Select appropriate msys2/MinGW shell on Windows

[akhra]

msys2 provides three shells: the full-POSIX msys2 shell, plus 64- and 32-bit MinGW shells which provide a GNU tool set but use a lighter, more optimized translation layer. Matching this, there are up to three versions of any msys2 pacman package: the unprefixed msys2 version, and architecture-prefixed MinGW versions.

Currently, stack uses the msys2 shell in all cases. This is poor behavior; in fact, the msys2 docs suggest only using that shell for system upgrades and pacman itself, and in practice I've found that MinGW packages install fine from the bit-appropriate MinGW shell. Correct behavior would be to pick MinGW64 or MinGW32 to match the GHC version in the active build strategy, and only use the raw msys2 shell when performing automated installation/upgrades (e.g. as suggested in #1586).

This is not a mere matter of optimization or correctness for its own sake: sdl2 fails to build with stack build on the Windows commandline, as it doesn't even have an msys2 package version; but with the MinGW64 pkg-config and sdl2 packages installed, manually launching the mingw64_shell.bat installed by stack and running stack --skip-msys build works fine. (Note if you want to reproduce this: there's a buggy header in sdl2-2.0.3, to get a successful build you'll need the replacement here).

[borsboom]

@snoyberg, do you have an opinion on this? I don't know the history of how the current design was reached.

[snoyberg]

I'm open to any suggested changes here from people who know better. I did not understand any of the details of these "multiple shells" when setting up the code. @tejon As you seem to have a pretty good understanding of this, would you like to take a crack at a PR on this?

[akhra]

Apologies for being slow getting to this, but yes, I'm giving it a shot. My Haskell's still at the rough end of intermediate, though. I don't suppose you could give me a quick run-down of the relevant code paths? I.e. those involved in selecting a build strategy (particularly selecting x86 vs x64), determining the host is Windows and locating/invoking msys. Figuring out where to look for things is often the longest and most tedious part of the process for me, heh.

[mgsloan]

@tejon Here's the bits for installing msys https://github.com/commercialhaskell/stack/blob/master/src/Stack/Setup.hs#L384 . Just below that section for installing it, it adds it to the PATH. Otherwise I don't think there's much to the usage of msys (could be wrong, but a search doesn't reveal much else)

[mgsloan]

@tejon Feel free to reopen if not fully addressed by your PR