Use Cabal PackageInfo_warp instead of Paths_warp

[lrworth]

Use of Paths_warp causes a large dependency closure when building with nix. See NixOS/nixpkgs#264434 for example.

This reduces the closure size of our application from 756Mb to 53Mb by virtue of not including GHC and GCC.

It makes a significant change to cabal-version and I'm not sure what compatibility we're trying to maintain in this project; also I made some incidental changes so that it would build with GHC 9.12.

This is not merge-ready — the PR is meant to open a discussion about using PackageInfo_.

[Vlix]

Looks fine, though does the cabal-version bump mean it won't build with GHC 9.10 anymore? That might be unacceptable.

We'd need to add conditionals etc. to add the Path/PackageInfo modules.

[Vlix]

Requires an increase to the lower version bound of http2 (>= 5.2)

[Vlix]

I expect this to fail on any GHC < 9.10, so this might need CPP around it.

[Vlix]

How did this end up in this module?

Can you move this to Network.Wai.Handler.Warp.Internal and re-export from here with a comment that it's for backwards compatibility?

[Vlix]

After moving warpVersion to Network.Wai.Handler.Warp.Internal, please import from there and use here.

[Vlix]

Also please merge the current master branch, as it has fixed some CI issues.

[lrworth]

The lts-21 / GHC 9.4 configuration listed in CI doesn't work:

$ stack test --system-ghc --resolver lts-21 --stack-yaml stack-lts-21.yaml

Warning: nix (Nix) 2.29.0 is on the PATH (at /nix/var/nix/profiles/default/bin/nix) but Stack's Nix integration is disabled. To mute this message in future, set notify-if-nix-on-path: false in Stack's
         configuration.

Selected snapshot: lts-21.25.
[...]
Populating package index cache ...
Package index cache populated

Error: [S-242]
       Unable to parse Cabal file from package $PWD/wai/warp/warp.cabal:
       * 0:0: Unsupported cabal-version 3.12. See https://github.com/haskell/cabal/issues/4899.

       The Cabal file uses the Cabal specification version 3.12, but we only support up to version 3.8. Recommended action: upgrade your build tool (e.g. stack upgrade).

If in principle this work might be merged at a later date, we are happy to maintain a fork in a mergeable state. What do you say, @Vlix?

[Vlix]

I think we could work around this and keep the cabal-version at its current >= 1.10.

Adding a flag to warp that switches between using the Paths_warp or PackageInfo_warp generated module would be sufficient, no?
And we'll want to move over to the PackageInfo_warp module in the future, of course, since we only use the version, but while we still want to support earlier GHC's, this is not enforceable.

flag package-info
  description: Use 'PackageInfo_warp' instead of 'Paths_warp' (needs Cabal >= 3.10)
  default: False
  manual: True

if flag(package-info)
    other-modules: PackageInfo_warp
else
    other-modules: Paths_warp

[lrworth]

haskell/cabal#9481 guards against using PackageInfo_* with cabal-version: >= 1.10, so that change would work with old versions of Cabal but not the ones that actually support PackageInfo_*. Building bellroy@5b6eb2c with Cabal 3.14.2 gives me:

> [autogen-guard] To use the autogenerated module PackageInfo_* you need to specify `cabal-version: 3.12` or higher.

[lrworth]

#1044 is a much more conservative approach — optionally removing dependency on Paths_* by disabling runtime version support altogether.