//! Generic job-DAG queue + desired-state reconciliation — the host-side
//! wrapper over the domain-agnostic [`hive_jobq`] scheduler. Jobs are nodes in
//! per-request DAGs (see [`templates`]); the special cases (graceful-stop
//! watcher, deferred-start follow-up, meta-update cascade) collapse into DAG
//! *shapes* over the shared node primitives ([`model::NodeKind`]).
//!
//! [`hive_jobq`] owns the graph, the two-class resource pool, and the roll-up
//! settle loop; this module maps hive-c0re's concepts onto it:
//! - [`model::NodeKind`] **is** the crate payload `N` directly — each variant
//!   carries the agent it targets ([`NodeKind::agent`]); the two resource
//!   classes are [`resource::Resource`] (`BuildSlot` node-held, `Agent` lease
//!   subtree-held), declared per node at its construction site;
//! - a **DAG is a single container node** ([`NodeKind::Dag`], `parent = None`)
//!   carrying the group's metadata, with the work nodes hung under it as
//!   its subtree (the **parent axis** groups; `deps` order). So the container's
//!   `NodeId` is the DAG id, its rolled-up state is the DAG state, and membership
//!   is a graph walk — there are no host grouping side-tables. The lease is owned
//!   by a subtree root and borrowed by its descendants (continuity);
//! - per-DAG terminal work is an ordinary **tail node**
//!   ([`NodeKind::ResolveApproval`] / [`NodeKind::EmitRebuilt`]) that the builder
//!   appends in [`templates`], edged onto the DAG's other group roots by the
//!   outcome it reports. Templates emit one tail per outcome and the graph runs
//!   exactly one, so nothing branches at runtime.
//!
//! The queue is runtime-only (no persistence): an empty graph on boot; desired
//! state is re-derived by the reconcile sweep. A single scheduler task
//! ([`scheduler::run_worker`]) drives it; concurrency comes from the build-slot
//! capacity, not multiple workers. Design: `docs/coordinator.md::Job queue`.

pub mod exec;
pub mod model;
pub mod resource;
pub mod scheduler;
pub mod submit;
pub mod templates;
#[cfg(test)]
mod tests;

use std::sync::{Arc, Mutex};

use chrono::{DateTime, Utc};
use hive_host_sock::jobs::NodeView;
use hive_jobq::resources::ResourceTable;
use hive_jobq::scheduler::{Outcome, Scheduler};
use hive_jobq::{Dep, Graph, NodeId};
use hive_jobq_wire::{GraphNode, GraphWire};
use tokio::sync::Notify;

pub use hive_jobq::TerminalState;
pub use model::{DagView, NodeKind, PermPayload, Source, State};
use resource::Resource;

/// A job under construction: `hive_jobq`'s builder over this queue's payload
/// ([`NodeKind`]) and resource ([`Resource`]) types. Templates declare into a
/// borrowed one; only `hive_jobq` can make or insert it.
pub type JobBuilder = hive_jobq::JobBuilder<NodeKind, Resource>;

/// A handle to one node a template declared — where its edges, grouping and
/// resources are declared. `Copy`; naming a node as a dependency does not
/// consume the ability to name it again.
pub type Handle<'a> = hive_jobq::NodeRef<'a, NodeKind, Resource>;

/// How many terminal DAGs (`Done` / `Failed` / `Cancelled`) the snapshot
/// retains, newest first. A flat cap over the whole sorted list: the
/// dashboard renders one recent-builds list, so one number bounds it.
const MAX_HISTORY_DAGS: usize = 50;

/// Cap on stored node error strings.
const MAX_ERROR_LEN: usize = 2_000;

/// One live transient pill, derived from a running node.
///
/// A named struct rather than a tuple because three of its four fields are
/// easy to confuse at a call site: two are strings and two answer questions
/// nobody should have to guess at ("is this the agent or the label?", "does
/// this bool mean deliberate or running?").
#[derive(Debug, Clone)]
pub struct RunningTransient {
    /// The agent whose lease the node declared.
    pub agent: String,
    /// The node's own wire tag, rendered as the pill.
    pub label: String,
    /// Whether this operation is expected to take the container down — the
    /// crash watcher's input. See [`NodeKind::takes_container_down`].
    pub takes_container_down: bool,
    /// When the node started running, so the dashboard can tick elapsed
    /// seconds. Taken from the node itself, which is the true start of the
    /// operation rather than the moment a watcher noticed it.
    pub since: DateTime<Utc>,
}

