dial9-core 0.5.1

//! On-trigger pipeline runs.
//!
//! By default the background worker processes sealed trace segments
//! continuously. Wiring a trigger flips the same pipeline into on-demand
//! operation: segments keep accumulating in the ring (memory or disk), and
//! the pipeline only runs when the application explicitly requests a dump.
//!
//! A runtime wires a trigger at build time and reaches it through the ambient
//! handle. See `with_dump_trigger` on the runtime builder for a worked example.
//!
//! [`DumpTrigger::dump_current_data`](crate::dump::DumpTrigger::dump_current_data) and
//! [`DumpTrigger::dump_time_range`](crate::dump::DumpTrigger::dump_time_range) build a
//! [`DumpRun`](crate::dump::DumpRun); the request is dispatched when that run is awaited
//! or dropped, whichever comes first (so `.with_metadata(...)` can mutate it before it
//! is sent). In the common temporary-statement form
//! (`trigger.dump_current_data();`) the run drops at the end of the statement and
//! dispatches right there; if you bind it to a variable, dispatch waits until that
//! binding is awaited or goes out of scope. Awaiting is optional and only retrieves the
//! [`DumpReceipt`](crate::dump::DumpReceipt).
//! Dumps are strictly best-effort: a window wider than what the ring
//! retained captures whatever survived, with no error and no effect on the
//! live stream.
//!
//! # Concurrent dumps
//!
//! Dumps are independent: triggering two at once registers two dumps, each
//! with its own [`DumpId`](crate::dump::DumpId) and (off S3) its own manifest. A segment whose
//! span overlaps both windows is captured by both. There is no coordination
//! by default - this is intentional, so unrelated subsystems can dump
//! without stepping on each other.
//!
//! When a single source fires repeatedly (a watcher that re-trips every
//! poll, a hot path that dumps on every slow request), configure
//! [`DumpTriggerConfig::debounce`](crate::dump::DumpTriggerConfig::debounce) to coalesce a burst
//! into one dump: triggers
//! within the debounce window after a dump dispatched resolve
//! [`DumpError::Coalesced`](crate::dump::DumpError::Coalesced), naming the dump they folded
//! into instead of
//! starting a new one. The gate lives on the trigger stored on the recorder,
//! so every [`dump_trigger`](crate::handle::Dial9Handle::dump_trigger)
//! clone shares it. (A *cooldown* that rejects extra triggers outright,
//! rather than folding them, is a possible future addition.)

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::{Duration, Instant, SystemTime};

use tokio::sync::{mpsc, oneshot};

use crate::pipeline::ProcessErrorKind;
use crate::primitives::sync::Arc;

/// Mint a dump trigger + receiver pair. The builder wires the receiver into
/// the worker and stashes the trigger in the recorder so it can be reached via
/// [`Dial9Handle::dump_trigger`](crate::handle::Dial9Handle::dump_trigger).
pub fn channel() -> (DumpTrigger, DumpRx) {
    let (tx, rx) = mpsc::unbounded_channel();
    (DumpTrigger { tx, debounce: None }, DumpRx { rx })
}

/// On-demand dump configuration, passed to
/// `with_dump_trigger`.
///
/// Flips the worker from continuous processing into on-demand operation:
/// segments keep accumulating in the ring and the pipeline only runs when the
/// application requests a dump. Configure coalescing with
/// [`debounce`](Self::debounce); the resulting [`DumpTrigger`] is then reached
/// through any [`Dial9Handle`](crate::handle::Dial9Handle) for the runtime
/// via [`dump_trigger`](crate::handle::Dial9Handle::dump_trigger).
#[derive(Debug, Default, Clone)]
pub struct DumpTriggerConfig {
    debounce: Option<Duration>,
}

impl DumpTriggerConfig {
    /// Default trigger: on-demand dumps with no debounce.
    pub fn new() -> Self {
        Self::default()
    }

    /// Coalesce duplicate triggers within `window` into a single dump.
    ///
    /// The first trigger in a quiet period dispatches normally; any trigger
    /// arriving within `window` of that dispatch resolves
    /// [`DumpError::Coalesced`] (naming the dump it folded into) without
    /// starting a new dump. The gate is shared by every
    /// [`dump_trigger`](crate::handle::Dial9Handle::dump_trigger) clone,
    /// so the effective rate is at most one dump per `window` across all
    /// callers.
    pub fn debounce(&mut self, window: Duration) {
        self.debounce = Some(window);
    }

    pub fn debounce_window(&self) -> Option<Duration> {
        self.debounce
    }
}

