//! This daemon's OIDC identity: the one it connects to the queue with and
//! mints the auth bridge's and the OTLP receiver's bearer tokens from.
//!
//! The queue's address, the token endpoint and the client id come from the
//! environment. The client secret comes from the swarm's secret store, at
//! `swarm_secret_client::queue::controller_client_path`, read once at start
//! under this daemon's own certificate and held only in memory — it is never
//! on this host's disk. `swarm-secret-publish`, on the host that runs
//! authelia, puts it there.
//!
//! [`init`] runs once in `main`, before anything calls [`get`].

use std::future::Future;
use std::path::PathBuf;
use std::sync::OnceLock;
use std::time::Duration;

use anyhow::{Context, Result, bail};
use swarm_queue_client::{ClientSecret, QueueConfig};
use swarm_secret_client::queue::{self, ControllerCredential};

/// The environment prefix the nix module's `queueEnv` sets.
const PREFIX: &str = "SWARM_CONTROLLER";

/// Seconds to wait after each failed fetch before the next one. About a minute
/// in all, which rides out a store restarting alongside this daemon. A longer
/// outage fails the start, and the unit's `Restart=` tries again from the top.
const RETRY_DELAYS_S: [u64; 6] = [1, 2, 4, 8, 16, 30];

static IDENTITY: OnceLock<Option<QueueConfig>> = OnceLock::new();

/// Everything in a [`QueueConfig`] except the secret.
#[derive(Debug, PartialEq, Eq)]
struct Coordinates {
    url: String,
    token_endpoint: String,
    client_id: String,
    ca_file: Option<PathBuf>,
}

impl Coordinates {
    fn with_secret(self, secret: String) -> QueueConfig {
        QueueConfig {
            url: self.url,
            token_endpoint: self.token_endpoint,
            client_id: self.client_id,
            client_secret: ClientSecret::Value(secret),
            ca_file: self.ca_file,
        }
    }
}

/// Resolve the identity and hold it for [`get`].
///
/// # Errors
/// A half-set environment, which is a deployment bug and not an absent queue;
/// a secret the store still would not give up after [`RETRY_DELAYS_S`]; or a
/// second call.
pub async fn init() -> Result<()> {
    let identity = match coordinates_from(|name| std::env::var(name).ok())? {
        None => None,
        Some(coordinates) => {
            let delays = RETRY_DELAYS_S.map(Duration::from_secs);
            Some(coordinates.with_secret(fetch_with_retry(&delays, fetch_secret).await?))
        }
    };
    IDENTITY
        .set(identity)
        .map_err(|_| anyhow::anyhow!("the queue identity was initialised twice"))
}

/// The identity, or `None` when this deployment wired no queue up.
pub fn get() -> Option<&'static QueueConfig> {
    IDENTITY.get().and_then(Option::as_ref)
}

/// `<PREFIX>_NATS_URL`, `_OIDC_TOKEN_ENDPOINT` and `_OIDC_CLIENT_ID`, all or
/// none; `_OIDC_CA_FILE` optionally.
fn coordinates_from(var: impl Fn(&str) -> Option<String>) -> Result<Option<Coordinates>> {
    let url = var(&format!("{PREFIX}_NATS_URL"));
    let token_endpoint = var(&format!("{PREFIX}_OIDC_TOKEN_ENDPOINT"));
    let client_id = var(&format!("{PREFIX}_OIDC_CLIENT_ID"));
    let ca_file = var(&format!("{PREFIX}_OIDC_CA_FILE")).map(PathBuf::from);
    match (url, token_endpoint, client_id) {
        (None, None, None) => Ok(None),
        (Some(url), Some(token_endpoint), Some(client_id)) => Ok(Some(Coordinates {
            url,
            token_endpoint,
            client_id,
            ca_file,
        })),
        _ => bail!(
            "swarm queue is half-configured: {PREFIX}_NATS_URL, {PREFIX}_OIDC_TOKEN_ENDPOINT \
             and {PREFIX}_OIDC_CLIENT_ID must be set together or not at all"
        ),
    }
}

/// One login and one read. An empty value is refused like an absent one: it
/// would reach authelia as the secret and come back as `invalid_client`, which
/// names neither the store nor the publisher.
async fn fetch_secret() -> Result<String> {
    let path = queue::controller_client_path()?;
    let store = crate::store::connect()
        .await
        .context("logging in to the swarm secret store")?;
    let stored: Option<ControllerCredential> = store
        .read_optional(&path)
        .await
        .with_context(|| format!("reading {path}"))?;
    match stored {
        Some(c) if !c.value.trim().is_empty() => Ok(c.value.trim().to_owned()),
        _ => bail!(
            "nothing at {path}: swarm-secret-publish, on the host that runs authelia, has not \
             published this daemon's client secret"
        ),
    }
}

