[opam-devel] Managing $PKG_CONFIG_PATH in opam

[Tim Cuthbertson]

On the MirageOS mailing list, I submitted some patches[0] to make some
mirage libraries build without assuming a strict `opam` destination
directory layout. Mainly this was about the build scripts explicitly
setting PKG_CONFIG_PATH to `opam config var prefix`/lib/pkgconfig.
When building in a nixpkgs environment (using the `opam2nix` tool I'm
building[1]), there is no such path, but that's OK - the build
environment will have already set $PKG_CONFIG_PATH correctly.

[0] http://lists.xenproject.org/archives/html/mirageos-devel/2015-09/msg00000.html
[1] https://github.com/gfxmonk/opam2nix

It was generally agreed that having the build scripts perform this
task is not ideal, and Thomas Leonard suggested we could change `opam`
itself to export $PKG_CONFIG_PATH, rather than having build scripts
assume too much about their environment. That way a build script can
assume its pkg-config dependencies are available without caring
exactly where they live.

This seems like a good idea to me, and I've written up a fairly simple
patch which (I think) will do so:

https://github.com/ocaml/opam/compare/master...gfxmonk:pkg-config

I have only superficially tested it, but in thinking about that it
seems like this almost certainly won't be enough. All "pure" opam
packages providing pkg-config libraries should work just fine, however
there exist a large number of opam packages (conf-*) which exist
solely to force installation of system packages, and therefore most of
them actually rely on the system $PKG_CONFIG_PATH being used.

We could make `opam` prefix the opam pkgconfig path with the system
one, but this could lead to accidental impurity (in the case of these
mirage libraries, it would mean that dependencies might accidentally
be provided by the system rather than opam deps, which makes builds
fragile).

Any ideas how we can have opam manage PKG_CONFIG_PATH so that build
scripts don't have to?

(I've sent this to both the mirage & opam lists - apologies if that
causes too much noise, but I figure it affects both projects).

Cheers,
 - Tim.