its intended use follows standard C, not just
  features of currently supported platforms.
  Specifically, follow the C dialect specified in :pep:`7`.

  * Do not cast a function pointer to ``void*`` (a data pointer) or vice versa.

* Make sure the new API follows reference counting conventions. (Following them
  makes the API easier to reason about, and easier use in other Python
  implementations.)

  * Do not return borrowed references from functions.
  * Do not steal references to function arguments.

* Make sure the ownership rules and lifetimes of all applicable struct fields,
  arguments and return values are well defined.
* Think about ease of use for the user. (In C, ease of use itself is not very 
  important; what *is* useful is reducing boilerplate code needed to use the
  API. Bugs like to hide in boiler plates.)

  * If a function will be often called with specific value for an argument,
    consider making it default (used when ``NULL`` is passed in).

* Think about future extensions: for example, if it's possible that future
  Python versions will need to add a new field to your struct,
  how will that be done?

* Make as few assumptions as possible about details that might change in
  future CPython versions or differ across C API implementations:

    * The GIL
    * Garbage collection
    * Memory layout of PyObject, lists/tuples and other structures

If following these guidelines would hurt performance, add a fast function
(or macro) to the non-limited API and a stable equivalent to the Limited API.

If anything is unclear, or you have a good reason to break the guidelines,
consider discussing the change at the `capi-sig`_ mailing list.

.. _capi-sig: https://mail.python.org/mailman3/lists/capi-sig.python.org/

Procedure:

* Move the declaration to a header file directly under ``Include/``, into a
  ``#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03yy0000`` block
  (with the ``yy`` corresponding to the target CPython version).
* Make an entry in the Stable ABI manifest, ``Misc/stable_abi.txt``.
* Regenerate the autogenerated files using ``make regen-all``.
  (or the alternative for non-``make`` platforms)
* Build Python and run checks using ``make check-abi``.
  (or the alternative for non-``make`` platforms)


Advice for Extenders and Embedders
----------------------------------

The following notes will be added to documentation.

Extension authors should test with all Python versions they support,
and preferably build with the lowest such version.

Compiling with ``Py_LIMITED_API`` defined is *not* a guarantee that your code
conforms to the Limited API or the Stable ABI.
``Py_LIMITED_API`` only covers definitions, but an API also includes other
issues, such as expected semantics.

Examples of issues that ``Py_LIMITED_API`` does not guard against are:

* Calling a function with invalid arguments
* A function that started accepting ``NULL`` values for an argument
  in Python 3.9 will fail if ``NULL`` is passed to it under Python 3.8.
  Only testing with 3.8 (or lower versions) will uncover this issue.
* Some structs include a few fields that are part of the Stable ABI and other
  fields that aren't.
  ``Py_LIMITED_API`` does not filter out such “private” fields.
* Code that uses something that is not documented as part of the Stable ABI,
  but exposed even with ``Py_LIMITED_API`` defined, may break in the future.
  Despite the team's best efforts, such issues may happen.


Note for Redistributors of Python
---------------------------------

The Stable ABI promise relies on stable underlying ABI details, such as the
layout of structures in memory and function calling conventions, which
are affected by the compiler and its settings.
For the promise to hold, these details must not change between CPython 3.x
releases on a particular platform.




Backwards Compatibility
=======================

Backwards compatibility is one honking great idea!

This PEP aims at full compatibility with the existing Stable ABI and Limited
API, but defines them terms more explicitly.
It might not be consistent with some interpretations of what the existing
Stable ABI/Limited API is.


Security Implications
=====================

None known.


How to Teach This
=================

Technical documentation will be provided in ``Doc/c-api/stable``
and linked from the *What's New* document.
Docs for CPython core developers will be added to the devguide.


Reference Implementation
========================

Nothing presentable yet.


Ideas for the Future
====================

The following issues are out of scope of this PEP, but show possible
future directions.

Defining a process for deprecations/removals
--------------------------------------------

While this PEP acknowledges that parts of the Limited API might be deprecated
or removed in the future, a process to do this is not in scope, and is left
to a possible future PEP.


C syntax for the ABI manifest
-----------------------------

It might be useful to have the ABI manifest be a C header file, or to
generate header files from the manifest.
Again, either are options for the future.


Open Issues
===========

None so far.


References
==========

.. _Devguide: https://devguide.python.org/


Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.


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