/// Run `attempt`, waiting each of `delays` after a failure, then once more.
/// Answers with the first success, or the last attempt's error.
async fn fetch_with_retry<F, Fut>(delays: &[Duration], mut attempt: F) -> Result<String>
where
    F: FnMut() -> Fut,
    Fut: Future<Output = Result<String>>,
{
    for delay in delays {
        match attempt().await {
            Ok(secret) => return Ok(secret),
            Err(e) => {
                tracing::warn!(
                    error = %format!("{e:#}"),
                    retry_in = ?delay,
                    "fetching the queue client secret from the swarm secret store failed"
                );
                tokio::time::sleep(*delay).await;
            }
        }
    }
    attempt().await.with_context(|| {
        format!(
            "fetching the queue client secret from the swarm secret store, after {} attempts",
            delays.len() + 1
        )
    })
}

#[cfg(test)]
mod tests {
    use std::collections::HashMap;

    use super::*;

    fn env(pairs: &[(&str, &str)]) -> impl Fn(&str) -> Option<String> {
        let map: HashMap<String, String> = pairs
            .iter()
            .map(|(k, v)| ((*k).to_owned(), (*v).to_owned()))
            .collect();
        move |name| map.get(name).cloned()
    }

    const FULL: [(&str, &str); 3] = [
        ("SWARM_CONTROLLER_NATS_URL", "tls://nats.example:4222"),
        (
            "SWARM_CONTROLLER_OIDC_TOKEN_ENDPOINT",
            "https://auth.example/api/oidc/token",
        ),
        ("SWARM_CONTROLLER_OIDC_CLIENT_ID", "swarm-controller"),
    ];

    #[test]
    fn no_coordinates_is_no_queue() {
        assert_eq!(
            coordinates_from(env(&[])).expect("absent is not an error"),
            None
        );
    }

    #[test]
    fn a_half_set_environment_is_an_error_naming_the_variables() {
        let err = coordinates_from(env(&FULL[..2])).expect_err("a partial set is not absent");
        assert!(
            format!("{err}").contains("SWARM_CONTROLLER_OIDC_CLIENT_ID"),
            "{err}"
        );
    }

    #[test]
    fn a_full_environment_resolves_with_its_optional_ca() {
        let mut with_ca = FULL.to_vec();
        with_ca.push(("SWARM_CONTROLLER_OIDC_CA_FILE", "/etc/ca.pem"));
        let c = coordinates_from(env(&with_ca))
            .expect("complete")
            .expect("configured");
        assert_eq!(c.client_id, "swarm-controller");
        assert_eq!(c.ca_file, Some(PathBuf::from("/etc/ca.pem")));

        let c = coordinates_from(env(&FULL))
            .expect("complete")
            .expect("configured");
        assert_eq!(c.ca_file, None);
    }

    #[test]
    fn the_fetched_secret_is_held_as_a_value_not_a_path() {
        let cfg = coordinates_from(env(&FULL))
            .expect("complete")
            .expect("configured")
            .with_secret("s3cret".to_owned());
        assert!(
            matches!(&cfg.client_secret, ClientSecret::Value(v) if v == "s3cret"),
            "{:?}",
            cfg.client_secret
        );
    }

    #[tokio::test]
    async fn a_store_that_comes_back_within_the_window_is_waited_for() {
        let mut calls = 0;
        let got = fetch_with_retry(&[Duration::ZERO; 3], || {
            calls += 1;
            let n = calls;
            async move {
                if n < 3 {
                    bail!("store sealed")
                }
                Ok("s3cret".to_owned())
            }
        })
        .await
        .expect("the third attempt succeeds");
        assert_eq!(got, "s3cret");
        assert_eq!(calls, 3);
    }

    #[tokio::test]
    async fn a_store_that_stays_down_fails_after_one_attempt_per_delay_plus_one() {
        let mut calls = 0;
        let err = fetch_with_retry(&[Duration::ZERO; 3], || {
            calls += 1;
            async { bail!("store sealed") }
        })
        .await
        .expect_err("every attempt fails");
        assert_eq!(calls, 4);
        let msg = format!("{err:#}");
        assert!(msg.contains("after 4 attempts"), "{msg}");
        assert!(msg.contains("store sealed"), "{msg}");
    }
}