/// The crate scheduler, specialised to this host's node + resource types.
///
/// A **DAG is a single container node** ([`NodeKind::Dag`], `parent = None`)
/// whose subtree is the DAG's work — so the container's `NodeId` is the DAG id,
/// its rolled-up state is the DAG state, and there are no grouping side-tables:
/// membership + meta are graph queries ([`container`] + the `hive_jobq::Graph`
/// accessors, with the meta read straight off the container's payload). One
/// shared crate [`Graph`] holds every DAG.
///
/// There is deliberately **no wrapper struct and no per-node side map**. The
/// last map held the `build_logs` row id; that link now lives on the log row
/// itself (`build_logs.node_id`). With nothing else to guard, the mutex holds
/// the scheduler *directly* — which is what lets `hive_jobq` drive the run loop
/// (it takes `&Arc<Mutex<Scheduler<..>>>`, a type a host-side wrapper could not
/// satisfy).
type Sched = Scheduler<NodeKind, Resource>;

/// The queue. Lives on `Coordinator` (one per hive-c0re process); a single
/// scheduler task ([`scheduler::run_worker`]) drives it.
pub struct JobQueue {
    /// The scheduler, held directly rather than behind a host-side wrapper —
    /// `hive_jobq`'s run-loop seam takes `&Arc<Mutex<Scheduler<..>>>`, so this
    /// *is* the type the crate drives.
    sched: Arc<Mutex<Sched>>,
    /// Wakes the scheduler when something new arrives or state changed.
    pub(crate) notify: Notify,
}

impl std::fmt::Debug for JobQueue {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("JobQueue").finish_non_exhaustive()
    }
}

impl Default for JobQueue {
    fn default() -> Self {
        Self::new(1)
    }
}

/// A node runner's `Result` as the scheduler's [`Outcome`].
///
/// The failure reason + `finished_at` are stamped onto the graph `Node` by the
/// scheduler (the reason rides `Outcome::Failed`); there is no host-side copy,
/// so nothing needs clearing on success.
fn outcome_of(result: Result<(), String>) -> Outcome {
    match result {
        Ok(()) => Outcome::Done,
        Err(e) => Outcome::Failed(truncate_error(&e)),
    }
}

/// Insert a declared `job` into the shared graph, returning the inserted ids.
///
/// A node that declared no parent hangs under `group_parent` — the DAG
/// container for a template, the emitting node for a runtime-appended
/// subgraph. Templates declare the parent axis + sibling ordering directly, so
/// there is no dep-on-root to drop and no lease to hoist: each node declares
/// its own resources, and the crate's borrow model keeps a resource continuous
/// across a subtree (a root owns it, descendants borrow it). Independent group
/// roots carry no cross-links, so a multi-agent DAG's per-agent subgraphs run
/// concurrently, each on its own lease.
///
/// # Errors
/// Propagates a crate graph-insert error (malformed dep/parent / dep-scope).
fn insert_group(
    inner: &mut Sched,
    declare: impl FnOnce(&JobBuilder),
    group_parent: Option<NodeId>,
) -> anyhow::Result<()> {
    inner
        .insert_job(group_parent, |b| {
            declare(b);
            // c0re names no handles: a DAG is addressed by its container node,
            // which `submit` inserts itself, and nothing downstream looks an
            // individual step up by id.
            Vec::new()
        })
        .map_err(|e| anyhow::anyhow!("job_queue: graph insert failed: {e}"))?;
    Ok(())
}

impl JobQueue {
    #[must_use]
    pub fn new(build_slots: usize) -> Self {
        let mut table = ResourceTable::new();
        table.set_capacity(
            Resource::BuildSlot,
            u32::try_from(build_slots.max(1)).unwrap_or(u32::MAX),
        );
        Self {
            sched: Arc::new(Mutex::new(Scheduler::new(Graph::new(), table))),
            notify: Notify::new(),
        }
    }

    fn lock(&self) -> std::sync::MutexGuard<'_, Sched> {
        self.sched.lock().expect("job_queue mutex poisoned")
    }

