Issue 40421: [C API] Add getter functions for PyFrameObject and maybe move PyFrameObject to the internal C API

Similarly to bpo-39573 (make PyObject opaque) and bpo-39947 (make PyThreadState opaque), I propose to add getter functions to access PyFrameObject members without exposing the PyFrameObject structure in the C API.

The first step is to identify common usage of the PyFrameObject structure inside CPython code base to add getter functions, and maybe a few setter functions as well.

frameobject.h is not part of Python.h, but it's part of the public C API. The long term plan is to move PyFrameObject structure to the internal C API to hide implementation details from the public C API.

New changeset 7c59d7c9860cdbaf4a9c26c9142aebd3259d046e by Victor Stinner in branch 'master':
bpo-40421: Add pyframe.h header file (GH-19755)
https://github.com/python/cpython/commit/7c59d7c9860cdbaf4a9c26c9142aebd3259d046e

New changeset b8f704d2190125a7750b50cd9b67267b9c20fd43 by Victor Stinner in branch 'master':
bpo-40421: Add Include/cpython/code.h header file (GH-19756)
https://github.com/python/cpython/commit/b8f704d2190125a7750b50cd9b67267b9c20fd43

New changeset a42ca74fa30227e2f89a619332557cf093a937d5 by Victor Stinner in branch 'master':
bpo-40421: Add PyFrame_GetCode() function (GH-19757)
https://github.com/python/cpython/commit/a42ca74fa30227e2f89a619332557cf093a937d5

I looked how Cython uses PyFrameObject:

* read f_lasti
* read/write f_back
* write f_lineno
* read f_localsplus
* read/write f_trace

Details:

* Cython/Debugger/libpython.py: code using the Python API of gdb to read PyFrameObject.f_lasti. It it used to compute the line number of a frame. The python_step() function puts a watch point on "f->f_lasti".
* Cython/Utility/Coroutine.c: set PyFrameObject.f_back using "f->f_back = tstate->frame;" and "Py_CLEAR(f->f_back);".
* Cython/Utility/ModuleSetupCode.c, __Pyx_PyFrame_SetLineNumber(): set PyFrameObject.f_lineno member. The limited C API flavor of this function does nothing, since this member cannot be set in the limited C API.
* Cython/Utility/ObjectHandling.c, __Pyx_PyFrame_GetLocalsplus(): complex implementation to access PyFrameObject.f_localsplus:

  // Initialised by module init code.
  static size_t __pyx_pyframe_localsplus_offset = 0;

  #include "frameobject.h"
  // This is the long runtime version of
  //     #define __Pyx_PyFrame_GetLocalsplus(frame)  ((frame)->f_localsplus)
  // offsetof(PyFrameObject, f_localsplus) differs between regular C-Python and Stackless Python.
  // Therefore the offset is computed at run time from PyFrame_type.tp_basicsize. That is feasible,
  // because f_localsplus is the last field of PyFrameObject (checked by Py_BUILD_ASSERT_EXPR below).
  #define __Pxy_PyFrame_Initialize_Offsets()  \
    ((void)__Pyx_BUILD_ASSERT_EXPR(sizeof(PyFrameObject) == offsetof(PyFrameObject, f_localsplus) + Py_MEMBER_SIZE(PyFrameObject, f_localsplus)), \
     (void)(__pyx_pyframe_localsplus_offset = ((size_t)PyFrame_Type.tp_basicsize) - Py_MEMBER_SIZE(PyFrameObject, f_localsplus)))
  #define __Pyx_PyFrame_GetLocalsplus(frame)  \
    (assert(__pyx_pyframe_localsplus_offset), (PyObject **)(((char *)(frame)) + __pyx_pyframe_localsplus_offset))

* Cython/Utility/Profile.c, __Pyx_TraceLine(): read PyFrameObject.f_trace.

* Cython/Utility/Profile.c, __Pyx_TraceSetupAndCall(): set PyFrameObject.f_trace.

New changeset 703647732359200c54f1d2e695cc3a06b9a96c9a by Victor Stinner in branch 'master':
bpo-40421: Add PyFrame_GetBack() function (GH-19765)
https://github.com/python/cpython/commit/703647732359200c54f1d2e695cc3a06b9a96c9a

"maybe a few setter functions as well"
Please don't add any setter functions, that would a major change to the VM and would need a PEP.

