//! Helpers for retrying a fallible operation according to a backoff schedule.

//!

//! [`Runner::run`] retries the specified operation according to the [`BackoffSchedule`] of the

//! [`Runner`]. Users can customize the backoff behavior by implementing [`BackoffSchedule`].

// TODO: this is a (somewhat) general-purpose utility, so it should probably be factored out of

// tor-hsservice

use std::pin::Pin;

use futures::future::FusedFuture;

use tor_rtcompat::TimeoutError;

use super::*;

/// A runner for a fallible operation, which retries on failure according to a [`BackoffSchedule`].

pub(super) struct Runner<B: BackoffSchedule, R: Runtime> {

    /// A description of the operation we are trying to do.

    doing: String,

    /// The backoff schedule.

    schedule: B,

    /// The runtime.

    runtime: R,

}

impl<B: BackoffSchedule, R: Runtime> Runner<B, R> {

    /// Create a new `Runner`.

    /// Run `fallible_fn`, retrying according to the [`BackoffSchedule`] of this `Runner`.

    ///

    /// If `fallible_fn` eventually returns `Ok(_)`, return that output. Otherwise,

    /// keep retrying until either `fallible_fn` has failed too many times, or until

    /// a fatal error occurs.

    #[allow(clippy::cognitive_complexity)] // TODO: Refactor

        // When this timeout elapses, the `Runner` will stop retrying the fallible operation.

        //

        // A `overall_timeout` of `None` means there is no time limit for the retries.

        }

        loop {

            // Bail if we've exceeded the number of allowed retries.

            {

                () = overall_timeout => {

                    // The timeout has elapsed, so stop retrying and return the errors

                    // accumulated so far.

                }

                    // TODO: the error branches in the match below have different error types,

                    // so we must compute should_retry and delay separately, on each branch.

                    //

                    // We could refactor this to extract the error using

                    // let err = match res { ... } and call err.should_retry()

                    // and next_delay() after the match, but this will involve

                    // rethinking the BackoffSchedule trait and/or RetriableError

                    // (currently RetriableError is Clone, so it's not object safe).

                            );

                        }

                        },

                    };

                        };

                        // Introduce the specified delay between retries

                        // Try again unless the entire operation has timed out.

                },

            }

        }

}

/// Wrap a [`Future`] with an optional timeout.

///

/// If `timeout` is `Some`, returns a [`Timeout`](tor_rtcompat::Timeout)

/// that resolves to the value of `future` if the future completes within `timeout`,

/// or a [`TimeoutError`] if it does not.

/// If `timeout` is `None`, returns a new future which maps the specified `future`'s

/// output type to a `Result::Ok`.

    }

/// A trait that specifies the parameters for retrying a fallible operation.

pub(super) trait BackoffSchedule {

    /// The maximum number of retries.

    ///

    /// A return value of `None` indicates is no upper limit for the number of retries, and that

    /// the operation should be retried until [`BackoffSchedule::overall_timeout`] time elapses (or

    /// indefinitely, if [`BackoffSchedule::overall_timeout`] returns `None`).

    fn max_retries(&self) -> Option<usize>;

    /// The total amount of time allowed for the retriable operation.

    ///

    /// A return value of `None` indicates the operation should be retried until

    /// [`BackoffSchedule::max_retries`] number of retries are exceeded (or indefinitely, if

    /// [`BackoffSchedule::max_retries`] returns `None`).

    fn overall_timeout(&self) -> Option<Duration>;

    /// The total amount of time allowed for a single operation.

    fn single_attempt_timeout(&self) -> Option<Duration>;

    /// Return the delay to introduce before the next retry.

    ///

    /// The `error` parameter contains the error returned by the fallible operation. This enables

    /// implementors to (optionally) implement adaptive backoff. For example, if the operation is

    /// sending an HTTP request, and the error is a 429 (Too Many Requests) HTTP response with a

    /// `Retry-After` header, the implementor can implement a backoff schedule where the next retry

    /// is delayed by the value specified in the `Retry-After` header.

    fn next_delay<E: RetriableError>(&mut self, error: &E) -> Option<Duration>;

}

/// The type of error encountered while running a fallible operation.

#[derive(Clone, Debug, thiserror::Error)]

pub(super) enum BackoffError<E> {

    /// A fatal (non-transient) error occurred.

    #[error("A fatal (non-transient) error occurred")]

    FatalError(RetryError<E>),

    /// Ran out of retries.

