pacta_executor/

lib.rs

//! Pacta-native execution abstractions.

#![forbid(unsafe_code)]
#![warn(missing_docs)]

use pacta_contract::Pact;

pub use pacta_contract::{Outcome, Settlement};

/// A single attempt to fulfill a claimed pact.
///
/// This is the executor's designated input seam. It is `#[non_exhaustive]` so it can
/// gain execution-context fields in a later minor release without a breaking change;
/// construct it through [`Execution::new`].
#[derive(Debug, Clone)]
#[non_exhaustive]
pub struct Execution {
    /// Pact being executed.
    pub pact: Pact,
}

impl Execution {
    /// Build an execution from a claimed pact.
    #[must_use]
    pub fn new(pact: Pact) -> Self {
        Self { pact }
    }
}

/// Public role responsible for executing claimed pacts through middleware.
pub trait Executor {
    /// Error returned when the execution infrastructure fails.
    type Error: std::error::Error;

    /// Execute a claimed pact.
    fn execute(&mut self, execution: Execution) -> Result<Outcome, Self::Error>;
}

/// The decision a [`Policy`] renders for one infrastructure failure.
///
/// This is a closed, two-variant decision (deliberately not `#[non_exhaustive]`, unlike the
/// growing kernel protocol enums): it is the complete answer to one question — keep lapsing,
/// or concede — not a protocol that accretes new cases over time.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Verdict {
    /// Propagate the error so the claim is left unsettled: it lapses and may be reclaimed
    /// and attempted again by a later step.
    Continue,
    /// Give up: settle the claim as breached now instead of letting it lapse again.
    Concede,
}

/// A user-obligation trait deciding, after a claimed pact's execution fails with an
/// infrastructure error, whether to keep letting the claim lapse (to be reclaimed and
/// attempted again) or concede as a terminal breach.
///
/// This governs only infrastructure failures — an [`Executor::Error`] the
/// shipped `Driver` would otherwise leave unsettled to lapse and be reclaimed indefinitely.
/// It has no bearing on a clean business [`Outcome`]: the shipped `Driver` always settles a
/// clean `Outcome::Breached` as terminal, so there is no "retry a business breach" decision
/// for a `Policy` to make (see `BACKLOG.md`'s `lifecycle-persistence` entry: attempt limits
/// stay outside the registry, as user-owned policy — this trait is that policy's seam).
pub trait Policy<E> {
    /// Decide the verdict for the `attempts`-th consecutive infrastructure failure (counting
    /// the current one) with `error`.
    fn decide(&self, attempts: u32, error: &E) -> Verdict;
}

/// A Pacta-native decorator over execution: the Tower `Layer` analog. Because `wrap`
/// takes an `Executor` and returns an `Executor`, middleware compose arbitrarily
/// (the closure property), which is how orchestration is composed onto the seam.
pub trait Middleware<E> {
    /// The wrapped executor type.
    type Executor: Executor;

    /// Wrap an executor with this middleware.
    fn wrap(&self, executor: E) -> Self::Executor;
}

/// The no-op middleware: `wrap` returns the executor unchanged. `Identity` is the
/// neutral element of composition — the empty stack — so "zero middleware" is a
/// first-class, holdable value rather than an absence.
#[derive(Debug, Default, Clone, Copy)]
pub struct Identity;

impl<E: Executor> Middleware<E> for Identity {
    type Executor = E;

    fn wrap(&self, executor: E) -> Self::Executor {
        executor
    }
}

/// Two middleware composed into one, reifying the closure property as a value: because
/// `Stack` is itself a [`Middleware`], a composed stack can be named, stored, and passed
/// as one middleware *before* an executor exists. `outer` wraps the result of `inner`,
/// so `outer` is applied last and therefore observes each execution first.
#[derive(Debug, Default, Clone, Copy)]
pub struct Stack<Inner, Outer> {
    inner: Inner,
    outer: Outer,
}

impl<Inner, Outer> Stack<Inner, Outer> {
    /// Compose `inner` and `outer` into one middleware. `outer` wraps `inner`'s result,
    /// so `outer` observes each execution first.
    #[must_use]
    pub const fn new(inner: Inner, outer: Outer) -> Self {
        Self { inner, outer }
    }
}

impl<E, Inner, Outer> Middleware<E> for Stack<Inner, Outer>
where
    E: Executor,
    Inner: Middleware<E>,
    Outer: Middleware<Inner::Executor>,
{
    type Executor = Outer::Executor;

    fn wrap(&self, executor: E) -> Self::Executor {
        self.outer.wrap(self.inner.wrap(executor))
    }
}

