Work with GHC_PACKAGE_PATH

[ttuegel]

At the moment, Cabal will die if run with the GHC_PACKAGE_PATH environment variable set. I think this is an arbitrary restriction. Instead, it should

1. Parse the search path in GHC_PACKAGE_PATH,
2. treat these paths as if they were passed to the configure command by --package-db, and
3. unset GHC_PACKAGE_PATH before invoking GHC.

Cabal should do the same for GHCJS_PACKAGE_PATH.

See also #2711.

I am implementing this in reconfigure.

/cc @ezyang @23Skidoo

[ezyang]

OK, that sounds fine to me.

[dcoutts]

@ttuegel this patch is likely to break cabal new-build since it changes getInstalledPackages but not getInstalledPackagesMonitorFiles. But just changing getInstalledPackagesMonitorFiles would also be wrong for new-build because the decision of whether or not to use the GHC_PACKAGE_PATH can't be taken locally like that, we'd need to decide for the old code path or the new code path differently whether we want to use it or not, and for new-build if we do then we need to track the monitored files.

In fact this will really break cabal new-build since we only want to get the installed packages for the global db and ignore user and any other ones. So unless we think that GHC_PACKAGE_PATH is specifying the "global" db (which nix does) then we do want to ignore it.

[23Skidoo]

@ttuegel @dcoutts Can we add a failing test for this?

[ttuegel]

In fact this will really break cabal new-build since we only want to get the installed packages for the global db and ignore user and any other ones. So unless we think that GHC_PACKAGE_PATH is specifying the "global" db (which nix does) then we do want to ignore it.

GHC_PACKAGE_PATH can really only specify an immutable global database: the paths are added before the global database, can't be cleared from the command line, and can't be installation targets. If I update getInstalledPackagesMonitorFiles accordingly, I don't see why this would break new-build.

[ttuegel]

This is blocked on #3651 (Support Nix in cabal-install). To really do this properly, cabal-install needs to pay attention to when certain environment variables change and reconfigure; this will break the usual workflow for Nix users unless I implement Nix integration first.