    /// Submit a DAG: insert a [`NodeKind::Dag`] **container node** carrying the
    /// group's metadata, then insert the template's nodes as its subtree (their
    /// roots re-parented to the container). Returns the container's id as the
    /// DAG id — its rolled-up state is the DAG state.
    ///
    /// The container is an ordinary node: it declares no resources, so the
    /// scheduler claims it on the next pass, runs its (empty) logic and parks
    /// it in `Finishing`, at which point its children become runnable. Nothing
    /// here completes it by hand — a node with no work of its own still goes
    /// the way every other node goes.
    ///
    /// `source` and `reason` are the container node's own payload — they are
    /// arguments here rather than fields of a spec struct because that is all
    /// they ever were. `declare` is the recipe, taken by generic and run
    /// against a builder `hive_jobq` owns: it goes from the template straight
    /// into this call, so there is nothing to allocate for.
    ///
    /// # Errors
    /// Propagates a graph-insert error (dependencies that aren't
    /// dependency-topological).
    pub fn submit(
        &self,
        source: Source,
        reason: String,
        declare: impl FnOnce(&JobBuilder),
    ) -> anyhow::Result<u64> {
        let mut inner = self.lock();
        let container = inner
            .append(NodeKind::Dag { source, reason }, Vec::new(), None)
            .map_err(|e| anyhow::anyhow!("job_queue: container insert failed: {e}"))?;
        insert_group(&mut inner, declare, Some(container))?;
        drop(inner);
        self.notify.notify_one();
        Ok(container.get())
    }

    /// The scheduler itself, for `hive_jobq`'s run-loop seam
    /// (`Scheduler::claim_next`), which takes exactly this type.
    ///
    /// Handing out the `Arc` rather than wrapping each crate call keeps the
    /// host from growing a parallel API: the run loop uses `hive_jobq`'s
    /// functions directly, and this module stays the thin glue it is being
    /// reduced to.
    pub(crate) fn sched(&self) -> &Arc<Mutex<Sched>> {
        &self.sched
    }

    /// The DAG container id owning `node`, for log lines and the dashboard.
    /// Derived from the graph rather than carried alongside the node — the
    /// parent axis already knows it.
    #[must_use]
    pub fn dag_of(&self, node: NodeId) -> Option<u64> {
        self.lock().graph().root_of(node).map(NodeId::get)
    }

    /// Cancel a DAG that hasn't started yet: every work node is still `Pending`,
    /// so each is cancelled. `false` once any work node is running or terminal —
    /// an in-flight nix build isn't interruptible.
    ///
    /// **Nodes that explicitly observe cancellation are spared** — a node whose
    /// edge names [`hive_jobq::TerminalState::Cancelled`] is asking to run when
    /// the work it follows was dropped, which is exactly what an approval tail
    /// needs: cancel the work, and the tail still fires to resolve the approval
    /// row rather than leaving it dangling forever.
    ///
    /// Nothing is special-cased by node kind. `AFTER_ANY` deliberately does *not*
    /// accept `Cancelled`, so an ordinary weak-edged step (rebuild's `Reconcile`,
    /// say) is cancelled along with everything else — there is nothing to converge
    /// when no node ever ran. Only a node that named `Cancelled` survives, and it
    /// survives because it asked to.
    ///
    /// `id` names **any node**, not specifically a DAG. Cancelling a group root
    /// drops that whole group (the cascade is the scheduler's), which is what
    /// the dashboard's whole-DAG cancel does; cancelling an interior node drops
    /// just that branch. Nothing here knows about DAGs.
    pub fn cancel(&self, id: u64) -> bool {
        let mut inner = self.lock();
        let Some(node) = inner.graph().resolve_id(id) else {
            return false;
        };
        if !inner.cancel_node(node) {
            return false;
        }
        drop(inner);
        self.notify.notify_one();
        true
    }