/// A blind, ordered assembly of middleware composed over [`Identity`]. It is itself a
/// [`Middleware`], so applying it is just `wrap`.
///
/// It is deliberately *blind*: [`Composition::then`] accepts any `Middleware` through a
/// single generic operation and inspects nothing, and the type offers no method named for
/// an orchestration policy (retry, timeout, backoff, circuit, quota, rate-limit) — that
/// policy is a consumer or sibling concern, never a core convenience.
///
/// # Order
///
/// Middleware are applied outermost-first in the order added: the **first** middleware
/// added with [`then`](Composition::then) is the outermost and observes each execution
/// **first**; the executor is innermost. This mirrors the assembly convention of the
/// prior art the mechanism is distilled from.
#[derive(Debug, Default, Clone, Copy)]
pub struct Composition<M> {
    middleware: M,
}

impl Composition<Identity> {
    /// Start an empty composition over [`Identity`].
    #[must_use]
    pub const fn new() -> Self {
        Self {
            middleware: Identity,
        }
    }
}

impl<M> Composition<M> {
    /// Add a middleware to the composition. Blind: it accepts any `Middleware` and
    /// inspects nothing. The first middleware added is outermost (observes each
    /// execution first); the executor is innermost.
    #[must_use]
    pub fn then<N>(self, next: N) -> Composition<Stack<N, M>> {
        Composition {
            middleware: Stack::new(next, self.middleware),
        }
    }
}

impl<E, M> Middleware<E> for Composition<M>
where
    E: Executor,
    M: Middleware<E>,
{
    type Executor = M::Executor;

    fn wrap(&self, executor: E) -> Self::Executor {
        self.middleware.wrap(executor)
    }
}

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

    use pacta_contract::Uuid;

    use super::*;

    #[derive(Debug)]
    struct DummyError;

    impl std::fmt::Display for DummyError {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            write!(f, "dummy error")
        }
    }

    impl std::error::Error for DummyError {}

    struct DummyExecutor;
    impl Executor for DummyExecutor {
        type Error = DummyError;
        fn execute(&mut self, _execution: Execution) -> Result<Outcome, Self::Error> {
            Ok(Outcome::Fulfilled)
        }
    }

    struct IdentityExecutor<E> {
        inner: E,
    }
    impl<E: Executor> Executor for IdentityExecutor<E> {
        type Error = E::Error;
        fn execute(&mut self, execution: Execution) -> Result<Outcome, Self::Error> {
            self.inner.execute(execution)
        }
    }

    struct IdentityMiddleware;
    impl<E: Executor> Middleware<E> for IdentityMiddleware {
        type Executor = IdentityExecutor<E>;
        fn wrap(&self, executor: E) -> Self::Executor {
            IdentityExecutor { inner: executor }
        }
    }

    struct BreachExecutor<E> {
        _inner: E,
    }
    impl<E: Executor> Executor for BreachExecutor<E> {
        type Error = E::Error;
        fn execute(&mut self, _execution: Execution) -> Result<Outcome, Self::Error> {
            Ok(Outcome::Breached)
        }
    }

    struct BreachMiddleware;
    impl<E: Executor> Middleware<E> for BreachMiddleware {
        type Executor = BreachExecutor<E>;
        fn wrap(&self, executor: E) -> Self::Executor {
            BreachExecutor { _inner: executor }
        }
    }

    fn dummy_execution() -> Execution {
        Execution::new(Pact::new(
            Default::default(),
            "dummy_docket".to_string(),
            "dummy_kind".to_string(),
            vec![],
        ))
    }

    #[test]
    fn identity_middleware_preserves_fulfilled() {
        let middleware = IdentityMiddleware;
        let mut executor = middleware.wrap(DummyExecutor);
        let outcome = executor.execute(dummy_execution()).unwrap();
        assert_eq!(outcome, Outcome::Fulfilled);
    }

    #[test]
    fn breach_middleware_alters_outcome() {
        let middleware = BreachMiddleware;
        let mut executor = middleware.wrap(DummyExecutor);
        let outcome = executor.execute(dummy_execution()).unwrap();
        assert_eq!(outcome, Outcome::Breached);
    }

    #[test]
    fn stacked_middleware_composes() {
        // The closure property: `Middleware` wraps `Executor` into `Executor`, so
        // two middleware stack and still yield a working executor. This proves the
        // "compose the rest" seam holds beyond a single wrap, and guards the
        // `Middleware<E>` generic shape from regressing so it can no longer stack.
        let inner = IdentityMiddleware.wrap(DummyExecutor);
        let mut stacked = IdentityMiddleware.wrap(inner);
        let outcome = stacked.execute(dummy_execution()).unwrap();
        assert_eq!(outcome, Outcome::Fulfilled);
    }

