huskies/server/examples/pipeline_state_sketch_statig.rs

//! Pipeline state machine — design sketch (story 520) — STATIG version.
//!
//! Parallel to `pipeline_state_sketch_bare.rs`. Same domain types, same
//! transitions, same event semantics — but the state machine is built using
//! the `statig` crate (https://crates.io/crates/statig) instead of being
//! hand-rolled.
//!
//! Run with:
//!     cargo run  --example pipeline_state_sketch_statig -p huskies
//! Test with:
//!     cargo test --example pipeline_state_sketch_statig -p huskies
//!
//! Why both versions?
//!
//! - The **bare** version shows that plain Rust enums + a transition function
//!   are *enough* to make impossible states unrepresentable. No framework.
//! - The **statig** version shows what we'd gain by adopting a state-machine
//!   crate: hierarchical states (the `active` superstate factors out the
//!   cross-cutting Block/ReviewHold/Abandon/Supersede transitions, which the
//!   bare version had to duplicate inline with `|` patterns), generated
//!   `State` enum with type-safe data-carrying constructors, and stateful
//!   `handle(&event)` dispatch. Type safety is preserved either way:
//!   `State::merge(BranchName, NonZeroU32)` requires both args at the
//!   constructor, just like `Stage::Merge { feature_branch, commits_ahead }`
//!   in the bare version.
//!
//! Trade-off: statig adds a dependency and a proc-macro layer, which makes
//! the code harder to read for someone unfamiliar with the crate. The
//! framework-free version is more transparent but requires manual
//! pattern-matching and inline duplication for cross-cutting transitions.

use chrono::{DateTime, Utc};
use statig::prelude::*;
use std::num::NonZeroU32;

// ── Newtypes (same as bare version) ──────────────────────────────────────────

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct StoryId(pub String);

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct BranchName(pub String);

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct GitSha(pub String);

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct AgentName(pub String);

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct NodePubkey(pub [u8; 32]);

// ── Archive reason (same as bare version) ────────────────────────────────────

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum ArchiveReason {
    Completed,
    Abandoned,
    Superseded { by: StoryId },
    Blocked { reason: String },
    MergeFailed { reason: String },
    ReviewHeld { reason: String },
}

// ── Pipeline events (same as bare version) ───────────────────────────────────

#[derive(Debug, Clone)]
pub enum PipelineEvent {
    DepsMet,
    GatesStarted,
    GatesPassed {
        feature_branch: BranchName,
        commits_ahead: NonZeroU32,
    },
    GatesFailed {
        reason: String,
    },
    QaSkipped {
        feature_branch: BranchName,
        commits_ahead: NonZeroU32,
    },
    MergeSucceeded {
        merge_commit: GitSha,
    },
    MergeFailedFinal {
        reason: String,
    },
    Accepted,
    Block {
        reason: String,
    },
    Unblock,
    Abandon,
    Supersede {
        by: StoryId,
    },
    ReviewHold {
        reason: String,
    },
}

// ── The state machine ────────────────────────────────────────────────────────
//
// statig requires a "context" struct (the `Self` of the impl block). For us
// it's empty — all per-state data lives ON the state itself, carried forward
// by the auto-generated `State::xxx(...)` constructors.

#[derive(Default)]
pub struct PipelineMachine;

#[state_machine(
    initial = "State::backlog()",
    state(derive(Debug, Clone, PartialEq, Eq))
)]
impl PipelineMachine {
    // ── Active stages: backlog, coding, qa, merge ────────────────────────
    //
    // Each is a child of the `active` superstate, which handles the
    // cross-cutting transitions (Block / ReviewHold / Abandon / Supersede)
    // exactly once instead of being duplicated per state.

