How to add pkgconfig packages without forking haskell.nix

[ramirez7]

This is caused by #1641 (@hamishmack)

The cabalPkgConfigWrapper works fine for packages in nixpkgs and manually mapped in pkgconf-nixpkgs-map.nix, but if a user adds their own pkgconfig-based packages via overlay, it will fail to find them. This is a regression from the previous behavior (which relied on a cabal bug).

I have an example in this PR (examples/sdl-gpu-minimal): https://gitlab.com/macaroni.dev/macaroni.nix/-/merge_requests/12

This is the error I get (as expected - SDL_FontCache is a C library I packaged myself):

Resolving dependencies...
Error: cabal: Could not resolve dependencies:
[__0] next goal: sdl-gpu (user goal)
[__0] rejecting: sdl-gpu-0.1.0.0 (conflict: pkg-config package
SDL_FontCache-any, not found in the pkg-config database)
[__0] fail (backjumping, conflict set: sdl-gpu)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: sdl-gpu

Exposing a way for the user to inject their own packages to the mapping in their cabalProject call would be a nice fix. I can take a stab at that.

[ramirez7]

#1667 This accomplishes this nicely.

Sadly, pkgconfig-depends in general is broken for cross-compilation #1666, so I can't test it against my mingwW64 examples. (It isn't broken but I did need to make changes to fix it).

[exarkun]

#1641 also broke my overlay-based pkg-config. I read the documentation to see if there were changes to pkg-config-support and I didn't see anything - I suppose because the option newly introduced here isn't documented yet. I didn't find this ticket until I bisected haskell.nix to find the revision that introduced the regression, then followed the outlinks to various tickets, including this one.

[ramirez7]

Yeah sorry about that. My fix got merged without accompanying docs. But there is a PR up for them now if you'd like to give any feedback as to whether they would have helped you! #1670