    /// The first failed node's error in `dag_id`, if any has failed yet.
    ///
    /// Unlike the roll-up summary this is readable *mid-flight*, which is the
    /// point: a compensation node runs `AfterAny` its subject, so when it asks,
    /// the DAG is still `Finishing` (the compensation node itself is running)
    /// while the node it is compensating for has already settled `Failed`. That
    /// lets the compensation annotate its bookkeeping with the reason the deploy
    /// failed, instead of having the error handed down from the node that hit
    /// it. `None` when nothing has failed — the ordinary success path.
    #[must_use]
    pub fn first_error(&self, dag_id: u64) -> Option<String> {
        let inner = self.lock();
        let node = find_node(&inner, dag_id)?;
        inner.graph().first_error(node).map(ToOwned::to_owned)
    }

    /// `(agent, label, takes_container_down)` for the live transient-pill set,
    /// recomputed from the nodes **actually running** — not from an intent a
    /// template declared at submit time. (A rebuild used to report `rebuilding`
    /// for its whole life: prebuild, stop, swap, tail and reconcile alike.)
    ///
    /// **Status is the only test**: every `Running` node that names an agent is
    /// in the set. Naming is targeting, not lease-holding — `Prebuild` /
    /// `MetaSync` are lease-exempt (the container keeps serving through them)
    /// but they *are* work on that agent, and the operator wants to see it.
    ///
    /// ⚠️ **So there can be more than one entry per agent**, which is the whole
    /// difference from the older lease-declaration test: lease-exemption is
    /// exactly what lets one DAG build for `a` while another holds `a`'s lease,
    /// so both are running and both name `a`. Anything keying this set by agent
    /// alone will silently drop one — see [`super::scheduler`].
    ///
    /// `label` is the node's own wire tag ([`NodeKind::as_str`]), the vocabulary
    /// [`NodeView::kind`] already ships, so a pill and a DAG node name an
    /// operation identically. `takes_container_down` is the crash watcher's
    /// input and does **not** ride the wire to the frontend — a `Start` pill and
    /// a `Stop` pill are both pills; only one means a vanished container is
    /// expected.
    ///
    /// Not the lease *owner* either: `resource_state()` answers "who holds the
    /// slot", a different question.
    #[must_use]
    pub fn running_transients(&self) -> Vec<RunningTransient> {
        let inner = self.lock();
        inner
            .graph()
            .nodes()
            .filter(|n| matches!(n.state, State::Running))
            .filter_map(|n| {
                // Status is the only test. The agent comes off the node's own
                // payload, not off a declared `Resource::Agent` edge: the
                // lease-exempt kinds (`Prebuild` / `MetaSync`) name an agent
                // without declaring its lease, and they are work on that agent
                // that the operator wants to see.
                //
                // Empty means an agentless container kind (`MetaLock`, `Dag`),
                // which targets no agent and lights nothing.
                let agent = n.payload.agent();
                if agent.is_empty() {
                    return None;
                }
                Some(RunningTransient {
                    agent: agent.to_owned(),
                    label: n.payload.as_str().to_owned(),
                    takes_container_down: n.payload.takes_container_down(),
                    // `started_at` is set when a node enters `Running`, and this
                    // only sees `Running` nodes — the fallback is unreachable in
                    // practice, and "just now" is the honest answer if it isn't.
                    since: n.started_at.unwrap_or_else(Utc::now),
                })
            })
            .collect()
    }

    /// Every node of every visible group, as generic graph nodes.
    ///
    /// **Nothing is hidden.** Group roots ride as ordinary nodes (so a
    /// consumer needs no special case for "the container" and reads the
    /// root's own `state` as the group's answer), and `Done` nodes stay (so a
    /// finished step is visible rather than vanishing from the payload, which
    /// is what makes a fast rebuild render as a single node).
    ///
    /// The projection itself is [`hive_jobq_wire`]'s; all this layer supplies
    /// is *which* groups to show — see [`visible_roots`] for why the graph
    /// can't decide that for itself.
    #[must_use]
    pub fn graph_snapshot(&self) -> Vec<GraphNode> {
        let inner = self.lock();
        inner.graph().wire_snapshot(visible_roots(&inner))
    }