    #[state(superstate = "active")]
    fn backlog(event: &PipelineEvent) -> Response<State> {
        match event {
            PipelineEvent::DepsMet => Transition(State::coding()),
            _ => Super, // defer to `active` (and ultimately to "unhandled")
        }
    }

    #[state(superstate = "active")]
    fn coding(event: &PipelineEvent) -> Response<State> {
        match event {
            PipelineEvent::GatesStarted => Transition(State::qa()),
            PipelineEvent::QaSkipped {
                feature_branch,
                commits_ahead,
            } => Transition(State::merge(feature_branch.clone(), *commits_ahead)),
            _ => Super,
        }
    }

    #[state(superstate = "active")]
    fn qa(event: &PipelineEvent) -> Response<State> {
        match event {
            PipelineEvent::GatesPassed {
                feature_branch,
                commits_ahead,
            } => Transition(State::merge(feature_branch.clone(), *commits_ahead)),
            PipelineEvent::GatesFailed { .. } => Transition(State::coding()),
            _ => Super,
        }
    }

    #[state(superstate = "active")]
    fn merge(
        _feature_branch: &mut BranchName,
        _commits_ahead: &mut NonZeroU32,
        event: &PipelineEvent,
    ) -> Response<State> {
        // Note: the type signature of this state function REQUIRES both
        // _feature_branch and _commits_ahead. There is no way to construct
        // a Merge state without them. NonZeroU32 makes "merge with zero
        // commits ahead" structurally unrepresentable (bug 519 fixed by
        // construction, same as the bare version).
        //
        // The fields are prefixed with `_` because this state function only
        // transitions forward and doesn't read them — but they're available
        // to inspect via the State::Merge variant generated by the macro.
        match event {
            PipelineEvent::MergeSucceeded { merge_commit } => {
                Transition(State::done(Utc::now(), merge_commit.clone()))
            }
            PipelineEvent::MergeFailedFinal { reason } => Transition(State::archived(
                Utc::now(),
                ArchiveReason::MergeFailed {
                    reason: reason.clone(),
                },
            )),
            _ => Super,
        }
    }

    // ── Cross-cutting superstate ─────────────────────────────────────────
    //
    // This is the statig payoff: ONE place defines what Block/ReviewHold/
    // Abandon/Supersede do across all four active stages. The bare version
    // had to duplicate this with `|` patterns. Adding a new active stage
    // here means just adding it as a child of `active`; the cross-cutting
    // transitions come for free.

    #[superstate]
    fn active(event: &PipelineEvent) -> Response<State> {
        let now = Utc::now();
        match event {
            PipelineEvent::Block { reason } => Transition(State::archived(
                now,
                ArchiveReason::Blocked {
                    reason: reason.clone(),
                },
            )),
            PipelineEvent::ReviewHold { reason } => Transition(State::archived(
                now,
                ArchiveReason::ReviewHeld {
                    reason: reason.clone(),
                },
            )),
            PipelineEvent::Abandon => Transition(State::archived(now, ArchiveReason::Abandoned)),
            PipelineEvent::Supersede { by } => Transition(State::archived(
                now,
                ArchiveReason::Superseded { by: by.clone() },
            )),
            _ => Handled, // unhandled events are silently ignored
        }
    }

    // ── Done is special: it's not a child of `active` because Block and ──
    // ── ReviewHold are NOT valid from Done (per the bare version's rules).
    // ── Abandon and Supersede ARE valid, so we have to handle them inline.

    #[state]
    fn done(
        merged_at: &mut DateTime<Utc>,
        merge_commit: &mut GitSha,
        event: &PipelineEvent,
    ) -> Response<State> {
        let now = Utc::now();
        let _ = merged_at; // currently unused; available for queries
        let _ = merge_commit;
        match event {
            PipelineEvent::Accepted => Transition(State::archived(now, ArchiveReason::Completed)),
            PipelineEvent::Abandon => Transition(State::archived(now, ArchiveReason::Abandoned)),
            PipelineEvent::Supersede { by } => Transition(State::archived(
                now,
                ArchiveReason::Superseded { by: by.clone() },
            )),
            _ => Handled,
        }
    }

    // ── Archived is terminal except for Unblock from Blocked → Backlog ───

    #[state]
    fn archived(
        archived_at: &mut DateTime<Utc>,
        reason: &mut ArchiveReason,
        event: &PipelineEvent,
    ) -> Response<State> {
        let _ = archived_at;
        match event {
            PipelineEvent::Unblock => {
                if matches!(reason, ArchiveReason::Blocked { .. }) {
                    Transition(State::backlog())
                } else {
                    Handled // unblock only valid from Blocked
                }
            }
            _ => Handled,
        }
    }
}

// ── Per-node execution state machine ─────────────────────────────────────────
//
// Independent of the pipeline stage machine. Tracks "what is THIS node doing
// about this story right now." Lives in its own sub-module so its generated
// `State` enum doesn't collide with `PipelineMachine`'s.
//
// In a real implementation, multiple nodes can have different ExecutionState
// for the same story_id at the same time — and that's fine, because each
// node owns its own subspace in the CRDT (keyed by node pubkey).

