huskies/server/src/pipeline_state/events.rs

//! Event bus for pipeline state transitions.

use chrono::{DateTime, Utc};
use std::sync::OnceLock;
use tokio::sync::broadcast;

use super::{PipelineEvent, Stage, StoryId};

// ── Static transition broadcast channel ─────────────────────────────────────

static TRANSITION_TX: OnceLock<broadcast::Sender<TransitionFired>> = OnceLock::new();

fn get_or_init_tx() -> &'static broadcast::Sender<TransitionFired> {
    TRANSITION_TX.get_or_init(|| {
        let (tx, _) = broadcast::channel(256);
        tx
    })
}

/// Subscribe to all pipeline stage transitions.
///
/// Every call to [`apply_transition`][super::apply_transition] broadcasts the
/// resulting [`TransitionFired`] on this channel.  Returns a new receiver that
/// replays events from the moment of subscription.  Lagged receivers silently
/// skip missed events — callers should handle
/// [`broadcast::error::RecvError::Lagged`].
pub fn subscribe_transitions() -> broadcast::Receiver<TransitionFired> {
    get_or_init_tx().subscribe()
}

/// Broadcast `fired` to all active transition subscribers.
///
/// Called from [`apply_transition`][super::apply] after writing the new stage
/// to the CRDT.  No-ops safely when there are no subscribers.
pub(super) fn try_broadcast(fired: &TransitionFired) {
    let _ = get_or_init_tx().send(fired.clone());
}

/// Replay the current CRDT pipeline state as a burst of synthetic
/// [`TransitionFired`] events at server startup.
///
/// Reads every item from the CRDT and broadcasts a self-transition
/// (`before == after`) for each one so that all existing subscribers
/// (worktree lifecycle, merge-failure auto-spawn, auto-assign) react
/// identically to a live event.  This replaces the legacy scan-based
/// `reconcile_on_startup` path.
///
/// Idempotent: a second call produces another burst of events, but every
/// subscriber already guards against duplicate work (e.g.
/// `is_story_assigned_for_stage` returns true once an agent is running,
/// and worktree creation is a no-op when the worktree already exists).
pub fn replay_current_pipeline_state() {
    for item in super::read_all_typed() {
        let fired = TransitionFired {
            story_id: item.story_id.clone(),
            before: item.stage.clone(),
            after: item.stage,
            event: super::PipelineEvent::DepsMet,
            at: chrono::Utc::now(),
        };
        try_broadcast(&fired);
    }
}

/// Fired when a pipeline stage transition completes.
#[derive(Debug, Clone)]
pub struct TransitionFired {
    pub story_id: StoryId,
    pub before: Stage,
    pub after: Stage,
    pub event: PipelineEvent,
    pub at: DateTime<Utc>,
}

/// Trait for side-effect handlers that react to pipeline transitions.
pub trait TransitionSubscriber: Send + Sync {
    fn name(&self) -> &'static str;
    fn on_transition(&self, fired: &TransitionFired);
}

/// Collects [`TransitionSubscriber`]s and dispatches [`TransitionFired`] events to each.
pub struct EventBus {
    subscribers: Vec<Box<dyn TransitionSubscriber>>,
}

impl EventBus {
    /// Create an empty event bus with no subscribers.
    pub fn new() -> Self {
        Self {
            subscribers: Vec::new(),
        }
    }

    /// Register a subscriber to receive all future transition events.
    pub fn subscribe<S: TransitionSubscriber + 'static>(&mut self, subscriber: S) {
        self.subscribers.push(Box::new(subscriber));
    }

    /// Fire a transition event, calling every registered subscriber in order.
    pub fn fire(&self, event: TransitionFired) {
        for sub in &self.subscribers {
            sub.on_transition(&event);
        }
    }
}

impl Default for EventBus {
    fn default() -> Self {
        Self::new()
    }
}