    /// Snapshot every live + retained DAG for `/api/state` + `RebuildQueueChanged`.
    #[must_use]
    pub fn snapshot(&self) -> Vec<DagView> {
        let inner = self.lock();
        let mut ids = visible_dags(&inner);
        ids.sort_unstable_by_key(|c| c.get());
        ids.into_iter()
            .filter_map(|c| dag_view(&inner, c))
            .collect()
    }

    /// One or more nodes plus their live subtrees, as generic wire nodes —
    /// the `QueueNodes` polling surface behind `hivectl`'s wait/progress
    /// loop. Sibling of [`Self::snapshot`] (which serves the same graph
    /// through the typed `DagView`/`NodeView` projection for the
    /// dashboard's `/api/state.rebuild_queue`), this one goes through
    /// [`GraphWire::wire_snapshot`] instead — no `Done`-node filtering, no
    /// roll-up field (a node's own `state` answers that, see
    /// `hive_jobq_wire`'s doc comment). Looks each id up by identity
    /// alone — no assumption that it names a DAG container or a root;
    /// "just show whatever the backend sends" for whatever ids the caller
    /// asks about. Multiple ids in one call is the normal shape for a
    /// batch op (e.g. restarting every agent submits one root per agent) —
    /// callers should request the whole batch together rather than poll
    /// one id per round-trip.
    ///
    /// An id with no matching node in the graph is silently dropped from
    /// the result rather than erroring the whole batch — some ids in a
    /// batch may already be evicted while others are still live. Today
    /// that only happens for a genuinely unknown id: nothing prunes the
    /// graph yet (bounded-prune is a Stage-C follow-up, see
    /// [`visible_dags`]), so a *completed* DAG's nodes keep riding here
    /// with a terminal `state` rather than disappearing — callers
    /// watching for "done" should read the root's `state`, not absence.
    #[must_use]
    pub fn node_subtrees(&self, ids: &[u64]) -> Vec<GraphNode> {
        let inner = self.lock();
        let roots: Vec<NodeId> = ids.iter().filter_map(|id| find_node(&inner, *id)).collect();
        inner.graph().wire_snapshot(roots)
    }
}

/// The graph node whose id equals `id`, whatever its kind or depth.
/// `NodeId` is un-fabricable from a raw `u64`, so this is a search.
fn find_node(sched: &Sched, id: u64) -> Option<NodeId> {
    sched
        .graph()
        .nodes()
        .find_map(|n| (n.id.get() == id).then_some(n.id))
}

