building GHC overwrites the installed package database if GHC_PACKAGE_PATH is set

[heatsink]

When building GHC, if GHC_PACKAGE_PATH is set to a single path, then the build process will register packages in that path instead of in the build tree. Since GHC_PACKAGE_PATH points to the host compiler's package database, the build system overwrites the host compiler's package database, rendering the host compiler unusable.

To reproduce:

1. Get a binary distribution of GHC 7.4.1 and a source distribution of GHC 7.4.2

2. Configure the binary distribution to install in a temporary directory: ./configure --prefix=$HOME/workspace1

3. Install the binary distribution

4. Set up the environment to use only the installed compiler: export PATH=$HOME/workspace1/bin:$PATH; export GHC_PACKAGE_PATH=$HOME/workspace1/lib/ghc-7.4.1/package.conf.d

5. Configure the source distribution to install in a different directory: ./configure --prefix=$HOME/workspace2

6. Build the source distribution: make

Building will eventually run commands that modify the package database in GHC_PACKAGE_PATH. One of these commands is "inplace/bin/ghc-pkg" update --force rts/package.conf.inplace. I confirmed that the database is being modified by making ghc-pkg print the name of the file it's about to update.

[igloo]

Thanks for the report.

[igloo]

Hmm, having thought about it, isn't that exactly what you'd expect to happen?

I wonder if we should just deprecate/remove GHC_PACKAGE_PATH. You already can't use cabal if it's set (cabal will just fail).

[heatsink]

I expect building software to pose no risk of breaking any installed software. It is understandable that installing software may break other software, but building should not.

I've been using GHC_PACKAGE_PATH to install two versions of GHC that both call themselves ghc-7.4.2. By default, they both look in the same user package database, which breaks at least one of the two versions because packages are not compatible with both GHCs. Setting GHC_PACKAGE_PATH fixes that problem. Cabal works if given options --global --package-db=$FOO. If this way of managing paths is deprecated, then I'd like to have an alternative.