    #[error("Ran out of retries")]

    MaxRetryCountExceeded(RetryError<E>),

    /// Exceeded the maximum allowed time.

    #[error("Timeout exceeded")]

    Timeout(RetryError<E>),

    /// The [`BackoffSchedule`] told us to stop retrying.

    #[error("Stopped retrying as requested by BackoffSchedule")]

    ExplicitStop(RetryError<E>),

}

impl<E> From<BackoffError<E>> for RetryError<E> {

}

/// A trait for representing retriable errors.

pub(super) trait RetriableError: StdError + Clone {

    /// Whether this error is transient.

    fn should_retry(&self) -> bool;

}

impl RetriableError for TimeoutError {

}

#[cfg(test)]

mod tests {

    // @@ begin test lint list maintained by maint/add_warning @@

    #![allow(clippy::bool_assert_comparison)]

    #![allow(clippy::clone_on_copy)]

    #![allow(clippy::dbg_macro)]

    #![allow(clippy::mixed_attributes_style)]

    #![allow(clippy::print_stderr)]

    #![allow(clippy::print_stdout)]

    #![allow(clippy::single_char_pattern)]

    #![allow(clippy::unwrap_used)]

    #![allow(clippy::unchecked_duration_subtraction)]

    #![allow(clippy::useless_vec)]

    #![allow(clippy::needless_pass_by_value)]

    //! <!-- @@ end test lint list maintained by maint/add_warning @@ -->

    use super::*;

    use std::sync::Arc;

    use std::iter;

    use std::sync::RwLock;

    use oneshot_fused_workaround as oneshot;

    use tor_rtcompat::{SleepProvider, ToplevelBlockOn};

    use tor_rtmock::MockRuntime;

    const SHORT_DELAY: Duration = Duration::from_millis(10);

    const TIMEOUT: Duration = Duration::from_millis(100);

    const SINGLE_TIMEOUT: Duration = Duration::from_millis(50);

    const MAX_RETRIES: usize = 5;

    macro_rules! impl_backoff_sched {

        ($name:ty, $max_retries:expr, $timeout:expr, $single_timeout:expr, $next_delay:expr) => {

            impl BackoffSchedule for $name {

                fn max_retries(&self) -> Option<usize> {

                    $max_retries

                }

                fn overall_timeout(&self) -> Option<Duration> {

                    $timeout

                }

                fn single_attempt_timeout(&self) -> Option<Duration> {

                    $single_timeout

                }

                #[allow(unused_variables)]

                fn next_delay<E: RetriableError>(&mut self, error: &E) -> Option<Duration> {

                    $next_delay

                }

            }

        };

    }

    struct BackoffWithMaxRetries;

    impl_backoff_sched!(

        BackoffWithMaxRetries,

        Some(MAX_RETRIES),

        None,

        None,

        Some(SHORT_DELAY)

    );

    struct BackoffWithTimeout;

    impl_backoff_sched!(

        BackoffWithTimeout,

        None,

        Some(TIMEOUT),

        None,

        Some(SHORT_DELAY)

    );

    struct BackoffWithSingleTimeout;

    impl_backoff_sched!(

        BackoffWithSingleTimeout,

        Some(MAX_RETRIES),

        None,

        Some(SINGLE_TIMEOUT),

        Some(SHORT_DELAY)

    );

    /// A potentially retriable error.

    #[derive(Debug, Copy, Clone, thiserror::Error)]

    enum TestError {

        /// A fatal error

        #[error("A fatal test error")]

        Fatal,

        /// A transient error

        #[error("A transient test error")]

        Transient,

    }

    impl RetriableError for TestError {

        fn should_retry(&self) -> bool {

            match self {

                Self::Fatal => false,

                Self::Transient => true,

            }

        }

    }

    /// Run a single [`Runner`] test.

    fn run_test<E: RetriableError + Send + Sync + 'static>(

        sleep_for: Option<Duration>,

        schedule: impl BackoffSchedule + Send + 'static,

        errors: impl Iterator<Item = E> + Send + Sync + 'static,

        expected_run_count: usize,

        description: &'static str,

        expected_duration: Duration,

    ) {

        let runtime = MockRuntime::new();

        runtime.clone().block_on(async move {

            let runner = Runner {

                doing: description.into(),

                schedule,

                runtime: runtime.clone(),

            };

            let retry_count = Arc::new(RwLock::new(0));

            let (tx, rx) = oneshot::channel();

            let start = runtime.now();

            runtime

                .mock_task()

