//! The first read of a client secret, retried through a store that is
//! restarting alongside the service reading it.
//!
//! Every service that logs in to the queue reads its secret once at start.
//! The store may still be coming up, so a failed read waits and tries again;
//! a store down for longer than the schedule fails the start, and the unit's
//! `Restart=` tries again.

use std::fmt::Display;
use std::future::Future;
use std::time::Duration;

use backon::{ExponentialBuilder, Retryable as _};

/// The wait after the first failed read.
const FIRST_RETRY: Duration = Duration::from_secs(1);

/// The longest wait between two reads.
const MAX_RETRY: Duration = Duration::from_secs(30);

/// Reads after the first one: waits of 1, 2, 4, 8, 16 and 30 seconds.
const RETRIES: usize = 6;

/// Run `read`, waiting after each failure up to six times, and answer
/// with the first success or the last attempt's error. Each failure is logged
/// as a warning naming `what`.
///
/// # Errors
/// The error of the final attempt, once every read has failed.
pub async fn read_with_startup_retry<T, E, F, Fut>(what: &str, read: F) -> Result<T, E>
where
    F: FnMut() -> Fut,
    Fut: Future<Output = Result<T, E>>,
    E: Display,
{
    read.retry(
        ExponentialBuilder::default()
            .with_min_delay(FIRST_RETRY)
            .with_max_delay(MAX_RETRY)
            .with_max_times(RETRIES),
    )
    .notify(|e: &E, wait| {
        tracing::warn!(error = %format!("{e:#}"), retry_in = ?wait, "{what} failed");
    })
    .await
}

#[cfg(test)]
mod tests {
    use std::sync::atomic::{AtomicUsize, Ordering};

    use tokio::time::Instant;

    use super::*;

    /// A store that fails `failures` times and then answers.
    fn read_after(
        failures: usize,
        calls: &AtomicUsize,
    ) -> std::future::Ready<Result<&'static str, String>> {
        std::future::ready(if calls.fetch_add(1, Ordering::SeqCst) < failures {
            Err("store is down".to_owned())
        } else {
            Ok("secret")
        })
    }

    #[tokio::test(start_paused = true)]
    async fn a_store_that_answers_at_once_costs_no_wait() {
        let calls = AtomicUsize::new(0);
        let started = Instant::now();
        let got = read_with_startup_retry("reading", || read_after(0, &calls)).await;
        assert_eq!(got, Ok("secret"));
        assert_eq!(calls.load(Ordering::SeqCst), 1);
        assert_eq!(started.elapsed(), Duration::ZERO);
    }

    #[tokio::test(start_paused = true)]
    async fn a_store_that_comes_up_late_is_waited_for() {
        let calls = AtomicUsize::new(0);
        let started = Instant::now();
        let got = read_with_startup_retry("reading", || read_after(3, &calls)).await;
        assert_eq!(got, Ok("secret"));
        assert_eq!(calls.load(Ordering::SeqCst), 4);
        assert_eq!(started.elapsed(), Duration::from_secs(1 + 2 + 4));
    }

    #[tokio::test(start_paused = true)]
    async fn a_store_that_stays_down_fails_after_the_whole_schedule() {
        let calls = AtomicUsize::new(0);
        let started = Instant::now();
        let got = read_with_startup_retry("reading", || read_after(usize::MAX, &calls)).await;
        assert_eq!(got, Err("store is down".to_owned()));
        assert_eq!(calls.load(Ordering::SeqCst), RETRIES + 1);
        assert_eq!(
            started.elapsed(),
            Duration::from_secs(1 + 2 + 4 + 8 + 16 + 30)
        );
    }
}