#[cfg(test)]
mod tests {
    use super::super::{BranchName, PlanState};
    use super::*;
    use std::num::NonZeroU32;

    fn nz(n: u32) -> NonZeroU32 {
        NonZeroU32::new(n).unwrap()
    }
    fn fb(name: &str) -> BranchName {
        BranchName(name.to_string())
    }
    fn sid(s: &str) -> StoryId {
        StoryId(s.to_string())
    }

    #[test]
    fn event_bus_fires_to_all_subscribers() {
        use std::sync::Arc;
        use std::sync::atomic::{AtomicU32, Ordering};

        struct CountingSub(Arc<AtomicU32>);
        impl TransitionSubscriber for CountingSub {
            fn name(&self) -> &'static str {
                "counter"
            }
            fn on_transition(&self, _: &TransitionFired) {
                self.0.fetch_add(1, Ordering::SeqCst);
            }
        }

        let counter = Arc::new(AtomicU32::new(0));
        let mut bus = EventBus::new();
        bus.subscribe(CountingSub(counter.clone()));
        bus.subscribe(CountingSub(counter.clone()));

        bus.fire(TransitionFired {
            story_id: StoryId("test".into()),
            before: Stage::Backlog,
            after: Stage::Coding {
                claim: None,
                plan: PlanState::Missing,
            },
            event: PipelineEvent::DepsMet,
            at: Utc::now(),
        });

        assert_eq!(counter.load(Ordering::SeqCst), 2);
    }

    // ── Bug 502 regression: agent field is not part of Stage ────────────

    #[test]
    fn bug_502_agent_not_in_stage() {
        // Bug 502 was caused by a coder agent being assigned to a story in
        // Merge stage. In the typed system, Stage has no `agent` field at all.
        // Agent assignment is per-node ExecutionState. This test documents that
        // the old failure mode is structurally impossible.
        let merge = Stage::Merge {
            feature_branch: BranchName("feature/story-1".into()),
            commits_ahead: NonZeroU32::new(3).unwrap(),
            claim: None,
        };
        // Stage::Merge has exactly two fields: feature_branch and commits_ahead.
        // There is no way to attach an agent name to it. The type system
        // prevents bug 502 by construction.
        assert!(matches!(merge, Stage::Merge { .. }));
    }

    // ── TransitionError Display ─────────────────────────────────────────

    // ── replay_current_pipeline_state ──────────────────────────────────

    /// AC1: replay broadcasts a synthetic event for every item in the CRDT.
    #[test]
    fn replay_broadcasts_event_for_crdt_item_in_coding_stage() {
        crate::crdt_state::init_for_test();
        crate::db::ensure_content_store();

        let story_id = "9901_replay_coding";
        crate::db::write_item_with_content(
            story_id,
            "2_current",
            "---\nname: Replay Coding\n---\n",
            crate::db::ItemMeta::named("Replay Coding"),
        );

        let mut rx = subscribe_transitions();
        replay_current_pipeline_state();

        let mut found = false;
        while let Ok(fired) = rx.try_recv() {
            if fired.story_id.0 == story_id && matches!(fired.after, Stage::Coding { .. }) {
                found = true;
            }
        }
        assert!(
            found,
            "replay must broadcast a Coding event for a story in 2_current"
        );
    }

    /// AC3: calling replay_current_pipeline_state twice fires events both times.
    ///
    /// Pool-state idempotency (no duplicate agents) is enforced by subscribers,
    /// not by the replay function itself.  This test verifies that replay is safe
    /// to call multiple times without panicking.
    #[test]
    fn replay_twice_does_not_panic() {
        crate::crdt_state::init_for_test();
        crate::db::ensure_content_store();

        let story_id = "9902_replay_idem";
        crate::db::write_item_with_content(
            story_id,
            "3_qa",
            "---\nname: Replay QA\n---\n",
            crate::db::ItemMeta::named("Replay QA"),
        );

        // Two successive replays must not panic.
        replay_current_pipeline_state();
        replay_current_pipeline_state();
    }
}