Cygwin build broken due to use of &PyType_Type in static declaration in _abc module

[embray]

BPO	34211
Nosy	@embray, @jdemeyer
PRs	bpo-34211: Do not pass &PyType_Type to PyVarObject_HEAD_INIT #8445 [3.7] bpo-34211: fix _abc.c compile error on Cygwin (GH-8445) #12004

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-02-24.01:55:01.014>
created_at = <Date 2018-07-24.17:00:16.267>
labels = ['3.8', 'build', '3.7']
title = 'Cygwin build broken due to use of &PyType_Type in static declaration in  _abc module'
updated_at = <Date 2019-02-24.01:55:01.013>
user = 'https://github.com/embray'

bugs.python.org fields:

activity = <Date 2019-02-24.01:55:01.013>
actor = 'methane'
assignee = 'none'
closed = True
closed_date = <Date 2019-02-24.01:55:01.014>
closer = 'methane'
components = ['Build']
creation = <Date 2018-07-24.17:00:16.267>
creator = 'erik.bray'
dependencies = []
files = []
hgrepos = []
issue_num = 34211
keywords = ['patch', '3.7regression']
message_count = 5.0
messages = ['322315', '322823', '322857', '322858', '323062']
nosy_count = 2.0
nosy_names = ['erik.bray', 'jdemeyer']
pr_nums = ['8445', '12004']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue34211'
versions = ['Python 3.7', 'Python 3.8']

[embray]

This is essentially the same as bpo-21124, but introduced more recently with the addition of the _abc_data_type in the _abc module.

The workaround is simply to use PyVarObject_HEAD_INIT(NULL, 0) instead of PyVarObject_HEAD_INIT(&PyType_Type, 0); PyType_Ready should take care of the rest.

P.S. I'm trying to get going again on the project of adding an AppVeyor build, and eventually a buildbot for Cygwin. My previous attempt was doomed because I wanted to fix all failing tests on Cygwin *first*. This time I am going for a more "instant gratification" approach of just skipping the tests that fail (for now) so that I can at least catch new regressions, and then gradually re-enabled skipped tests as I can find fixes for them.

However, for that to happen we at least need a minimal build to work.

[jdemeyer]

For those who are not very aware of Cygwin issues: what is wrong with

PyVarObject_HEAD_INIT(&PyType_Type, 0);

[embray]

For those who are not very aware of Cygwin issues: what is wrong with

PyVarObject_HEAD_INIT(&PyType_Type, 0);

I'm glad you asked, because it actually got me thinking, and since I added a fix (albeit an unsatisfying one) for bpo-34212, this fix is no longer strictly necessary *so long as* the _abc module is always built as a core built-in (that is, linked into libpython).

IIUC that is the case since _abc is required for the core, but I'm not sure.

The problem is explained somewhat in bpo-21124, but what it comes down to is that if the linker can't resolve PyType_Type at link time it will make a complaint like "can't initialize global with a non-constant".

Because of bpo-34212, when compiling _abc.c it was using the wrong external linkage for PyType_Type (treating it as some data that needs to be imported from an other DLL, rather than data in the same DLL). But with a fix for bpo-34212 this is not a problem (again, so long as the _abc module is included in libpython).

[jdemeyer]

Linker issues are always tricky...

I understand that there is no problem within libpython, so the questions below refer to extension modules. I am asking them from the point of view of somebody writing Python extension modules who is clueless about Cygwin.

So you're saying that it's not allowed to refer to &PyType_Type in a static struct? But it is OK to refer to &PyType_Type in code?

I assume that there is nothing special about PyType_Type and that this apply for all variables. Are functions OK? For example, in functools.c I see

static PyGetSetDef partial_getsetlist[] = {
{"__dict__", PyObject_GenericGetDict, PyObject_GenericSetDict},
{NULL} /* Sentinel */
};

What makes functions different from variables? Aren't they essentially just pointers?

[embray]

What makes functions different from variables? Aren't they essentially just pointers?

You're on the right track by noting a difference between functions and data variables. I can tell you off the bat that when it comes to data imported from DLLs, non-functions are handled (somewhat by necessity) quite differently from functions.

That said, since you asked, I struggled to articulate *exactly* why this exact problem occurs on Cygwin (actually on Windows in general), so I thought about it for a while and wrote up an explanation in a blog post: http://iguananaut.net/blog/programming/windows-data-import.html

The TL;DR though is that limitations of how the runtime dynamic loader on Windows works are such that it's impossible to initialize static data with a pointer to some data in an external library. The compiler knows this and prevents you from doing it. The workaround is simple enough for most cases: Complete the initialization at runtime. In the case of PyType_Type objects, PyType_Ready can set their base type at runtime just fine.