hyperhive/swarm-controller/src/forge/runner_token.rs

//! The forge's runner registration token, published to the swarm secret store
//! at [`ci::registration_token_path`], where the CI runner's fetch unit
//! (`nix/host-modules/swarm-ci-runner.nix`) reads it under the runner's own
//! certificate.
//!
//! Forgejo answers `GET /admin/runners/registration-token` with the same
//! instance-wide token until someone resets it. The runner re-registers on
//! every new value it reads, which leaves its previous entry on the forge, so
//! a pass writes only when the forge's token differs from the one this process
//! last wrote. The controller's store grant on the path is write-only, so that
//! last write is held in memory: a restarted controller writes the current
//! token once.

use std::sync::Arc;

use anyhow::{Context, Result};
use swarm_secret_client::ci;

use super::Client;

/// How often [`spawn`] re-reads the forge's token.
const RECONCILE_INTERVAL: std::time::Duration = std::time::Duration::from_mins(5);

/// Whether `current`, the forge's token now, has to be written, given the one
/// this process last wrote (`None` before its first write).
fn needs_write(last_written: Option<&str>, current: &str) -> bool {
    last_written != Some(current)
}

impl Client {
    /// The forge's instance-wide runner registration token.
    ///
    /// # Errors
    /// When the forge refuses the call, or answers without a token.
    pub async fn runner_registration_token(&self) -> Result<String> {
        let answer = self
            .api
            .admin_get_registration_token()
            .await
            .context("GET /admin/runners/registration-token")?;
        answer
            .token
            .filter(|t| !t.is_empty())
            .context("the forge answered without a runner registration token")
    }
}

/// One pass: read the forge's token and write it when it is not the one
/// `last_written` holds. Returns the token it wrote, if it wrote one.
async fn pass(client: &Client, last_written: Option<&str>) -> Result<Option<String>> {
    let token = client.runner_registration_token().await?;
    if !needs_write(last_written, &token) {
        return Ok(None);
    }
    let path = ci::registration_token_path()?;
    let store = crate::store::connect()
        .await
        .context("logging in to the swarm secret store")?;
    store
        .write(
            &path,
            &ci::Credential {
                value: token.clone(),
            },
        )
        .await
        .with_context(|| format!("writing {path}"))?;
    Ok(Some(token))
}

/// Publish the forge's runner registration token now and every
/// [`RECONCILE_INTERVAL`] after. A pass that fails is logged and retried on
/// the next tick; it never stops the daemon.
pub fn spawn(client: Arc<Client>) {
    tokio::spawn(async move {
        let mut ticker = tokio::time::interval(RECONCILE_INTERVAL);
        let mut last_written: Option<String> = None;
        loop {
            ticker.tick().await;
            match pass(&client, last_written.as_deref()).await {
                Ok(Some(written)) => {
                    tracing::info!("runner registration token: written to the store");
                    last_written = Some(written);
                }
                Ok(None) => tracing::debug!("runner registration token: unchanged"),
                Err(e) => tracing::warn!(
                    error = %format!("{e:#}"),
                    retry_in_s = RECONCILE_INTERVAL.as_secs(),
                    "runner registration token: pass failed; retrying next tick"
                ),
            }
        }
    });
}

#[cfg(test)]
mod tests {
    use super::needs_write;

    #[test]
    fn the_first_pass_writes() {
        assert!(needs_write(None, "t1"));
    }

    #[test]
    fn an_unchanged_token_is_not_rewritten() {
        // A rewrite of the same value is harmless to the runner, but it is a
        // new store version every five minutes for nothing.
        assert!(!needs_write(Some("t1"), "t1"));
    }

    #[test]
    fn a_reset_token_is_written() {
        assert!(needs_write(Some("t1"), "t2"));
    }
}