Add support for building on Windows / Cygwin

[fingolfin]

Similar to issue #17, but probably more work, as one also needs a way to install a cygwin environment -- I hope that we can reuse some of the work done on gap-system/gap for this, though

[ssiccha]

(I've had a long day so I might be talking nonsense here)

I'm not sure whether this is possible ATM. Composite actions do not support using if on a step-level, see this issue. So I don't know how we would figure out whether we have to use bash or cmd.

Wait, one can use shell: bash also when running on a windows machine? Nice, see here I'm putting this here so that I don't forget about it.

[fingolfin]

@wilfwilson has been very active on this. Some PRs that convert packages to using his work: gap-packages/ace#28 and gap-packages/datastructures#136

One concern about that is that it requires various custom versions of our GH actions, e.g. gap-actions/setup-gap@cygwin-v2 instead of gap-actions/setup-gap@v2. These are maintained in branches right now.

The main reason is that we can't use shell: bash in actions on Windows, as it gives us the "wrong" bash shell. You can see this on https://github.com/gap-actions/setup-gap/pull/22/files . This relies on the custom shell support in GH Actions workflow files, and right now that can't be controlled by e.g. env variables.

One way out might be to install a helper script bash.bat into a suitable directory in the PATH before bash.exe, which could look roughly like this (untested):

set CHERE_INVOKING=1
C:\cygwin64\bin\bash.exe --login -o igncr %*
exit /b %ERRORLEVEL%

So it would invoke the cygwin bash.exe with the right arguments, and also forward the exit code. Then we might be able to write shell: bash for cygwin, too.

[wilfwilson]

Thanks for the summary of the current obstacles (which matches my understanding), and for suggesting a reasonable sounding solution! (Although, having battled with this stuff for a while, I wouldn't be surprised if it turns out to be unreasonably tricky!)

[fingolfin]

I've just learned that one can prepend a dir to PATH via https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#adding-a-system-path which sounds as if it ought to work on Windows, too. Indeed, this is confirmed by this post

[fingolfin]

I just discovered https://github.com/cygwin/cygwin-install-action which I think has everything we need to get rid of our shell: C:\cygwin64\bin\bash.exe --login -o igncr '{0}' hack:

1. running git config --global core.autocrlf input once at the start (e.g. as part of gap-actions/setup-cygwin) may be enough to get rid of the need for -o igncr
2. alternatively, we could once at the start set igncr in the SHELLOPTS environment variable (this in turn can be done via echo "SHELLOPTS=igncr" >> $GITHUB_ENV though we may also wish to set additional options)
3. it explains why CHERE_INVOKING has to be set: to "prevent the profile script from changing directory" at startup; perhaps we can document that with a comment?
4. it may be possible for the setup-cygwin action to set this for all subsequent steps/actions via echo "CHERE_INVOKING=1" >> $GITHUB_ENV but this has to be tested
5. these days we can prepend a directory to PATH by modifying the $GITHUB_PATH file; the quoted action does that, and we can do, via echo "C:\cygwin64\bin" >> $GITHUB_PATH
6. apparently by setting the CYGWIN_NOWINPATH environment variable we can prevent the Cygwin bash from adding a bunch of non-cygwin dirs to its PATH? So echo CYGWIN_NOWINPATH=1 >> $GITHUB_ENV.

So to find out if this works, we ought to make a modified version of setup-cygwin that does all of this. Then try to use it with "stock" versions of the other actions.