/// Identifier minted for each dump request.
///
/// A ULID: time-sortable, encoded as Crockford base32 in its `Display`
/// form. Surfaces as `dump-id` user metadata on each S3 object the dump
/// produces and names the dump's manifest.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub struct DumpId(ulid::Ulid);

impl DumpId {
    pub(crate) fn new() -> Self {
        Self(ulid::Ulid::new())
    }

    /// The instant the dump was triggered, embedded in the id.
    pub fn timestamp(&self) -> SystemTime {
        SystemTime::UNIX_EPOCH + Duration::from_millis(self.0.timestamp_ms())
    }
}

impl std::fmt::Display for DumpId {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.0.fmt(f)
    }
}

impl std::str::FromStr for DumpId {
    type Err = ulid::DecodeError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        Ok(Self(s.parse()?))
    }
}

/// How far back a dump looks from its trigger time.
#[derive(Debug, Clone, Copy)]
pub(crate) enum Lookback {
    /// Everything the ring still holds (`dump_current_data`).
    Unbounded,
    /// Segments with creation epoch `>= trigger - window`.
    Window(Duration),
}

/// A dump request as it travels over the trigger channel to the worker.
#[derive(Debug)]
pub(crate) struct DumpRequest {
    pub(crate) id: DumpId,
    pub(crate) triggered_at: SystemTime,
    pub(crate) lookback: Lookback,
    pub(crate) lookforward: Duration,
    pub(crate) metadata: Vec<(String, String)>,
    pub(crate) receipt_tx: oneshot::Sender<Result<DumpReceipt, DumpError>>,
}

impl DumpRequest {
    pub(crate) fn elapsed_since_trigger(&self) -> Duration {
        crate::primitives::time::elapsed_since(self.triggered_at)
    }
}

/// Leading-edge debounce gate shared across [`DumpTrigger`] clones.
///
/// Records when the last accepted request was *built* and the [`DumpId`] it
/// was given. The gate is armed in [`DumpTrigger::request`] (the coalescing
/// decision has to be synchronous there so a folded trigger can return a
/// [`DumpError::Coalesced`] run), not at the later drop/await that actually
/// sends the request; the two coincide in the common temporary-statement
/// usage. A request arriving within `window` of that instant coalesces into
/// that id rather than starting a new dump. The window is measured from the
/// last accepted request and is not extended by coalesced requests, so a
/// burst all folds into the first dump and the effective rate is at most one
/// dump per `window`.
#[derive(Debug)]
struct Debounce {
    window: Duration,
    last: crate::primitives::sync::Mutex<Option<(Instant, DumpId)>>,
}

/// Sending half of the trigger channel.
///
/// Cloneable; reach it from any thread owned by the runtime via
/// [`Dial9Handle::dump_trigger`](crate::handle::Dial9Handle::dump_trigger)
/// and hand it to whatever subsystem decides when to dump (an idle-ratio
/// watcher, a panic hook, a `/dump` HTTP handler, ...). Every clone shares the
/// debounce gate configured by [`DumpTriggerConfig::debounce`].
#[derive(Debug, Clone)]
pub struct DumpTrigger {
    tx: mpsc::UnboundedSender<DumpRequest>,
    /// `Some` once a debounce window is configured via
    /// [`DumpTriggerConfig::debounce`]; shared by reference so every clone honors one
    /// gate.
    debounce: Option<Arc<Debounce>>,
}

impl DumpTrigger {
    /// Coalesce duplicate triggers within `window` into a single dump.
    ///
    /// Applied once at build time from [`DumpTriggerConfig::debounce`]; every
    /// [`dump_trigger`](crate::handle::Dial9Handle::dump_trigger) clone
    /// then shares one gate. The first trigger in a quiet period dispatches
    /// normally; any trigger arriving within `window` of that dispatch resolves
    /// [`DumpError::Coalesced`] (naming the dump it folded into) without
    /// starting a new dump. Useful when a single source - a watcher that
    /// re-trips every poll, a hot path that dumps per slow request - would
    /// otherwise fire a burst of near-identical dumps.
    pub fn with_debounce(mut self, window: Duration) -> Self {
        self.debounce = Some(Arc::new(Debounce {
            window,
            last: crate::primitives::sync::Mutex::new(None),
        }));
        self
    }

