# swarm-builder

The swarm builder's build service, in the `swarm-builder` container on the
host that sets `deploy.builder.enable`. The design is
[`docs/swarm/builder.md`](../docs/swarm/builder.md).

swarm-controller sends one request per config PR head: an agent, a commit and
the PR. This service builds that agent config on the agent base this host was
deployed with, through the host's nix daemon, and answers with the system
closure's store path or the tail of what `nix` printed. The controller turns
the answer into a `swarm-builder` commit status on the PR's head.

## What a build is

- `nix build` of `nix/builder/check-flake`'s `check` output, with its three
  inputs overridden: `hyperhive` and `nixpkgs` to this host's own sources,
  and `agent` to `git+https://<forge>/agent-configs/<agent>?ref=refs/pull/<pr>/head&rev=<rev>`.
- The evaluation runs here, pure; the builds run in the host daemon's sandbox
  as an untrusted client, so a config can neither set daemon options nor read
  this service's environment.
- Nothing is pushed and nothing is recorded: the closure stays in the host's
  store until it is collected.

## Credentials

The container holds one credential: its own store certificate, which nspawn
hands in at start without a copy on disk. With it the service reads its
queue client secret at start and again when authelia refuses it, and its
read-only forge token at every build.
`git` gets the token through a credential helper reading the child's
environment, never in a URL or an argument, and the helper answers only an
`https` request for the forge's host. Its store role reads exactly
those two paths.

## Archetypes

The service also answers archetype evaluations: `nix eval` of the `meta` of
every `hyperhiveArchetypes` entry in this host's hyperhive source, with
nixpkgs pointed at this host's. It fetches nothing and uses no token, and it
doesn't wait for a running build. See
[`docs/swarm/archetypes.md`](../docs/swarm/archetypes.md).