/// Project a DAG into its wire [`DagView`]: a near-raw view of the
/// container's work nodes, with `Done` nodes excluded. Lifecycle
/// (`state` / `started_at` / `finished_at` / `error`) is read straight
/// off each `hive_jobq::Node`; the client derives the DAG label, roll-up
/// state, and DAG timestamps from the node set. Non-derivable per-node
/// payload (`approval_id`, meta `inputs`) rides the owning node. Returns
/// `None` when every work node is `Done` or `Skipped` — a fully-settled
/// DAG drops out of the snapshot entirely (a `Failed` one lingers until
/// aged out).
fn dag_view(sched: &Sched, container: NodeId) -> Option<DagView> {
    // Read straight off the container's payload: the domain fields have a
    // single home there, so an intermediate owned copy of them was a second
    // type describing the same data rather than a grouping side-table.
    //
    // `created_at` is NOT among them — it comes off the container node itself
    // below, where the graph stamps it for every node. Keeping a payload copy
    // would record one instant in two places.
    let node = sched.graph().node(container)?;
    let NodeKind::Dag { source, reason } = &node.payload else {
        return None;
    };
    let all: Vec<_> = sched.graph().descendants(container).collect();
    // DAG-level timestamps are taken over *all* subtree nodes (including the
    // `Done` ones excluded from the wire) — the client can't derive them
    // from a `Done`-filtered node set, so the host computes them here.
    let mut started: Vec<DateTime<Utc>> = Vec::new();
    let mut finished: Vec<DateTime<Utc>> = Vec::new();
    for node in &all {
        if let Some(s) = node.started_at {
            started.push(s);
        }
        if let Some(f) = node.finished_at {
            finished.push(f);
        }
    }
    // Decide which nodes ride the wire *before* projecting any of them: a
    // `NodeView` costs a `build_logs` lookup, so building one for a node
    // that's about to be dropped would be a query per finished step.
    let shown = shown_on_wire(&all.iter().map(|n| n.state).collect::<Vec<_>>())?;
    let mut nodes = Vec::new();
    for node in shown.into_iter().map(|i| all[i]) {
        let id = node.id;
        let deps: Vec<u64> = node
            .deps
            .iter()
            .filter_map(|d| match d {
                Dep::Node { id, .. } => Some(id.get()),
                Dep::Resource { .. } => None,
            })
            .collect();
        // Non-derivable per-node payload rides the node that owns it. Every
        // deploy phase carries the approval id, but only the subtree root
        // projects it onto the wire — hanging the approval link off all of
        // them would render the same card once per phase.
        let approval_id = match &node.payload {
            NodeKind::DeployWindow { approval_id, .. } => Some(*approval_id),
            _ => None,
        };
        let inputs = match &node.payload {
            NodeKind::MetaLock { inputs, .. } => inputs.clone(),
            _ => Vec::new(),
        };
        // Looked up from the log row itself (`build_logs.node_id`), not a
        // host-side map. One indexed query per node in the snapshot; the
        // node set is bounded by `MAX_HISTORY_DAGS` and the store is a
        // local sqlite file, so this is cheaper than the lock contention
        // a second shared map would reintroduce.
        let build_log_id = crate::build_logs::global().and_then(|h| h.id_for_node(id.get()));
        // `node.parent` is the structural jobq parent. Top-level nodes
        // have `parent == Some(container)` (direct children of the Dag
        // container); those become `parent: None` on the wire since the
        // container itself is not part of the work-node payload. Sub-nodes
        // carry the id of their containing parent work-node.
        let parent = node
            .parent
            .filter(|&p| p != container)
            .map(hive_jobq::NodeId::get);
        nodes.push(NodeView {
            id: id.get(),
            agent: node.payload.agent().to_owned(),
            kind: node.payload.as_str().to_owned(),
            deps,
            state: node.state,
            started_at: node.started_at,
            finished_at: node.finished_at,
            error: node.error.clone(),
            approval_id,
            inputs,
            build_log_id,
            parent,
        });
    }
    let is_terminal = sched.graph().is_settled(container) == Some(true);
    Some(DagView {
        id: container.get(),
        source: *source,
        reason: reason.clone(),
        created_at: node.created_at,
        started_at: started.into_iter().min(),
        finished_at: is_terminal.then(|| finished.into_iter().max()).flatten(),
        nodes,
    })
}

/// Which of a DAG's work nodes ride the wire, by index into `states` — or
/// `None` when the DAG has nothing left worth showing and drops out of the
/// snapshot entirely.
///
/// Two separate decisions, and conflating them pins every completed deploy in
/// the queue view forever:
/// - **`Done` drops off the wire.** A finished step isn't interesting.
///   `Skipped` stays: which branch a run *didn't* take is the readable half of
///   an outcome-branched DAG.
/// - **`Skipped` alone doesn't hold a DAG in the snapshot.** So "the node list
///   is non-empty" and "there's still something here worth showing" are
///   different questions, and only the second one may drop the DAG.
///
/// Takes states rather than projected nodes so the caller can skip the work of
/// projecting what it's about to discard, and so this is testable without a
/// graph — the states it keys on are ones only a run can produce.
fn shown_on_wire(states: &[State]) -> Option<Vec<usize>> {
    let worth_showing = states
        .iter()
        .any(|s| !matches!(s, State::Done | State::Skipped));
    if !worth_showing {
        return None;
    }
    Some(
        states
            .iter()
            .enumerate()
            .filter(|(_, s)| !matches!(s, State::Done))
            .map(|(i, _)| i)
            .collect(),
    )
}