    /// Capture everything the ring still holds, right now. No forward
    /// window.
    pub fn dump_current_data(&self) -> DumpRun<'_> {
        self.request(Lookback::Unbounded, Duration::ZERO)
    }

    /// Capture the window `[trigger - lookback, trigger + lookforward]`.
    /// Either side may be `Duration::ZERO`.
    ///
    /// `lookback` captures pre-trigger segments; you can look back only as
    /// far as the ring still retains, so a `lookback` wider than the
    /// retained history is best-effort and captures what survived.
    /// `lookforward` keeps the dump open until `trigger + lookforward`,
    /// attaching segments as they seal; it is bounded only by the process
    /// lifetime and is best-effort under upload pressure. The actual
    /// covered span is reported on [`DumpReceipt::time_range`]. This never
    /// errors and never resizes or pins the ring.
    pub fn dump_time_range(&self, lookback: Duration, lookforward: Duration) -> DumpRun<'_> {
        self.request(Lookback::Window(lookback), lookforward)
    }

    fn request(&self, lookback: Lookback, lookforward: Duration) -> DumpRun<'_> {
        let id = DumpId::new();

        // Leading-edge debounce: a trigger within `window` of the last
        // accepted request coalesces into it instead of starting a new one.
        // Armed here at request-build time so a folded trigger can return a
        // `Coalesced` run synchronously (see `Debounce`).
        if let Some(debounce) = &self.debounce {
            let now = Instant::now();
            let mut last = debounce.last.lock().expect("debounce mutex poisoned");
            match *last {
                Some((at, into)) if now.duration_since(at) < debounce.window => {
                    return DumpRun::preempted(&self.tx, DumpError::Coalesced { into });
                }
                _ => *last = Some((now, id)),
            }
        }

        let (receipt_tx, receipt_rx) = oneshot::channel();
        DumpRun {
            request: Some(DumpRequest {
                id,
                triggered_at: SystemTime::now(),
                lookback,
                lookforward,
                metadata: Vec::new(),
                receipt_tx,
            }),
            tx: &self.tx,
            receipt_rx: Some(receipt_rx),
            preempt: None,
        }
    }
}

/// Receiving half of the trigger channel; the builder wires it into the worker.
#[derive(Debug)]
pub struct DumpRx {
    pub(crate) rx: mpsc::UnboundedReceiver<DumpRequest>,
}

/// In-flight dump request.
///
/// The dump is dispatched within the statement that created it: either
/// when this handle is awaited or when it is dropped, whichever comes
/// first. The handle is only needed to retrieve the [`DumpReceipt`];
/// dropping it does not cancel the dump. Chain
/// [`with_metadata`](Self::with_metadata) before awaiting to attach
/// correlation pairs.
#[derive(Debug)]
pub struct DumpRun<'a> {
    request: Option<DumpRequest>,
    tx: &'a mpsc::UnboundedSender<DumpRequest>,
    receipt_rx: Option<oneshot::Receiver<Result<DumpReceipt, DumpError>>>,
    /// Set when the run never dispatches (debounced): awaiting resolves this
    /// error directly. `request` is `None` for a preempted run, so
    /// [`dispatch`](Self::dispatch) and `Drop` are no-ops.
    preempt: Option<DumpError>,
}

impl<'a> DumpRun<'a> {
    /// A run that never dispatches; awaiting resolves `err`.
    fn preempted(tx: &'a mpsc::UnboundedSender<DumpRequest>, err: DumpError) -> Self {
        DumpRun {
            request: None,
            tx,
            receipt_rx: None,
            preempt: Some(err),
        }
    }

    /// Attach a caller-supplied correlation pair. Chainable. Each pair is
    /// stamped onto every captured segment's metadata (namespaced as
    /// `dump.{key}`) before the pipeline runs; pipeline stages decide what
    /// to do with them (the S3 stage surfaces them as additional user
    /// metadata).
    pub fn with_metadata(mut self, key: impl Into<String>, value: impl Into<String>) -> Self {
        if let Some(req) = self.request.as_mut() {
            req.metadata.push((key.into(), value.into()));
        }
        self
    }

