Feature request: Option to make a compiler available for all users

[ygale]

On a build server used by many devs, the number of compilers installed by stack is basically O(n^2) - number of compilers needed times number of devs.

Proposal:

• Support a global programs directory in the same place where the global config.yaml lives, e.g. /etc/stack/programs.
• The global programs directory is structured the same way as the user programs directory (e.g. ~/.stack/programs).
• If stack doesn't find the compiler it is looking for in the user programs directory, then instead of immediately failing it first looks in the global programs directory.
• The --global option to stack setup tells stack to install the compiler in the global programs directory instead of the user programs directory.

Notes:

• The build server problem can currently be solved by the "symlink hack", but that is awful in many ways.
• It could also be solved by Feature request: like "system-ghc" but for multiple preinstalled versions of ghc #3931, but that requires manual individual configuration for each user, creating IT and support overhead.

[borsboom]

This sounds like a good approach to me.

[DanBurton]

I don't quite understand. Doesn't this functionality basically already exist, in the form of the STACK_ROOT environment variable?

As an added bonus, not only are the compilers shared, but all libraries built in a snapshot package-db are shared as well.

https://docs.haskellstack.org/en/stable/GUIDE/#setting-stack-root-location

The only potential downside is that users presumably need write access to the shared location in order to use it this way. Is the idea here that users only have read access to the "global programs"?

[borsboom]

STACK_ROOT also isn't safe for multiple `stack` processes to use at the same time, so that would be a problem on a multi-user machine.

[ygale]

@DanBurton Right, normally the global programs will be accessed read-only. When a new GHC version is about to come into play, an admin would run sudo stack setup ... --global once, and we're set.

@borsboom Yes. And besides that, we want to avoid the complexities of per-user configuration, and shell-script/environment-variable hackery.

[snoyberg]

Moved to wishlist https://github.com/commercialhaskell/stack/wiki/Wishlist