Improve handling of temp directory in GAP for Windows

[ChrisJefferson]

This improves the Cygwin release in a few ways. This should only be merged once we have decided we are definitely moving to my new Windows release, because this breaks the current release.

The improvements are:

1) Use 'cygstart' to open webpages. This deals with unix vs windows filenames. It doesn't let us handle anchors, but after some googling this appears impossible in windows, as # can appear in filenames.

2) always use sh to spawn external programs (sh can still run windows programs)

3. Don't try to hard-wire the temp directory (and on many computers the guess is wrong). Instead we reconfigure cygwin to tell it to bind /tmp to the correct temp directory (this is done by the installer).

[fingolfin]

This is marked as "do no merge", and I also cannot test it, but overall it looks sensible to me. I could approve it if required, but I'd feel more comfortable if somebody who actually can test it (e.g. @PaulaHaehndel ) were to test and review it; but as long as it is marked "do not merge", I am not sure that's sensible?

[fingolfin]

Also, could you explain in how far this breaks the current release if used in a release made using the current process for making a Windows release?

[ChrisJefferson]

So, the changes are:

1) Use the program cygstart. We don't currently package that, and if we did we wouldn't set up the paths correctly.

2) Similarly, run external commands via sh, we don't distribute that either.

3. Use /tmp for temp files -- at the moment that won't work because we don't set up standard unix directories. Further, in the new installer we make cygwin point /tmp at the windows temp directory.

[codecov]

Codecov Report

Merging #3939 (f8ece80) into master (ce4b50f) will increase coverage by 10.22%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master    #3939       +/-   ##
===========================================
+ Coverage   82.26%   92.48%   +10.22%     
===========================================
  Files         686      714       +28     
  Lines      296672   790718   +494046     
===========================================
+ Hits       244045   731321   +487276     
- Misses      52627    59397     +6770     

Impacted Files	Coverage Δ
lib/helpview.gi	37.24% <0.00%> (-0.69%)	⬇️
lib/process.gi	27.20% <0.00%> (-42.80%)	⬇️
src/streams.c	76.93% <ø> (-5.17%)	⬇️
lib/helpt2t.gi	0.23% <0.00%> (-83.14%)	⬇️
lib/autsr.gi	1.09% <0.00%> (-66.09%)	⬇️
lib/csetpc.gi	39.42% <0.00%> (-48.58%)	⬇️
src/vec8bit.h	51.78% <0.00%> (-42.86%)	⬇️
lib/gpfpiso.gi	38.48% <0.00%> (-37.26%)	⬇️
lib/permdeco.gi	37.93% <0.00%> (-34.65%)	⬇️
... and 216 more

[ChrisJefferson]

Hi @fingolfin or @wilfwilson , this very old PR is now ready to merge, it will improve future cygwin releases.

[wilfwilson]

@ChrisJefferson and I have just been talking about it this PR, and associated Cygwin-related stuff. I've crossed out some text in the previous parts of this discussion, because they're no longer applicable.

Given our discussion, I think this should be merged.

[wilfwilson]

In more detail, @ChrisJefferson said this PR doesn't break the existing GAP release system for Windows. It also does two things:

• The changes to the temporary directory that Chris mentioned above
• This redefines ARCH_IS_WINDOWS to mean 'is running in Cygwin but not in a full Cygwin installation' (see the changed documentation).