Add a page on changing the C API (PEP-652) (GH-682)

This repeats points currently said in `Include/README.rst`.
I intend to replace most of that README with a link to this document.

The main change from that document are:
- The three "rings" are not defined by where things are declared.
  It would be good to have them in the right place, but it's
  not yet the case, and for technical reasons we might never reach
  the ideal. (In `Include/README.rst` it makes more sense to make
  the locations more prominent.)
- The order is switched. For changing the Limited API you generally
  need to read (and follow guidelines in) the preceding sections
  as well.

On top of that, I addes points from PEP 652 (Maintaining the Stable ABI).


Co-Authored-By: Erlend Egeberg Aasland <[email protected]>

Author: encukou

c-api.rst

@@ -0,0 +1,194 @@
+=======================
+Changing Python's C API
+=======================
+
+The C API is divided into three sections:
+
+1. The internal, private API, available with ``Py_BUILD_CORE`` defined.
+   Ideally declared in ``Include/internal/``. Any API named with a leading
+   underscore is also considered private.
+2. The public C API, available when ``Python.h`` is included normally.
+   Ideally declared in ``Include/cpython/``.
+3. The Limited API, available with ``Py_LIMITED_API`` defined.
+   Ideally declared directly under ``Include/``.
+
+Each section has higher stability & maintenance requirements, and you will
+need to think about more issues when you add or change definitions in it.
+
+The compatibility guarantees for public C API are explained in the
+user documentation, ``Doc/c-api/stable.rst`` (:ref:`python:stable`).
+
+
+The internal API
+================
+
+Internal API is defined in ``Include/internal/`` and is only available
+for building CPython itself, as indicated by a macro like ``Py_BUILD_CORE``.
+
+While internal API can be changed at any time, it's still good to keep it
+stable: other API or other CPython developers may depend on it.
+
+With PyAPI_FUNC or PyAPI_DATA
+-----------------------------
+
+Functions or structures in ``Include/internal/`` defined with
+``PyAPI_FUNC`` or ``PyAPI_DATA`` are internal functions which are
+exposed only for specific use cases like debuggers and profilers.
+
+
+With the extern keyword
+-----------------------
+
+Functions in ``Include/internal/`` defined with the ``extern`` keyword
+*must not and can not* be used outside the CPython code base.  Only
+built-in stdlib extensions (built with the ``Py_BUILD_CORE_BUILTIN``
+macro defined) can use such functions.
+
+When in doubt, new internal C functions should be defined in
+``Include/internal`` using the ``extern`` keyword.
+
+Private names
+--------------
+
+Any API named with a leading underscore is also considered internal.
+There are two main use cases for using such names rather than putting the
+definition in ``Include/internal/`` (or directly in a ``.c`` file):
+
+* Internal helpers for other public API; users should not use these directly;
+* “Provisional” API, included in a Python release to test real-world usage
+  of new API. Such names should be renamed when stabilized; preferably with
+  a macro aliasing the old name to the new one.
+  See `"Finalizing the API" in PEP 590`_ for an example.
+
+.. _"Finalizing the API" in PEP 590: https://www.python.org/dev/peps/pep-0590/#finalizing-the-api
+
+
+.. _public-capi:
+
+Public C API
+============
+
+CPython's public C API is available when ``Python.h`` is included normally
+(that is, without defining macros to select the other variants).
+
+It should be defined in ``Include/cpython/`` (unless part of the Limited API,
+see below).
+
+Guidelines for expanding/changing the public API:
+
+- 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.)
+
+  - Functions *must not* steal references
+  - Functions *must not* return borrowed references
+  - Functions returning references *must* return a strong reference
+
+- Make sure the ownership rules and lifetimes of all applicable struct
+  fields, arguments and return values are well defined.
+
+
+Limited API
+===========
+
+The Limited API is a subset of the C API designed to guarantee ABI
+stability across Python 3 versions.
+Defining the macro ``Py_LIMITED_API`` will limit the exposed API to
+this subset.
+
+No changes that break the Stable ABI are allowed.
+
+The Limited API should be defined in ``Include/``, excluding the
+``cpython`` and ``internal`` subdirectories.
+
+Guidelines for changing the Limited API
+---------------------------------------
+
+- Guidelines for the general :ref:`public-capi` apply.
+
+- New Limited API should only be defined if ``Py_LIMITED_API`` is set
+  to the version the API was added in or higher.
+  (See below for the proper ``#if`` guard.)
+
+- All parameter types, return values, struct members, etc. need to be part
+  of the Limited API.
+
+  - Functions that deal with ``FILE*`` (or other types with ABI portability
+    issues) should not be added.
+
+- Think twice when defining macros.
+
+  - Macros should not expose implementation details
+  - Functions must be exported as actual functions, not (only)
+    as functions-like macros.
+  - If possible, avoid macros. This makes the Limited API more usable in
+    languages that don't use the C preprocessor.
+
+- Please start a public discussion before expanding the Limited API
+
+- The Limited API and must follow standard C, not just features of currently
+  supported platforms. The exact C dialect is described in :pep:`7`.
+
+  - Documentation examples (and more generally: the intended use of the API)
+    should also follow standard C.
+  - In particular, do not cast a function pointer to ``void*`` (a data pointer)
+    or vice versa.
+
+- 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).
+  - The Limited API needs to be well documented.
+
+- Think about future extensions
+
+  - If it's possible that future Python versions will need to add a new
+    field to your struct, make sure it can be done.
+  - Make as few assumptions as possible about implementation details that
+    might change in future CPython versions or differ across C API
+    implementations. The most important CPython-specific implementation
+    details involve:
+
+    - The GIL
+    - :ref:`Garbage collection <gc>`
+    - 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/
+
+Adding a new definition to the Limited API
+------------------------------------------
+
+- Add the declaration to a header file directly under ``Include/``, into a
+  block guarded with the following:
+
+  .. code-block:: c
+
+    #if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03yy0000
+
+  with the ``yy`` corresponding to the target CPython version, e.g.
+  ``0x030A0000`` for Python 3.10.
+- Append an entry to the Stable ABI manifest, ``Misc/stable_abi.txt``.
+- Regenerate the autogenerated files using ``make regen-limited-abi``.
+  On platforms without ``make``, run this command directly:
+
+  .. code-block:: shell
+
+     ./python ./Tools/scripts/stable_abi.py --generate-all ./Misc/stable_abi.txt
+
+- Build Python and check the using ``make check-limited-abi``.
+  On platforms without ``make``, run this command directly:
+
+  .. code-block:: shell
+
+    ./python ./Tools/scripts/stable_abi.py --all ./Misc/stable_abi.txt

index.rst

@@ -324,6 +324,7 @@ Full Table of Contents
    compiler
    garbage_collector
    extensions
+   c-api
    coverity
    clang
    buildworker