#[derive(Debug, Clone)]
pub enum ExecutionEvent {
    SpawnRequested { agent: AgentName },
    SpawnedSuccessfully,
    Heartbeat,
    HitRateLimit { resume_at: DateTime<Utc> },
    Exited { exit_code: i32 },
    Stopped,
    Reset,
}

pub mod execution {
    use super::{AgentName, DateTime, ExecutionEvent, Utc};
    use statig::prelude::*;

    #[derive(Default)]
    pub struct ExecutionMachine;

    #[state_machine(initial = "State::idle()", state(derive(Debug, Clone, PartialEq, Eq)))]
    impl ExecutionMachine {
        // ── Idle: no agent on this node is working on this story ──────────

        #[state(superstate = "any")]
        fn idle(event: &ExecutionEvent) -> Response<State> {
            match event {
                ExecutionEvent::SpawnRequested { agent } => {
                    Transition(State::pending(agent.clone(), Utc::now()))
                }
                _ => Super,
            }
        }

        // ── Pending: agent has been requested but hasn't started yet ──────

        #[state(superstate = "any")]
        fn pending(
            agent: &mut AgentName,
            _since: &mut DateTime<Utc>,
            event: &ExecutionEvent,
        ) -> Response<State> {
            match event {
                ExecutionEvent::SpawnedSuccessfully => {
                    let now = Utc::now();
                    Transition(State::running(agent.clone(), now, now))
                }
                ExecutionEvent::HitRateLimit { resume_at } => {
                    Transition(State::rate_limited(agent.clone(), *resume_at))
                }
                ExecutionEvent::Exited { exit_code } => {
                    Transition(State::completed(agent.clone(), *exit_code, Utc::now()))
                }
                _ => Super,
            }
        }

        // ── Running: agent's subprocess is alive ──────────────────────────
        //
        // Heartbeat is a self-transition: we update last_heartbeat in-place
        // via the &mut reference and return `Handled` (no actual stage change).
        // This is statig's idiomatic way to mutate state-local data without
        // transitioning.

        #[state(superstate = "any")]
        fn running(
            agent: &mut AgentName,
            _started_at: &mut DateTime<Utc>,
            last_heartbeat: &mut DateTime<Utc>,
            event: &ExecutionEvent,
        ) -> Response<State> {
            match event {
                ExecutionEvent::Heartbeat => {
                    *last_heartbeat = Utc::now();
                    Handled
                }
                ExecutionEvent::HitRateLimit { resume_at } => {
                    Transition(State::rate_limited(agent.clone(), *resume_at))
                }
                ExecutionEvent::Exited { exit_code } => {
                    Transition(State::completed(agent.clone(), *exit_code, Utc::now()))
                }
                _ => Super,
            }
        }

        // ── RateLimited: waiting for the API rate-limit window to clear ───

        #[state(superstate = "any")]
        fn rate_limited(
            agent: &mut AgentName,
            _resume_at: &mut DateTime<Utc>,
            event: &ExecutionEvent,
        ) -> Response<State> {
            match event {
                ExecutionEvent::SpawnedSuccessfully => {
                    let now = Utc::now();
                    Transition(State::running(agent.clone(), now, now))
                }
                ExecutionEvent::Exited { exit_code } => {
                    Transition(State::completed(agent.clone(), *exit_code, Utc::now()))
                }
                _ => Super,
            }
        }

        // ── Completed: agent finished, exit code captured ─────────────────

        #[state(superstate = "any")]
        fn completed(
            agent: &mut AgentName,
            exit_code: &mut i32,
            completed_at: &mut DateTime<Utc>,
            event: &ExecutionEvent,
        ) -> Response<State> {
            // Completed is mostly terminal; only Stopped/Reset (handled by
            // the `any` superstate) returns to Idle. Field names are kept
            // un-underscored so the generated State::Completed variant
            // exposes them as `exit_code` etc. for test pattern matching.
            let _ = (agent, exit_code, completed_at, event);
            Super
        }

