find sandbox from an environment variable

[redneb]

Currently, sandbox aware commands (such as cabal install) look for a sandbox in the current directory. I think it would be nice if there was an environment variable (e.g. CABAL_SANDBOX_PATH) that cabal examines in order to determine the path of the sandbox. This would allow a sandbox to be used while working in a directory different than the one where cabal sandbox init was issued from.

Here's an example scenario:

mkdir -p /tmp/dir1/dir2
cd /tmp/dir1
cabal sandbox init
cabal exec bash
cd dir2
cabal install somepackage

With cabal 1.20, the last command will install the package in ~/.cabal, despite the fact that we are inside a cabal exec environment.

hsenv handles the above situation nicely; once you enter an hsenv environment, you can issue a cabal install from any directory.

[tibbe]

You can set CABAL_SANDBOX_CONFIG to point to a sandbox config located elsewhere. The config points to the package DB.

That being said, this looks like a bug in exec. It is supposed to set an environment variable that makes sure GHC uses the sandbox's package DB for installs.

/cc @benarmston

[benarmston]

I can't reproduce the bug. Instead of installing cabal prints an error message and exits:

$ cabal install hlint
cabal: Use of GHC's environment variable GHC_PACKAGE_PATH is incompatible with
Cabal. Use the flag --package-db to specify a package database (it can be used
multiple times).

If I unset GHC_PACKAGE_PATH then I get the behaviour described in the bug: installs into the user database. If I then export CABAL_SANDBOX_CONFIG=/tmp/dir1/cabal.sandbox.config the install is into the sandbox.

So it would seem that we have two issues to fix here:

1. Set the CABAL_SANDBOX_CONFIG variable when running cabal exec.
2. Allow cabal to install into the sandbox when running under cabal exec. Perhaps instead of failing unconditionally if GHC_PACKAGE_PATH is set, we could check if CABAL_SANDBOX_CONFIG is also set and whether GHC_PACKAGE_PATH has the value that would be set for that sandbox.