    #[test]
    fn stacked_middleware_preserves_ordering() {
        // A breach layer wrapping an identity layer over a fulfilling executor still
        // composes to a working executor whose outcome is the outermost layer's.
        let inner = IdentityMiddleware.wrap(DummyExecutor);
        let mut stacked = BreachMiddleware.wrap(inner);
        let outcome = stacked.execute(dummy_execution()).unwrap();
        assert_eq!(outcome, Outcome::Breached);
    }

    #[test]
    fn identity_wrap_returns_the_executor_unchanged() {
        // Identity is the neutral element: wrapping adds nothing.
        let mut executor = Identity.wrap(DummyExecutor);
        assert_eq!(
            executor.execute(dummy_execution()).unwrap(),
            Outcome::Fulfilled
        );
    }

    #[test]
    fn stack_is_itself_a_middleware() {
        // Stack reifies the closure property as a value: a composed pair is one Middleware
        // that wraps an executor into an executor just like a single middleware does.
        let stack = Stack::new(IdentityMiddleware, BreachMiddleware);
        let mut executor = stack.wrap(DummyExecutor);
        // BreachMiddleware is `outer`, so it is applied last and observes execution first.
        assert_eq!(
            executor.execute(dummy_execution()).unwrap(),
            Outcome::Breached
        );
    }

    #[test]
    fn composition_assembles_and_drives_to_a_settlement() {
        // The blind assembler composes two pass-through middleware over Identity and
        // drives to a settlement — the reified mechanism proven to compose.
        let composed = Composition::new()
            .then(IdentityMiddleware)
            .then(IdentityMiddleware);
        let mut executor = composed.wrap(DummyExecutor);
        assert_eq!(
            executor.execute(dummy_execution()).unwrap(),
            Outcome::Fulfilled
        );
    }

    #[test]
    fn composition_orders_first_added_outermost() {
        use std::cell::RefCell;
        use std::rc::Rc;

        // A middleware that records both when its layer is *entered* (before delegating inward) and
        // *exited* (after the inner execution returns), so the full nesting is observable — not just
        // the final outcome. A regression that inverted the nesting but kept the outcome would still
        // be caught by comparing the whole trace.
        struct RecordingExecutor<E> {
            inner: E,
            label: &'static str,
            log: Rc<RefCell<Vec<String>>>,
        }
        impl<E: Executor> Executor for RecordingExecutor<E> {
            type Error = E::Error;
            fn execute(&mut self, execution: Execution) -> Result<Outcome, Self::Error> {
                self.log.borrow_mut().push(format!("{}:enter", self.label));
                let outcome = self.inner.execute(execution);
                self.log.borrow_mut().push(format!("{}:exit", self.label));
                outcome
            }
        }
        struct Recorder {
            label: &'static str,
            log: Rc<RefCell<Vec<String>>>,
        }
        impl<E: Executor> Middleware<E> for Recorder {
            type Executor = RecordingExecutor<E>;
            fn wrap(&self, inner: E) -> Self::Executor {
                RecordingExecutor {
                    inner,
                    label: self.label,
                    log: Rc::clone(&self.log),
                }
            }
        }

        // The innermost executor records that it ran, so the trace shows the executor is innermost.
        struct RecordingInner {
            log: Rc<RefCell<Vec<String>>>,
        }
        impl Executor for RecordingInner {
            type Error = DummyError;
            fn execute(&mut self, _execution: Execution) -> Result<Outcome, Self::Error> {
                self.log.borrow_mut().push("executor".to_string());
                Ok(Outcome::Fulfilled)
            }
        }

        let log: Rc<RefCell<Vec<String>>> = Rc::new(RefCell::new(Vec::new()));
        let composed = Composition::new()
            .then(Recorder {
                label: "first",
                log: Rc::clone(&log),
            })
            .then(Recorder {
                label: "second",
                log: Rc::clone(&log),
            });
        let mut executor = composed.wrap(RecordingInner {
            log: Rc::clone(&log),
        });
        executor.execute(dummy_execution()).unwrap();

        // Full trace: the first-added middleware is outermost — entered first, exited last; the
        // second is nested within it; the executor is innermost.
        assert_eq!(
            *log.borrow(),
            vec![
                "first:enter",
                "second:enter",
                "executor",
                "second:exit",
                "first:exit",
            ]
        );
    }

    // In-workspace reference validator for `Policy`, per `composition-governance`'s "trait's
    // in-workspace validator may be test-only scaffolding" scenario: never public API.