        // ── Cross-cutting: Stopped and Reset return to Idle from anywhere ─

        #[superstate]
        fn any(event: &ExecutionEvent) -> Response<State> {
            match event {
                ExecutionEvent::Stopped | ExecutionEvent::Reset => Transition(State::idle()),
                _ => Handled,
            }
        }
    }
}

// ── Side effects via statig's entry/exit actions (alternative to EventBus) ───
//
// The bare version uses an explicit EventBus + Subscriber trait + per-state
// publish-on-transition pattern. statig has a more native equivalent:
// `#[action]`-tagged functions that fire on state entry / exit / transition.
//
// We don't include a full action-based example here — it would roughly look
// like adding `entry_action = "log_entry"` to each #[state] attribute and
// defining `fn log_entry(...)` in the impl block. The trade-off is that
// statig's actions are tightly coupled to the state machine impl block,
// while the bare version's EventBus allows arbitrary external subscribers
// to plug in without touching the state machine code. Both patterns are
// valid; pick based on whether you want side-effect dispatch INSIDE the
// machine (statig actions) or OUTSIDE it (bare EventBus).

// ── Tests ────────────────────────────────────────────────────────────────────

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

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

    // ── Happy path ─────────────────────────────────────────────────────────

    #[test]
    fn happy_path_backlog_through_done() {
        let mut sm = PipelineMachine.state_machine();
        assert!(matches!(sm.state(), State::Backlog {}));

        sm.handle(&PipelineEvent::DepsMet);
        assert!(matches!(sm.state(), State::Coding {}));

        sm.handle(&PipelineEvent::QaSkipped {
            feature_branch: fb("feature/story-1"),
            commits_ahead: nz(3),
        });
        assert!(matches!(sm.state(), State::Merge { .. }));

        sm.handle(&PipelineEvent::MergeSucceeded {
            merge_commit: sha("abc123"),
        });
        assert!(matches!(sm.state(), State::Done { .. }));

        sm.handle(&PipelineEvent::Accepted);
        assert!(matches!(
            sm.state(),
            State::Archived {
                reason: ArchiveReason::Completed,
                ..
            }
        ));
    }

    #[test]
    fn qa_retry_loop() {
        let mut sm = PipelineMachine.state_machine();
        sm.handle(&PipelineEvent::DepsMet);
        sm.handle(&PipelineEvent::GatesStarted);
        assert!(matches!(sm.state(), State::Qa {}));

        sm.handle(&PipelineEvent::GatesFailed {
            reason: "tests failed".into(),
        });
        assert!(matches!(sm.state(), State::Coding {}));
    }

    // ── Bug 519 unrepresentability: Merge with zero commits ahead ──────────

    #[test]
    fn merge_with_zero_commits_is_unrepresentable() {
        // Identical to the bare version: NonZeroU32::new(0) returns None,
        // so a State::merge(branch, ZERO) literally cannot be constructed.
        assert!(NonZeroU32::new(0).is_none());
    }

    // ── Cross-cutting Block from any active stage (superstate proves it) ───

    #[test]
    fn block_from_backlog_via_superstate() {
        let mut sm = PipelineMachine.state_machine();
        sm.handle(&PipelineEvent::Block {
            reason: "stuck".into(),
        });
        assert!(matches!(
            sm.state(),
            State::Archived {
                reason: ArchiveReason::Blocked { .. },
                ..
            }
        ));
    }

    #[test]
    fn block_from_coding_via_superstate() {
        let mut sm = PipelineMachine.state_machine();
        sm.handle(&PipelineEvent::DepsMet);
        sm.handle(&PipelineEvent::Block {
            reason: "stuck".into(),
        });
        assert!(matches!(
            sm.state(),
            State::Archived {
                reason: ArchiveReason::Blocked { .. },
                ..
            }
        ));
    }

    #[test]
    fn block_from_qa_via_superstate() {
        let mut sm = PipelineMachine.state_machine();
        sm.handle(&PipelineEvent::DepsMet);
        sm.handle(&PipelineEvent::GatesStarted);
        sm.handle(&PipelineEvent::Block {
            reason: "stuck".into(),
        });
        assert!(matches!(
            sm.state(),
            State::Archived {
                reason: ArchiveReason::Blocked { .. },
                ..
            }
        ));
    }