    /// Send the request over the trigger channel, once. Returns `false`
    /// when the worker is gone (channel closed).
    fn dispatch(&mut self) -> bool {
        match self.request.take() {
            Some(req) => self.tx.send(req).is_ok(),
            // Already dispatched.
            None => true,
        }
    }
}

impl Drop for DumpRun<'_> {
    fn drop(&mut self) {
        // Dispatch even when the caller never awaits. A closed channel
        // means the worker is gone; nothing to do.
        let _ = self.dispatch();
    }
}

impl<'a> IntoFuture for DumpRun<'a> {
    type Output = Result<DumpReceipt, DumpError>;
    type IntoFuture = DumpFuture;

    fn into_future(mut self) -> Self::IntoFuture {
        if let Some(err) = self.preempt.take() {
            return DumpFuture {
                inner: DumpFutureInner::Preempted(err),
            };
        }
        let sent = self.dispatch();
        let inner = match (sent, self.receipt_rx.take()) {
            (true, Some(rx)) => DumpFutureInner::Waiting(rx),
            _ => DumpFutureInner::Stopped,
        };
        DumpFuture { inner }
    }
}

/// Future resolving to the dump's [`DumpReceipt`].
///
/// For a look-back-only dump it resolves once the last captured segment
/// finishes the pipeline; for a dump with a look-forward it resolves after
/// the forward deadline elapses and the last in-window segment finishes.
#[derive(Debug)]
pub struct DumpFuture {
    inner: DumpFutureInner,
}

#[derive(Debug)]
enum DumpFutureInner {
    Waiting(oneshot::Receiver<Result<DumpReceipt, DumpError>>),
    Stopped,
    /// Debounced: never dispatched, resolves this error.
    Preempted(DumpError),
}

impl Future for DumpFuture {
    type Output = Result<DumpReceipt, DumpError>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let inner = &mut self.get_mut().inner;
        match inner {
            DumpFutureInner::Waiting(rx) => match Pin::new(rx).poll(cx) {
                Poll::Ready(Ok(result)) => Poll::Ready(result),
                // Worker exited without resolving the receipt.
                Poll::Ready(Err(_)) => Poll::Ready(Err(DumpError::WorkerStopped)),
                Poll::Pending => Poll::Pending,
            },
            DumpFutureInner::Stopped => Poll::Ready(Err(DumpError::WorkerStopped)),
            // Move the error out; the future is not polled again after Ready.
            DumpFutureInner::Preempted(_) => {
                match std::mem::replace(inner, DumpFutureInner::Stopped) {
                    DumpFutureInner::Preempted(err) => Poll::Ready(Err(err)),
                    _ => unreachable!("matched Preempted above"),
                }
            }
        }
    }
}

/// Worker → pipeline-stage signal that a dump finished; passed to
/// [`SegmentProcessor::finalize_dump`](crate::pipeline::SegmentProcessor::finalize_dump)
/// so stages can flush per-dump state (the S3 stage writes the dump's
/// manifest from it).
#[derive(Debug)]
#[non_exhaustive]
pub struct DumpCompletion {
    /// Id of the dump that finished.
    pub dump_id: DumpId,
    /// When the dump was dispatched.
    pub triggered_at: SystemTime,
    /// Actual covered span (see [`DumpReceipt::time_range`]).
    pub time_range: (SystemTime, SystemTime),
    /// Count of segments that made it through the pipeline.
    pub segments_processed: usize,
    /// Caller correlation pairs from `with_metadata(...)`.
    pub metadata: Vec<(String, String)>,
    /// True when the dump resolves with [`DumpError::Pipeline`]: a captured
    /// segment failed terminally and nothing made it through. Stages still
    /// get to clear per-dump state, but should skip success artifacts (the
    /// S3 stage writes no manifest for a failed dump).
    pub failed: bool,
}

