wlroots/_build.py is installed/packaged only if no existing version is installed

[mgorny]

pywlroots/wlroots/ffi_build.py

Lines 67 to 70 in 1b39a33

	try:
	return importlib.import_module("wlroots._build").has_xwayland
	except ModuleNotFoundError:
	pass

This code means that the function won't be executed if wlroots._build can be imported from anywhere, including the previous version installed to the system. So if pywlroots are installed, the following bit won't ever be executed:

pywlroots/wlroots/ffi_build.py

Lines 86 to 87 in 1b39a33

	with (Path(__file__).parent / "_build.py").open("w") as f:
	f.write(f"has_xwayland = {has_xwayland}\n")

and the newly created wheel (or install to --root) won't contain the file. As a result, the Gentoo ebuild produces an install without the file every second run; which in turns breaks building qtile.

Downstream bug report: https://bugs.gentoo.org/919097

[mgorny]

Upon further investigation, this cache logic can't ever work properly!

Even if wlroots/_build.py exists in the source, wlroots._build cannot be imported because wlroots requires wlroots._ffi that doesn't exist yet!

 python3.11 -c 'import wlroots._build'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/tmp/portage/dev-python/pywlroots-0.16.6-r2/work/pywlroots-0.16.6/wlroots/__init__.py", line 8, in <module>
    from ._ffi import ffi, lib
ModuleNotFoundError: No module named 'wlroots._ffi'

So the only case when "caching" works is when it wrongly imports the earlier version…