                .spawn_identified(format!("retry runner task: {description}"), {

                    let retry_count = Arc::clone(&retry_count);

                    let errors = Arc::new(RwLock::new(errors));

                    let runtime = runtime.clone();

                    async move {

                        if let Ok(()) = runner

                            .run(|| async {

                                *retry_count.write().unwrap() += 1;

                                if let Some(dur) = sleep_for {

                                    runtime.sleep(dur).await;

                                }

                                Err::<(), _>(errors.write().unwrap().next().unwrap())

                            })

                            .await

                        {

                            unreachable!();

                        }

                        let () = tx.send(()).unwrap();

                    }

                });

            // The expected retry count may be unknown (for example, if we set a timeout but no

            // upper limit for the number of retries, it's impossible to tell exactly how many

            // times the operation will be retried)

            for i in 1..=expected_run_count {

                runtime.mock_task().progress_until_stalled().await;

                // If our fallible_op is sleeping, advance the time until after it times out or

                // finishes sleeping.

                if let Some(sleep_for) = sleep_for {

                    runtime

                        .mock_sleep()

                        .advance(std::cmp::min(SINGLE_TIMEOUT, sleep_for));

                }

                runtime.mock_task().progress_until_stalled().await;

                runtime.mock_sleep().advance(SHORT_DELAY);

                assert_eq!(*retry_count.read().unwrap(), i);

            }

            let () = rx.await.unwrap();

            let end = runtime.now();

            assert_eq!(*retry_count.read().unwrap(), expected_run_count);

            assert!(duration_close_to(end - start, expected_duration));

        });

    }

    /// Return true if d1 is in range [d2...d2 + 0.01sec]

    ///

    /// TODO: lifted from tor-circmgr

    fn duration_close_to(d1: Duration, d2: Duration) -> bool {

        d1 >= d2 && d1 <= d2 + SHORT_DELAY

    }

    #[test]

    fn max_retries() {

        run_test(

            None,

            BackoffWithMaxRetries,

            iter::repeat(TestError::Transient),

            MAX_RETRIES,

            "backoff with max_retries and no timeout (transient errors)",

            Duration::from_millis(SHORT_DELAY.as_millis() as u64 * MAX_RETRIES as u64),

        );

    }

    #[test]

    fn max_retries_fatal() {

        use TestError::*;

        /// The number of transient errors that happen before the final, fatal error.

        const RETRIES_UNTIL_FATAL: usize = 3;

        /// The total number of times we exoect the fallible function to be called.

        /// The first RETRIES_UNTIL_FATAL times, a transient error is returned.

        /// The last call corresponds to the fatal error

        const EXPECTED_TOTAL_RUNS: usize = RETRIES_UNTIL_FATAL + 1;

        run_test(

            None,

            BackoffWithMaxRetries,

            std::iter::repeat_n(Transient, RETRIES_UNTIL_FATAL)

                .chain([Fatal])

                .chain(iter::repeat(Transient)),

            EXPECTED_TOTAL_RUNS,

            "backoff with max_retries and no timeout (transient errors followed by a fatal error)",

            Duration::from_millis(SHORT_DELAY.as_millis() as u64 * EXPECTED_TOTAL_RUNS as u64),

        );

    }

    #[test]

    fn timeout() {

        use TestError::*;

        let expected_run_count = TIMEOUT.as_millis() / SHORT_DELAY.as_millis();

        run_test(

            None,

            BackoffWithTimeout,

            iter::repeat(Transient),

            expected_run_count as usize,

            "backoff with timeout and no max_retries (transient errors)",

            TIMEOUT,

        );

    }

    #[test]

    fn single_timeout() {

        use TestError::*;

        // Each attempt will time out after SINGLE_TIMEOUT time units,

        // and the backoff runner sleeps for SLEEP_DELAY units in between retries

        let expected_duration = Duration::from_millis(

            (SHORT_DELAY.as_millis() + SINGLE_TIMEOUT.as_millis()) as u64 * MAX_RETRIES as u64,

        );

        run_test(

            // Sleep for more than SINGLE_TIMEOUT units

            // to trigger the single_attempt_timeout() timeout

            Some(SINGLE_TIMEOUT * 2),

            BackoffWithSingleTimeout,

            iter::repeat(Transient),

            MAX_RETRIES,

            "backoff with single timeout and max_retries and no overall timeout",

            expected_duration,

        );

    }

    // TODO (#1120): needs tests for the remaining corner cases

}