A new CI story
The Problem ----------- When I started looking at the problem of providing CI for `head.hackage` I considered two possible designs: 1. Build upon `cabal-install` 2. Build upon Nix's Haskell infrastructure While I preferred (1), I found that integrating with `cabal-install` was quite difficult: * it [does not produce logs](https://github.com/haskell/cabal/issues/5901) for local packages, which was the obvious way to incorporate patched packages into the build plan * it is difficult to reconstruct why a package build failed (e.g. due to a planning failure, dependency failing to build, or an error in the package itself) For these reasons it so happened that (2) ended up being a tad easier to implement. However, it suffers from a number of problems: * Nix's Haskell infrastructure doesn't handle multiple versions of a single package at all, yet we now have patches for multiple package versions in `head.hackage` * Nix's Haskell infrastructure doesn't handle flags, which can complicate building some packages * The Nix expressions ended up being rather difficult to maintain The Solution ------------ This MR moves the CI infrastructure back in the direction of (1), facilitated by workarounds that I found for the two issues described above. The infrastructure revolves around the `head-hackage-ci` executable which provides a `test-patches` mode which `gitlab-ci.yml` invokes thusly: ``` head-hackage-ci test-patches --patches=./patches --with-compiler=$GHC ``` This mode does several things: 1. Builds a local package repository (using the same script used to build `https://ghc.gitlab.haskell.org/head.hackage/`). (N.B. by pulling patched packages from a proper repository instead of using local packages we side-step the fact that `cabal-install` doesn't produce logs for local packages) 2. Generate a `cabal.project` file containing: * a `remote-repository` stanza referring to this repository * constraints to ensure that we only choose patched package versions * some additional `package` stanzas to ensure that `Cabal` can find native library dependencies (these are defined in `ci/build-deps.nix`) 3. Run `cabal new-update` (as well as perform a dummy build of the `acme-box` package to ensure that the package index cache is built, otherwise parallel builds can randomly fail) 4. For each patched create a new working directory containing: * the previously generated `cabal.project` file * a `test-$PKGNAME.cabal` file defining a dummy package depending upon the library and perform the build. We use some heuristics depending upon: * the `plan.json` file * which log files exist * the contents of said log files to sort out what happened. 6. After all the packages have been built produce a final report of the result. While this is admittedly pretty hacky, in truth it's no worse than the somersaults which we had to perform in the Nix infrastructure. Reliably introspecting on failed builds seems to be messy business no matter which build system you use.
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