Also, could you remove PyFrame_GetLastInstr(PyFrameObject *frame)?
The only purpose of `f_lasti` is to get the line number and that can be done directly via `PyFrame_GetLineNumber(PyFrameObject *frame)`

Perhaps Stefan can tell us why Cython needs to access the internals of the frame object.

> Please don't add any setter functions, that would a major change to the VM and would need a PEP.

There is no such "major change". PyFrameObject structure was fully exposed in the public C API since the earliest Python version.

I don't see how adding setter is a major change, since it's already possible to directly modify *any* field of PyFrameObject without any kind of protection.

My plan is to have two milestones:

A) Make the structure opaque, so it's not longer possible to directly access it.

B) Deprecate or remove a few getter or setter functions, or move them to the internal C API.


I don't think that moving directly to step (B) is a realistic migration plan. So far, nobody analyzed all C extensions on PyPI to see how PyFrameObject is accessed.

I prefer to move slowly, step by step.

Breaking C extensions which currently *modify* directly PyFrameObject on purpose doesn't seem like a reasonable option to me.

--

I'm trying to distinguish functions which should be "safe"/"portable" from the ones which may be too "CPython specific":

* I added Include/pyframe.h which is included by Include/Python.h for "safe"/"portable" functions:

  * PyFrame_GetLineNumber()
  * PyFrame_GetCode()

* All other functions are added to Include/cpython/frameobject.h which is excluded from the limited C API:

  * PyFrame_GetBack()

Note: Compared to accessing directly PyFrameObject.f_code, PyFrame_GetCode() also avoids the issue of borrowed references since it returns a strong reference.

PyFrame_GetBack() looks specific to the current implementation of CPython. Another implementation might decide to not chain frames. Or maybe don't provide an easy way to traverse the chain of frames. Or at least, it might be a different API than PyFrame_GetBack().

--

> Also, could you remove PyFrame_GetLastInstr(PyFrameObject *frame)?

I didn't add it :-) It's the pending PR 19764.

I didn't merge it since it's unclear to me if it's a good idea to directly expose it or not. Cython uses it, but Cython also abuses CPython internals in general, usually for best performances :-)

*Maybe* one compromise would be to add a private _PyFrame_GetLastInstr() to ease migration to step (A) (replace direct access to the structure with function calls).

Adding to the list above:

"f_lasti" is used in "libpython.py", which is an almost exact copy of the one in CPython's "Tools/gdb/" for debugging Python code in gdb. If that implementation changes, we can probably adapt, especially since it uses runtime generated code. I don't find a C function for this necessary at this point, given that CPython will also need to access its own internals here in some way.

"f_localsplus" is used in the FASTCALL implementation and seems specific to Py3.6/7(?) which lacks the vectorcall protocol. So I guess it won't impact 3.8+ anymore. (The reason why its access is so complex is that StacklessPython has a slightly different frame struct, but we need to be binary(!) compatible with both.)

"f_back" is used in the coroutine implementation for correct exception traceback building, and the usage is pretty much copied from CPython. I doubt that there is much use for setting it outside of "code that tries to reimplement CPython behaviour", but Cython has to do exactly that here.

"f_lineno" is also used for traceback generation, because Cython creates frames when an exception occurs and needs to tell it what the current code line is. It's only used on newly created frames, although I can't guarantee that that will never change. Being able to read and set it seems reasonable.

"f_trace" is obviously used for tracing/profiling, and there isn't currently a good C-API for that, besides talking to frame struct fields (which is quite efficient when it comes to performance).

New changeset fcc6935384b933fbe1a1ef659ed455a3b74c849a by Victor Stinner in branch 'master':
Add symbols of the stable ABI to python3dll.c (GH-23598)
https://github.com/python/cpython/commit/fcc6935384b933fbe1a1ef659ed455a3b74c849a

New changeset 166286849048eccadecf02b242dbc4042b780944 by Victor Stinner in branch '3.9':
Add symbols of the stable ABI to python3dll.c (GH-23598) (GH-23801)
https://github.com/python/cpython/commit/166286849048eccadecf02b242dbc4042b780944