Windows build gotchas

[Martin Foster]

Hello all,

I've been spending some of my winter break trying my hand at compiling GHC,
with a mind to hopefully contributing down the line.

I've got it working, but I ran into a few things along the way that I
figure might be worth fixing and/or documenting. In the approximate order I
encountered them:

   - The first pacman mirror on the list bundled with MSYS2 is down, with
   the result that every download pacman makes takes ~10sec longer than it
   should. It downloads a lot, so that really adds up - but it's easy to fix,
   just "pacman -Sy pacman-mirrors" before doing anything else with it. Is
   that worth mentioning on the wiki? I was thinking a line on
   https://ghc.haskell.org/trac/ghc/wiki/Building/Preparation/Windows could
   be helpful.


   - That page mentions "If you see errors related to fork(), try closing
   and reopening the shell" - I've determined that you can reliably avoid that
   problem by following the instructions at
   http://sourceforge.net/p/msys2/wiki/MSYS2%20installation/#iii-updating-packages,
   ie by running "pacman --needed -S bash pacman msys2-runtime", then closing
   & re-opening the MSYS shell, before you tell pacman to install the GHC
   prerequisite packages.


   - A minor point: I found it helpful to include "man-db" in the list of
   packages to install - without it, "git help" breaks down with " failed to
   exec 'man'".


   - I note "./sync-all --help" says, under "Flags", that "--windows also
   clones the ghc-tarballs repository (enabled by default on Windows)", and
   I've confirmed that default behaviour experimentally - but
   https://ghc.haskell.org/trac/ghc/wiki/Building/GettingTheSources tells
   you to manually clone ghc-tarballs when on Windows. Is that line on the
   wiki obsolete, or am I overlooking something?


   -  And finally, the big one: cabal and/or ghc-pkg put some files outside
   the MSYS root directory, and caused me no end of trouble in doing so...

I made a bit of a mess at one point, and tried to fix it by starting over
completely from scratch. I expected uninstalling & reinstalling MSYS to
achieve this (it deletes its root directory when you uninstall it), but
that left me with a huge pile of errors when I tried to run "cabal install
-j --prefix=/usr/local alex happy", of the form "Could not find module
`...': There are files missing in the `...' package".

I noticed that the cabal output made reference to
"C:\Users\Martin\AppData\Roaming\cabal\", so tried moving that out of the
way, but it only made the problem worse. I did figure it out eventually: in
addition to that directory, "%APPDATA%\cabal", there were also files left
over in "%APPDATA%\ghc". Once I removed that directory as well, things
started working again - but it took me a lot of time & frustration to get
there.

I'm not entirely sure, but I think the copy of Cabal I already had from
installing the Platform may also have been storing files in those
directories, in which case this process completely mangled them - which
isn't great.

It seems to me that, ideally, the "build GHC inside MSYS" procedure would
keep itself entirely inside the MSYS directory structure: if it were wholly
self-contained, you'd know where everything is, and it couldn't break
anything outside. As far as I can tell, the only breach is those two
directories courtesy of Cabal, so I didn't think it would be too difficult
- but none of the things I've tried (the --package-db cabal flag, a custom
cabal --config-file, setting the GHC_PACKAGE_PATH environment variable,
maybe some others I've forgotten) had the desired effect. Is it possible?
Is it even a good idea?

If that's just how it has to be, I feel like there should be an obvious
note somewhere for the sake of the next person to trip over it.

I'd be happy to amend the wiki for any/all of the first four points, if
people think it'd be appropriate; I'm not sure at all what to do about the
last one.

Any thoughts?

- Martin
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