    #[test]
    fn block_from_merge_via_superstate() {
        let mut sm = PipelineMachine.state_machine();
        sm.handle(&PipelineEvent::DepsMet);
        sm.handle(&PipelineEvent::QaSkipped {
            feature_branch: fb("f"),
            commits_ahead: nz(1),
        });
        sm.handle(&PipelineEvent::Block {
            reason: "stuck".into(),
        });
        assert!(matches!(
            sm.state(),
            State::Archived {
                reason: ArchiveReason::Blocked { .. },
                ..
            }
        ));
    }

    // ── Block from Done is NOT valid (Done isn't a child of `active`) ──────

    #[test]
    fn block_from_done_is_ignored() {
        let mut sm = PipelineMachine.state_machine();
        sm.handle(&PipelineEvent::DepsMet);
        sm.handle(&PipelineEvent::QaSkipped {
            feature_branch: fb("f"),
            commits_ahead: nz(1),
        });
        sm.handle(&PipelineEvent::MergeSucceeded {
            merge_commit: sha("abc"),
        });
        // Now in Done. Block should NOT transition us anywhere.
        sm.handle(&PipelineEvent::Block {
            reason: "stuck".into(),
        });
        assert!(matches!(sm.state(), State::Done { .. }));
    }

    // ── Abandon from Done IS valid (handled inline in done()) ──────────────

    #[test]
    fn abandon_from_done_works() {
        let mut sm = PipelineMachine.state_machine();
        sm.handle(&PipelineEvent::DepsMet);
        sm.handle(&PipelineEvent::QaSkipped {
            feature_branch: fb("f"),
            commits_ahead: nz(1),
        });
        sm.handle(&PipelineEvent::MergeSucceeded {
            merge_commit: sha("abc"),
        });
        sm.handle(&PipelineEvent::Abandon);
        assert!(matches!(
            sm.state(),
            State::Archived {
                reason: ArchiveReason::Abandoned,
                ..
            }
        ));
    }

    // ── Unblock from Archived(Blocked) → Backlog ───────────────────────────

    #[test]
    fn unblock_returns_to_backlog() {
        let mut sm = PipelineMachine.state_machine();
        sm.handle(&PipelineEvent::Block {
            reason: "test".into(),
        });
        assert!(matches!(
            sm.state(),
            State::Archived {
                reason: ArchiveReason::Blocked { .. },
                ..
            }
        ));

        sm.handle(&PipelineEvent::Unblock);
        assert!(matches!(sm.state(), State::Backlog {}));
    }

    #[test]
    fn unblock_from_review_held_does_nothing() {
        // Unblock is specifically for Blocked, not for any Archived variant.
        let mut sm = PipelineMachine.state_machine();
        sm.handle(&PipelineEvent::ReviewHold {
            reason: "TBD".into(),
        });
        // Now in Archived(ReviewHeld). Unblock should NOT transition.
        sm.handle(&PipelineEvent::Unblock);
        assert!(matches!(
            sm.state(),
            State::Archived {
                reason: ArchiveReason::ReviewHeld { .. },
                ..
            }
        ));
    }

    // ── ExecutionMachine tests ─────────────────────────────────────────────

    use super::execution::{ExecutionMachine, State as ExecState};

    #[test]
    fn execution_happy_path() {
        let mut em = ExecutionMachine.state_machine();
        assert!(matches!(em.state(), ExecState::Idle {}));

        em.handle(&ExecutionEvent::SpawnRequested {
            agent: AgentName("coder-1".into()),
        });
        assert!(matches!(em.state(), ExecState::Pending { .. }));

        em.handle(&ExecutionEvent::SpawnedSuccessfully);
        assert!(matches!(em.state(), ExecState::Running { .. }));

        em.handle(&ExecutionEvent::Heartbeat);
        // Heartbeat updates last_heartbeat in-place; we stay in Running.
        assert!(matches!(em.state(), ExecState::Running { .. }));

        em.handle(&ExecutionEvent::Exited { exit_code: 0 });
        assert!(matches!(
            em.state(),
            ExecState::Completed { exit_code: 0, .. }
        ));
    }

