laminar-config

Last updated: 2019-08-15 07:54:14 +0100

Upstream URL: git clone http://chriswarbo.net/git/laminar-config.git

Contents of README follows

Warbo’s Laminar Configuration

Laminar is a simple continuous integration server. Laminar jobs are just scripts which live in a particular directory, which this repository provides.

We don’t point Laminar directly at this git repository, since the scripts might need a variety of dependencies, etc. which might even conflict (e.g. if jobs need different versions of GHC). Instead we define the jobs directory as a Nix package, which lets us manage each script’s dependencies independently.

The default.nix file defines the job directory package. Build it using e.g. nix-build.

The install script builds the job directory and points Laminar at the result.

Garbage Collection

We can free up space by performing garbage collection on the Nix store, which deletes paths that aren’t ‘live’ (in use) anymore. Usually, paths are live if they are installed on the system (or are a dependency of some installed package). We don’t use Nix to install our Laminar configuration, since doing so would treat a config update like an OS update, which drags in too many other concerns for our purposes. Hence we can’t rely on this to keep our config directory live.

Instead, the nix-build command automatically creates a symlink called result in the current directory. This is a ‘garbage collector root’, whose presence tells Nix that the Laminar directory is live. If this symlink is deleted or renamed, the config directory will no longer be seen as live, and may be deleted when running the garbage collector. Building a new config directory will overwrite the result symlink, which makes the old directory not live anymore.

Note that installing a new Laminar config is not an atomic operation, it consists of two atomic steps: building the new config, which also overwrites the result symlink; then moving the new config into place. In between these steps Laminar will be running a config that is no longer live, hence running the garbage collector may delete it. This should hopefully not cause any problems.