    #[derive(Clone, Copy)]
    struct FixedThreshold {
        threshold: u32,
    }

    impl Policy<DummyError> for FixedThreshold {
        fn decide(&self, attempts: u32, _error: &DummyError) -> Verdict {
            if attempts >= self.threshold {
                Verdict::Concede
            } else {
                Verdict::Continue
            }
        }
    }

    struct FailingExecutor;
    impl Executor for FailingExecutor {
        type Error = DummyError;
        fn execute(&mut self, _execution: Execution) -> Result<Outcome, Self::Error> {
            Err(DummyError)
        }
    }

    struct GiveUpExecutor<Inner, P> {
        inner: Inner,
        policy: P,
        attempts: HashMap<Uuid, u32>,
    }

    impl<Inner, P> Executor for GiveUpExecutor<Inner, P>
    where
        Inner: Executor,
        P: Policy<Inner::Error>,
    {
        type Error = Inner::Error;

        fn execute(&mut self, execution: Execution) -> Result<Outcome, Self::Error> {
            let id = execution.pact.id;
            match self.inner.execute(execution) {
                Ok(outcome) => {
                    self.attempts.remove(&id);
                    Ok(outcome)
                }
                Err(error) => {
                    let attempts = self.attempts.entry(id).or_insert(0);
                    *attempts += 1;
                    match self.policy.decide(*attempts, &error) {
                        Verdict::Continue => Err(error),
                        Verdict::Concede => Ok(Outcome::Breached),
                    }
                }
            }
        }
    }

    struct GiveUp<P> {
        policy: P,
    }

    impl<E, P> Middleware<E> for GiveUp<P>
    where
        E: Executor,
        P: Policy<E::Error> + Clone,
    {
        type Executor = GiveUpExecutor<E, P>;

        fn wrap(&self, executor: E) -> Self::Executor {
            GiveUpExecutor {
                inner: executor,
                policy: self.policy.clone(),
                attempts: HashMap::new(),
            }
        }
    }

    fn execution_with_id(id: Uuid) -> Execution {
        Execution::new(Pact::new(
            id,
            "dummy_docket".to_string(),
            "dummy_kind".to_string(),
            vec![],
        ))
    }

    #[test]
    fn below_threshold_keeps_propagating_the_error() {
        let mut executor = GiveUp {
            policy: FixedThreshold { threshold: 3 },
        }
        .wrap(FailingExecutor);
        let id = Uuid::from_u128(1);

        assert!(executor.execute(execution_with_id(id)).is_err());
        assert!(executor.execute(execution_with_id(id)).is_err());
    }

    #[test]
    fn reaching_threshold_concedes_as_breached() {
        let mut executor = GiveUp {
            policy: FixedThreshold { threshold: 3 },
        }
        .wrap(FailingExecutor);
        let id = Uuid::from_u128(2);

        assert!(executor.execute(execution_with_id(id)).is_err());
        assert!(executor.execute(execution_with_id(id)).is_err());
        assert_eq!(
            executor.execute(execution_with_id(id)).unwrap(),
            Outcome::Breached
        );
    }

    #[test]
    fn a_success_resets_the_failure_count() {
        struct FlakyThenFulfilling {
            fail_next: bool,
        }
        impl Executor for FlakyThenFulfilling {
            type Error = DummyError;
            fn execute(&mut self, _execution: Execution) -> Result<Outcome, Self::Error> {
                if self.fail_next {
                    Err(DummyError)
                } else {
                    Ok(Outcome::Fulfilled)
                }
            }
        }

        let mut executor = GiveUp {
            policy: FixedThreshold { threshold: 2 },
        }
        .wrap(FlakyThenFulfilling { fail_next: true });
        let id = Uuid::from_u128(3);

        // attempts = 1, below the threshold of 2.
        assert!(executor.execute(execution_with_id(id)).is_err());

        // A success in between resets the count instead of letting it carry toward the
        // threshold.
        executor.inner.fail_next = false;
        assert_eq!(
            executor.execute(execution_with_id(id)).unwrap(),
            Outcome::Fulfilled
        );

        // Failing again starts back at attempts = 1, still below the threshold of 2.
        executor.inner.fail_next = true;
        assert!(executor.execute(execution_with_id(id)).is_err());
    }

    #[test]
    fn distinct_pacts_are_tracked_independently() {
        let mut executor = GiveUp {
            policy: FixedThreshold { threshold: 2 },
        }
        .wrap(FailingExecutor);
        let first = Uuid::from_u128(4);
        let second = Uuid::from_u128(5);

        assert!(executor.execute(execution_with_id(first)).is_err());
        assert!(executor.execute(execution_with_id(second)).is_err());

        // Each pact's own count reaches the threshold independently, not a shared one.
        assert_eq!(
            executor.execute(execution_with_id(first)).unwrap(),
            Outcome::Breached
        );
        assert_eq!(
            executor.execute(execution_with_id(second)).unwrap(),
            Outcome::Breached
        );
    }
}