    #[test]
    fn execution_rate_limit_then_resume() {
        let mut em = ExecutionMachine.state_machine();
        em.handle(&ExecutionEvent::SpawnRequested {
            agent: AgentName("coder-1".into()),
        });
        em.handle(&ExecutionEvent::SpawnedSuccessfully);
        em.handle(&ExecutionEvent::HitRateLimit {
            resume_at: Utc::now() + chrono::Duration::minutes(5),
        });
        assert!(matches!(em.state(), ExecState::RateLimited { .. }));

        em.handle(&ExecutionEvent::SpawnedSuccessfully);
        assert!(matches!(em.state(), ExecState::Running { .. }));
    }

    #[test]
    fn execution_stop_from_running_returns_idle_via_superstate() {
        let mut em = ExecutionMachine.state_machine();
        em.handle(&ExecutionEvent::SpawnRequested {
            agent: AgentName("coder-1".into()),
        });
        em.handle(&ExecutionEvent::SpawnedSuccessfully);
        em.handle(&ExecutionEvent::Stopped);
        assert!(matches!(em.state(), ExecState::Idle {}));
    }

    #[test]
    fn execution_stop_from_pending_returns_idle_via_superstate() {
        let mut em = ExecutionMachine.state_machine();
        em.handle(&ExecutionEvent::SpawnRequested {
            agent: AgentName("coder-1".into()),
        });
        em.handle(&ExecutionEvent::Stopped);
        assert!(matches!(em.state(), ExecState::Idle {}));
    }

    #[test]
    fn execution_stop_from_rate_limited_returns_idle_via_superstate() {
        let mut em = ExecutionMachine.state_machine();
        em.handle(&ExecutionEvent::SpawnRequested {
            agent: AgentName("coder-1".into()),
        });
        em.handle(&ExecutionEvent::SpawnedSuccessfully);
        em.handle(&ExecutionEvent::HitRateLimit {
            resume_at: Utc::now() + chrono::Duration::minutes(5),
        });
        em.handle(&ExecutionEvent::Stopped);
        assert!(matches!(em.state(), ExecState::Idle {}));
    }

    #[test]
    fn nodepubkey_type_is_constructible() {
        // Just exercise the NodePubkey newtype so it isn't dead code.
        // In a real implementation it'd key the per-node ExecutionState
        // map inside the CRDT.
        let _ = NodePubkey([0u8; 32]);
    }
}

// ── main: a quick interactive demo ───────────────────────────────────────────

fn main() {
    println!("─── Pipeline state machine sketch (story 520) — STATIG version ───\n");

    let mut sm = PipelineMachine.state_machine();
    println!("Initial: {:?}\n", sm.state());

    println!("→ DepsMet");
    sm.handle(&PipelineEvent::DepsMet);
    println!("  state: {:?}\n", sm.state());

    println!("→ QaSkipped");
    sm.handle(&PipelineEvent::QaSkipped {
        feature_branch: BranchName("feature/story-100".into()),
        commits_ahead: NonZeroU32::new(3).unwrap(),
    });
    println!("  state: {:?}\n", sm.state());

    println!("→ MergeSucceeded");
    sm.handle(&PipelineEvent::MergeSucceeded {
        merge_commit: GitSha("abc1234".into()),
    });
    println!("  state: {:?}\n", sm.state());

    println!("→ Accepted");
    sm.handle(&PipelineEvent::Accepted);
    println!("  state: {:?}\n", sm.state());

    println!("─── Trying invalid transition: Done → Unblock ───");
    let mut sm2 = PipelineMachine.state_machine();
    sm2.handle(&PipelineEvent::DepsMet);
    sm2.handle(&PipelineEvent::QaSkipped {
        feature_branch: BranchName("feature/story-101".into()),
        commits_ahead: NonZeroU32::new(2).unwrap(),
    });
    sm2.handle(&PipelineEvent::MergeSucceeded {
        merge_commit: GitSha("def5678".into()),
    });
    println!("  before Unblock: {:?}", sm2.state());
    sm2.handle(&PipelineEvent::Unblock); // silently ignored — no transition
    println!(
        "  after  Unblock: {:?}  (no change — Unblock is a no-op from Done)",
        sm2.state()
    );
}