/// What a completed dump produced.
///
/// Best-effort: a dump where some matched segments make it through the
/// pipeline and others fail terminally still resolves `Ok`, with
/// [`segments_processed`](Self::segments_processed) counting only the
/// survivors (the failures are dropped silently, exactly like a segment the
/// ring evicted before the worker reached it). [`DumpError::Pipeline`] is
/// reserved for total failure: every captured segment failed and nothing
/// landed.
#[derive(Debug)]
#[non_exhaustive]
pub struct DumpReceipt {
    /// ULID minted when the dump was dispatched. Time-sortable; surfaces
    /// as `dump-id` user metadata on each S3 object.
    pub dump_id: DumpId,
    /// Count of segments that made it through the pipeline.
    pub segments_processed: usize,
    /// When the last segment finished the pipeline.
    pub finished_at: SystemTime,
    /// Actual covered span. May be shorter than the requested window on
    /// either side: look-back if the ring did not retain that much
    /// history, look-forward if the dump stopped before the deadline.
    pub time_range: (SystemTime, SystemTime),
    /// `Some({prefix}/dumps/{dump_id}.json)` when the pipeline ends at S3;
    /// `None` otherwise (no manifest is written off S3).
    pub manifest_key: Option<String>,
}

/// Why a dump failed.
#[derive(Debug)]
#[non_exhaustive]
pub enum DumpError {
    /// The worker is shutting down or already stopped.
    WorkerStopped,
    /// Every captured segment failed in a pipeline stage.
    Pipeline(ProcessErrorKind),
    /// The trigger was coalesced into an in-flight dump by the debounce gate
    /// (see [`DumpTrigger::with_debounce`]). No new dump ran; `into` names the
    /// dump that covers this trigger.
    Coalesced {
        /// Id of the dump this trigger folded into.
        into: DumpId,
    },
}

impl std::fmt::Display for DumpError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::WorkerStopped => write!(f, "worker is shutting down or already stopped"),
            Self::Pipeline(kind) => write!(f, "pipeline stage failed: {kind}"),
            Self::Coalesced { into } => write!(f, "coalesced into dump {into}"),
        }
    }
}

impl std::error::Error for DumpError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            Self::WorkerStopped => None,
            Self::Pipeline(ProcessErrorKind::Io(e)) => Some(e),
            Self::Pipeline(ProcessErrorKind::Transfer { source, .. }) => Some(source.as_ref()),
            Self::Coalesced { .. } => None,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[tokio::test]
    async fn dispatches_on_drop_without_await() {
        let (trigger, mut rx) = channel();
        {
            let _run = trigger
                .dump_time_range(Duration::from_secs(300), Duration::from_secs(60))
                .with_metadata("reason", "test")
                .with_metadata("incident", "i-123");
            // Not awaited; dispatch happens on drop at end of scope.
        }
        let req = rx.rx.try_recv().expect("request dispatched on drop");
        assert!(matches!(req.lookback, Lookback::Window(d) if d == Duration::from_secs(300)));
        assert_eq!(req.lookforward, Duration::from_secs(60));
        assert_eq!(
            req.metadata,
            vec![
                ("reason".to_string(), "test".to_string()),
                ("incident".to_string(), "i-123".to_string()),
            ]
        );
    }

    #[tokio::test]
    async fn dump_current_data_is_unbounded_lookback() {
        let (trigger, mut rx) = channel();
        trigger.dump_current_data();
        let req = rx.rx.try_recv().expect("dispatched");
        assert!(matches!(req.lookback, Lookback::Unbounded));
        assert_eq!(req.lookforward, Duration::ZERO);
    }

    #[tokio::test]
    async fn awaiting_dispatches_exactly_once_and_resolves_receipt() {
        let (trigger, mut rx) = channel();
        let run = trigger.dump_current_data();

        let worker = tokio::spawn(async move {
            let req = rx.rx.recv().await.expect("one request");
            assert!(rx.rx.try_recv().is_err(), "no second dispatch");
            let receipt = DumpReceipt {
                dump_id: req.id,
                segments_processed: 3,
                finished_at: SystemTime::now(),
                time_range: (req.triggered_at, req.triggered_at),
                manifest_key: None,
            };
            let _ = req.receipt_tx.send(Ok(receipt));
        });

        let receipt = run.await.expect("receipt");
        assert_eq!(receipt.segments_processed, 3);
        worker.await.unwrap();
    }

    #[tokio::test]
    async fn closed_channel_resolves_worker_stopped() {
        let (trigger, rx) = channel();
        drop(rx);
        let err = trigger.dump_current_data().await.unwrap_err();
        assert!(matches!(err, DumpError::WorkerStopped));
    }

