harmos-domain 0.1.4

/* ***
 *
 * Harmos
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * ***
 */

/* === Modules === */

/* === Re-Exports === */

/* === Imports === */

// Standard Library Imports
use std::time::Duration;
// Third Party Imports
use serde::{Deserialize, Serialize};
// Local Imports

/* === Constants === */

/* === Macros === */

/* === Type Aliases === */

/* === Structs === */

// The request half (`timeout`, `attempt_timeout`, `retries`) bounds one
// request-scoped invocation. The resident half (`restart`) governs observers
// and services, which run until stopped: for them `timeout` is the stop
// deadline and request deadlines never apply.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct OperationPolicy {
    pub timeout: Duration,
    pub attempt_timeout: Duration,
    pub retries: RetryPolicy,
    pub restart: RestartPolicy,
}

/* === Enums === */

#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case", deny_unknown_fields)]
pub enum RetryPolicy {
    Never,
    OnTransient { max_retries: usize, backoff: Backoff },
}

// Environmental faults of a resident spend restart credit; a healthy run of
// `sustained_health` refills it. `Never` deactivates on the first fault.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case", deny_unknown_fields)]
pub enum RestartPolicy {
    Never,
    OnTransient { credit: usize, sustained_health: Duration, backoff: Backoff },
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case", deny_unknown_fields)]
pub enum Backoff {
    Fixed { delay: Duration, jitter: Jitter },
    Exponential { initial: Duration, maximum: Duration, jitter: Jitter },
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case", deny_unknown_fields)]
pub enum Jitter {
    None,
    Deterministic,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum FailureKind {
    Permanent,
    Transient,
    Ambiguous,
    Unauthorized,
    Invalid,
    Cancelled,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case", deny_unknown_fields)]
pub enum ReplaySafety {
    Unknown,
    Idempotent,
    Deduplicated,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, thiserror::Error)]
pub enum PolicyError {
    #[error("operation deadlines must be positive (overall {timeout:?}, attempt {attempt_timeout:?})")]
    Deadline { timeout: Duration, attempt_timeout: Duration },

    #[error("a retry policy needs a positive retry count; use Never for zero")]
    EmptyRetries,

    #[error("a restart policy needs positive credit and sustained health; use Never for zero")]
    EmptyRestarts,

    #[error("retry delays must be positive and ordered (initial {initial:?}, maximum {maximum:?})")]
    Backoff { initial: Duration, maximum: Duration },
}

/* === Traits === */

/* === Implementations === */

impl OperationPolicy {
    pub fn validate(self) -> Result<(), PolicyError> {
        if self.timeout.is_zero() || self.attempt_timeout.is_zero() {
            return Err(PolicyError::Deadline { timeout: self.timeout, attempt_timeout: self.attempt_timeout });
        }

        self.retries.validate()?;
        self.restart.validate()
    }
}

impl RestartPolicy {
    pub fn validate(self) -> Result<(), PolicyError> {
        match self {
            Self::Never => Ok(()),
            Self::OnTransient { credit, sustained_health, .. } if credit == 0 || sustained_health.is_zero() => {
                Err(PolicyError::EmptyRestarts)
            }
            Self::OnTransient { backoff, .. } => backoff.validate(),
        }
    }

    pub const fn credit(self) -> usize {
        match self {
            Self::Never => 0,
            Self::OnTransient { credit, .. } => credit,
        }
    }
}

impl RetryPolicy {
    pub fn validate(self) -> Result<(), PolicyError> {
        match self {
            Self::Never => Ok(()),
            Self::OnTransient { max_retries: 0, .. } => Err(PolicyError::EmptyRetries),
            Self::OnTransient { backoff, .. } => backoff.validate(),
        }
    }

    pub fn retry_after(self, failed_attempt: usize, failure: FailureKind, replay: ReplaySafety, entropy: u64) -> Option<Duration> {
        let Self::OnTransient { max_retries, backoff } = self else {
            return None;
        };
        if self.validate().is_err() || failed_attempt >= max_retries {
            return None;
        }
        if !matches!(failure, FailureKind::Transient) && !(failure == FailureKind::Ambiguous && replay != ReplaySafety::Unknown) {
            return None;
        }

        backoff.delay(failed_attempt, entropy).ok()
    }
}

impl Backoff {
    pub fn validate(self) -> Result<(), PolicyError> {
        let (initial, maximum) = match self {
            Self::Fixed { delay, .. } => (delay, delay),
            Self::Exponential { initial, maximum, .. } => (initial, maximum),
        };
        if initial.is_zero() || maximum < initial {
            return Err(PolicyError::Backoff { initial, maximum });
        }

        Ok(())
    }

    pub fn delay(self, failed_attempt: usize, entropy: u64) -> Result<Duration, PolicyError> {
        self.validate()?;

        let (nominal, maximum, jitter) = match self {
            Self::Fixed { delay, jitter } => (delay, delay.saturating_add(from_nanos(delay.as_nanos() / 4)), jitter),
            Self::Exponential { initial, maximum, jitter } => {
                let shift = u32::try_from(failed_attempt).unwrap_or(u32::MAX);
                let nanos = if shift >= 128 || initial.as_nanos() > (u128::MAX >> shift) {
                    maximum.as_nanos()
                } else {
                    (initial.as_nanos() << shift).min(maximum.as_nanos())
                };

                (from_nanos(nanos), maximum, jitter)
            }
        };
        if jitter == Jitter::None {
            return Ok(nominal.min(maximum));
        }
        let mixed = u128::from(entropy)
            .wrapping_mul(0x9e37_79b9_7f4a_7c15)
            .wrapping_add((failed_attempt as u128).wrapping_mul(17).wrapping_add(11));
        let nanos = mixed % (nominal.as_nanos() / 4 + 1);

        Ok(nominal.saturating_add(from_nanos(nanos)).min(maximum))
    }
}

/* === Public Functions === */

/* === Private Functions === */

// Every caller bounds nanos by a representable Duration before this conversion.
fn from_nanos(nanos: u128) -> Duration {
    Duration::new((nanos / 1_000_000_000) as u64, (nanos % 1_000_000_000) as u32)
}

/* === Tests === */