//! Swarm-level controller daemon. Runs as the unprivileged
//! `swarm-controller` user on whichever host the operator flips
//! `services.hyperhive.swarm.controller.enable` on, and serves HTTP over a
//! unix socket that the hive-gateway's nginx proxies to.
//!
//! **Today it serves one endpoint and owns no state.** That is deliberate:
//! this slice exists to make the *unit* real — service user, runtime and
//! state directories, socket, nginx reachability — so the swarm-level
//! surfaces that follow have somewhere to land. Guessing those surfaces
//! now would bake in a shape nobody has agreed to.
//!
//! Distinct from `hive-c0re`, which is per-hive: c0re owns the agents on
//! one host, this owns what is true across hives.

use std::os::unix::fs::PermissionsExt as _;
use std::path::PathBuf;

use anyhow::{Context, Result};
use axum::{Router, routing::get};

/// Where the daemon binds, overridable via `SWARM_CONTROLLER_SOCKET`.
///
/// A compiled-in default is legitimate here and is *not* the mistake that
/// a hardcoded remote address would be: this is a path this process
/// **creates**, not an address it hopes to find something at. systemd's
/// `RuntimeDirectory=swarm-controller` makes the parent exist before
/// `ExecStart`, so the default names a directory the unit just produced.
///
/// The directory is its own — deliberately not shared with hive-c0re's
/// `/run/hyperhive`. The socket is `0666`, so its directory is the only
/// access control it has; co-locating it with c0re's admin socket would
/// put both within reach of whatever can reach either. nginx runs on the
/// host, so nothing narrows its reach for you.
const DEFAULT_SOCKET: &str = "/run/swarm-controller/controller.sock";

fn socket_path() -> PathBuf {
    std::env::var_os("SWARM_CONTROLLER_SOCKET")
        .map_or_else(|| PathBuf::from(DEFAULT_SOCKET), PathBuf::from)
}

/// Liveness probe. Returns the build's version so an operator can tell
/// *which* controller answered without shelling onto the host.
async fn health() -> &'static str {
    concat!("swarm-controller ", env!("CARGO_PKG_VERSION"), "\n")
}

#[tokio::main]
async fn main() -> Result<()> {
    tracing_subscriber::fmt()
        .with_env_filter(
            tracing_subscriber::EnvFilter::try_from_default_env()
                .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("info")),
        )
        .init();

    let path = socket_path();

    // `RuntimeDirectoryPreserve=yes` keeps the directory across a restart,
    // so a socket file from the previous run can outlive the process that
    // owned it and `bind` would fail with EADDRINUSE. Unlinking a stale
    // socket is safe precisely because the directory is ours alone: nothing
    // else can have put a file at this path.
    if let Err(e) = std::fs::remove_file(&path)
        && e.kind() != std::io::ErrorKind::NotFound
    {
        return Err(e).with_context(|| format!("clearing stale socket at {}", path.display()));
    }

    let listener = tokio::net::UnixListener::bind(&path)
        .with_context(|| format!("binding {}", path.display()))?;

    // `bind` leaves the socket 0755, and connecting needs write — the
    // gateway's nginx is a different user, so it would be locked out.
    // 0666 matches how hive-c0re publishes the per-agent sockets
    // (`socket_server::start`), and rests on the same argument: **the
    // containing directory is the access control, not the socket mode.**
    // This directory holds one socket and is bind-mounted into exactly
    // one container. That is also why it must not be shared with
    // hive-c0re's `/run/hyperhive` — with a 0666 socket, a directory
    // that carries more than it should is the whole vulnerability.
    std::fs::set_permissions(&path, std::fs::Permissions::from_mode(0o666))
        .with_context(|| format!("chmod {}", path.display()))?;
    tracing::info!(socket = %path.display(), "swarm-controller listening");

    let app = Router::new().route("/health", get(health));
    axum::serve(listener, app)
        .await
        .context("serving swarm-controller")
}

#[cfg(test)]
mod tests {
    use super::DEFAULT_SOCKET;
    use std::path::Path;

    /// The socket must not share a directory with anything else, because
    /// the socket is `0666` and the directory is therefore the only access
    /// control it has. `/run/hyperhive` in particular holds hive-c0re's
    /// **admin** socket, and nginx — a host service — is bounded by
    /// nothing but the directory itself.
    ///
    /// A test rather than a comment: the failure this guards against is a
    /// one-word edit that looks tidier and reads fine in review.
    #[test]
    fn socket_lives_in_its_own_runtime_dir() {
        let parent = Path::new(DEFAULT_SOCKET)
            .parent()
            .expect("socket path has a parent directory");
        assert_eq!(
            parent,
            Path::new("/run/swarm-controller"),
            "the socket's directory is its access control — moving it under a shared \
             directory (notably /run/hyperhive, which holds the host admin socket) \
             exposes everything else in that directory to the gateway's nginx"
        );
    }
}