    #[tokio::test]
    async fn dropped_receipt_sender_resolves_worker_stopped() {
        use std::future::IntoFuture;

        let (trigger, mut rx) = channel();
        let fut = trigger.dump_current_data().into_future();
        let req = rx.rx.try_recv().expect("dispatched at into_future");
        // Worker exiting without resolving the receipt drops `receipt_tx`.
        drop(req);
        let err = fut.await.unwrap_err();
        assert!(matches!(err, DumpError::WorkerStopped));
    }

    #[tokio::test]
    async fn debounce_coalesces_into_the_first_dump() {
        let (trigger, mut rx) = channel();
        let trigger = trigger.with_debounce(Duration::from_secs(60));

        // First trigger dispatches (drop dispatches the un-awaited run).
        let _ = trigger.dump_current_data();
        let first = rx.rx.try_recv().expect("first trigger dispatched");

        // Second trigger within the window folds into the first.
        let err = trigger.dump_current_data().await.unwrap_err();
        assert!(matches!(err, DumpError::Coalesced { into } if into == first.id));
        assert!(
            rx.rx.try_recv().is_err(),
            "coalesced trigger must not dispatch"
        );
    }

    #[tokio::test]
    async fn debounce_dispatches_again_after_window() {
        let (trigger, mut rx) = channel();
        let trigger = trigger.with_debounce(Duration::from_millis(30));

        let _ = trigger.dump_current_data();
        let first = rx.rx.try_recv().expect("first dispatched");

        tokio::time::sleep(Duration::from_millis(80)).await;

        let _ = trigger.dump_current_data();
        let second = rx.rx.try_recv().expect("dispatched again after window");
        assert_ne!(first.id, second.id, "post-window dump gets a fresh id");
    }

    #[tokio::test]
    async fn debounce_gate_is_shared_across_clones() {
        let (trigger, mut rx) = channel();
        let trigger = trigger.with_debounce(Duration::from_secs(60));
        let clone = trigger.clone();

        let _ = trigger.dump_current_data();
        let first = rx.rx.try_recv().expect("first dispatched");

        // A clone honors the same gate, so its trigger coalesces too.
        let err = clone.dump_current_data().await.unwrap_err();
        assert!(matches!(err, DumpError::Coalesced { into } if into == first.id));
    }

    #[tokio::test]
    async fn without_debounce_duplicate_triggers_both_dispatch() {
        let (trigger, mut rx) = channel();
        let _ = trigger.dump_current_data();
        let _ = trigger.dump_current_data();
        assert!(rx.rx.try_recv().is_ok(), "first dispatched");
        assert!(
            rx.rx.try_recv().is_ok(),
            "second dispatched (no coordination)"
        );
    }

    #[test]
    fn dump_id_is_time_sorted_and_timestamp_round_trips() {
        let before = SystemTime::now();
        let a = DumpId::new();
        std::thread::sleep(Duration::from_millis(2));
        let b = DumpId::new();
        let after = SystemTime::now();
        assert!(a < b);
        assert!(a.timestamp() >= before - Duration::from_millis(1));
        assert!(b.timestamp() <= after + Duration::from_millis(1));

        let parsed: DumpId = a.to_string().parse().expect("round-trip");
        assert_eq!(parsed, a);
    }

    #[cfg(shuttle)]
    mod shuttle_tests {
        use super::*;

        const CALLERS: usize = 4;

        crate::shuttle_test! {
            default;
            // Multiple threads race a shared, debounced trigger; exactly one
            // must dispatch a real request, the rest must coalesce into it.
            fn shuttle_debounce_gate() {
                let (trigger, mut rx) = channel();
                let trigger = trigger.with_debounce(Duration::from_secs(60));

                let handles: Vec<_> = (0..CALLERS)
                    .map(|_| {
                        let trigger = trigger.clone();
                        crate::primitives::thread::spawn(move || {
                            // Dropped without awaiting: per `DumpRun`'s `Drop`
                            // impl, dispatch (or no-op, if coalesced) happens
                            // right here.
                            drop(trigger.dump_current_data());
                        })
                    })
                    .collect();

                for h in handles {
                    h.join().unwrap();
                }

                let mut dispatched = 0;
                while rx.rx.try_recv().is_ok() {
                    dispatched += 1;
                }
                assert_eq!(
                    dispatched, 1,
                    "exactly one trigger within the debounce window must dispatch a real request"
                );
            }
        }
    }
}