hyperhive/docs/ci.md

# hive-ci: Forgejo Actions Runner

The `hive-ci` module runs a Forgejo Actions runner in a `hive-ci` nixos-container, executing CI jobs from `.forgejo/workflows/ci.yml` (e.g., `nix flake check` on every PR).

## Operator bootstrap

Set `services.hyperhive.ci.enable = true` in the host NixOS config. That's it — no manual token provisioning.

**Requirements:**
- `services.hyperhive.forge.enable = true` must also be set (the runner registers against hive-forge).
- Optional: tune `services.hyperhive.ci.name` (runner name in forge admin panel), `concurrency` (parallel job capacity), `labels` (workflow targeting).

## Container design

- **Shared host netns**: container reaches hive-forge at `http://127.0.0.1:<httpPort>` (same as hive-gateway).
- **Non-ephemeral**: runner credentials persist across restarts (written to container's stateDir on first registration, reused thereafter).
- **Sandbox fallback**: nspawn containers can't create user-namespaces, so nix's sandboxing would always fail. Module sets `nix.settings.sandbox-fallback = true` in the container — nix builds run unsandboxed (safe because the container is already isolated). See `docs/gotchas.md`.
- **Credential isolation**: the forge admin token (`forge-core-token`) never enters the container. A host-side oneshot service (`hive-ci-prefetch.service`) performs all forge API calls and writes only the runner registration token into the container via a read-only bind-mount at `/run/hive-ci/runner-token`.

## Auto-registration flow

`hive-ci-prefetch.service` is a host-side oneshot that runs on **every boot** before `nixos-container@hive-ci.service`. It handles both first-run registration and stale-credential detection. The core admin token is accessed only on the host and never bind-mounted into the container.

### Every boot

1. Check for the core admin token at `/var/lib/hyperhive/forge-core-token`. If absent (forge still initialising), write `TOKEN=placeholder` and exit — the runner service will fail gracefully until the next boot.
2. If `.runner` exists at `/var/lib/nixos-containers/hive-ci/var/lib/gitea-runner/hive/.runner`: validate the runner ID against `GET /api/v1/admin/runners/{id}` using the core token:
   - **200**: runner still registered — write `TOKEN=placeholder` to `/run/hive-ci/runner-token` and exit; runner reuses `.runner` credentials.
   - **404**: runner was deleted from forge (e.g. after a wipe) — delete `.runner`, proceed to re-registration below.
   - **000** (forge unreachable): keep existing `.runner`; write placeholder token; the runner itself will surface the connectivity error.
   - **other non-200**: treat as stale, delete `.runner` and re-register.
   - **malformed `.runner`** (no `id` field): delete and re-register.
3. If `.runner` is absent (first boot or purged above): fetch a fresh registration token from `GET /api/v1/admin/runners/registration-token`. Retries for 30s in case forge is still starting. Writes `TOKEN=<real>` to `/run/hive-ci/runner-token`.
4. Container starts with `/run/hive-ci/runner-token` bind-mounted read-only. `gitea-actions-runner` reads the token, registers itself, and persists credentials to `.runner`. On subsequent boots step 2 validates these credentials and fast-paths past registration.

## CI workflow

The single CI job is defined in `.forgejo/workflows/ci.yml`:

```yaml
name: CI
on:
  pull_request:
    branches: ["**"]
jobs:
  check:
    name: nix flake check
    runs-on: [hive-ci]
    steps:
      - uses: actions/checkout@v3
      - name: check
        run: nix flake check
```

This runs on every PR, executing all flake checks (treefmt, rustfmt, cargo test, cargo clippy, module evaluation). No `--no-build`: the checks' derivations are the canonical source of truth.

## Security: unsandboxed builds and trusted contributors

**hive-ci should only run CI for trusted contributors.** The security boundary is weaker than it looks:

### What unsandboxed builds mean

nspawn containers cannot create user-namespaces, so `nix.settings.sandbox-fallback = true` is set in the container. This means every `nix build` (and `nix flake check`) runs **without a build sandbox** — the build process has full access to the container filesystem, network, and any bind-mounts during the build phase.

A malicious `default.nix` or build script in a PR can therefore:

- **Make arbitrary network requests** to any address reachable from the container. The container shares host netns, so `http://127.0.0.1:<forgePort>` is reachable with the forge API — without admin credentials, but public/read endpoints are accessible.
- **Write to the container filesystem**, including corrupting the runner's state dir or `.runner` credentials.

The core admin token (`forge-core-token`) is **not** bind-mounted into the container. It is accessed only by the host-side `hive-ci-prefetch.service` before the container starts. A build process can still reach forge over the network, but cannot use the admin token to issue privileged API calls.

Note: `nix flake check --no-build` (eval-only) reduces the attack surface but does not eliminate it — `builtins.fetchGit`, `builtins.fetchurl`, and import-from-derivation can reach the network and filesystem during evaluation. The default CI workflow runs full `nix flake check` (builds derivations), which is the higher-risk path.

### Mitigation

For a hive used by a single operator or a small trusted team, the risk is low — all contributors are already trusted with forge access anyway.

For repos with external contributors or fork PRs:

- Use Forgejo's **fork PR approval workflow** (`repository.settings` → "Require approval for fork PRs from first-time contributors") to gate CI until a maintainer approves the first PR.
- Or restrict the CI workflow trigger to push events on branches (not `pull_request` from forks) — forks can't push to upstream branches.

The current design is appropriate for a trusted-team hive where all contributors have implicit forge access.

## References

- `nix/modules/hive-ci.nix`: runner configuration, auto-registration script, container setup.
- `.forgejo/workflows/ci.yml`: workflow definition.
- `docs/gotchas.md`: nix sandboxing limitations in containers.