/// When a DAG's work node finishes on `finished_at` — the max over its
/// subtree (read off the graph `Node`, as unix seconds), for the history
/// cap ordering.
fn dag_finished_at(sched: &Sched, container: NodeId) -> i64 {
    sched
        .graph()
        .descendants(container)
        .filter_map(|n| n.finished_at)
        .map(|t| t.timestamp())
        .max()
        .unwrap_or(0)
}

/// Every DAG container node id in the graph.
fn containers(sched: &Sched) -> Vec<NodeId> {
    sched
        .graph()
        .nodes()
        .filter(|n| n.parent.is_none() && matches!(n.payload, NodeKind::Dag { .. }))
        .map(|n| n.id)
        .collect()
}

/// The **visible** DAG set for the snapshot: every live (non-terminal) DAG,
/// plus the newest [`MAX_HISTORY_DAGS`] terminal ones. Crate nodes for
/// evicted DAGs linger in the graph (bounded-prune is a Stage-C follow-up);
/// this filter is what bounds what the dashboard sees.
fn visible_dags(sched: &Sched) -> Vec<NodeId> {
    let mut live: Vec<NodeId> = Vec::new();
    let mut terminal: Vec<(NodeId, i64, u64)> = Vec::new();
    for c in containers(sched) {
        if sched.graph().is_settled(c) == Some(true) {
            terminal.push((c, dag_finished_at(sched, c), c.get()));
        } else {
            live.push(c);
        }
    }
    retain_history(live, terminal, MAX_HISTORY_DAGS)
}

/// The visible **group** set for [`Queue::graph_snapshot`]: every live group
/// root, plus the newest [`MAX_HISTORY_DAGS`] settled ones.
///
/// Same policy as [`visible_dags`], selected *structurally* — a root is a node
/// with no parent. The `DagView` path next door keys on `NodeKind::Dag`
/// instead, which is fine for a projection that already only means anything to
/// hive-c0re, but would make the generic endpoint depend on one node kind that
/// is itself slated for removal.
///
/// **This bound is load-bearing, not tidiness.** Nothing ever removes a node
/// from the graph (bounded pruning is a Stage-C follow-up), so serving
/// `graph.roots()` directly would grow the payload without limit for the whole
/// uptime of the daemon.
fn visible_roots(sched: &Sched) -> Vec<NodeId> {
    let roots: Vec<NodeId> = sched.graph().roots().map(|n| n.id).collect();
    let mut live: Vec<NodeId> = Vec::new();
    let mut terminal: Vec<(NodeId, i64, u64)> = Vec::new();
    for root in roots {
        if sched.graph().is_settled(root) == Some(true) {
            terminal.push((root, group_finished_at(sched, root), root.get()));
        } else {
            live.push(root);
        }
    }
    retain_history(live, terminal, MAX_HISTORY_DAGS)
}

/// When a whole group last finished: the newest `finished_at` across the root
/// **and** its descendants. Unlike [`dag_finished_at`] the root itself counts,
/// because a generic group root can be an ordinary node with no children at
/// all — reading only descendants would date every such group to the epoch and
/// evict it first.
fn group_finished_at(sched: &Sched, root: NodeId) -> i64 {
    sched
        .graph()
        .node(root)
        .and_then(|n| n.finished_at)
        .into_iter()
        .chain(
            sched
                .graph()
                .descendants(root)
                .filter_map(|n| n.finished_at),
        )
        .map(|t| t.timestamp())
        .max()
        .unwrap_or(0)
}

/// [`visible_dags`]'s policy, split from the graph it reads: keep every live
/// DAG, plus the newest `cap` terminal ones.
///
/// `terminal` rows are `(handle, finished_at, tiebreak)`. The tiebreak orders
/// DAGs that settled inside the same wall-clock second — which is *most* of
/// them under a burst, and all of them in a test, so it is load-bearing rather
/// than a formality.
///
/// Generic over the handle purely so this is reachable without a graph: a
/// `NodeId` cannot be fabricated, so a test that had to pass real ones could
/// only get them by submitting and running DAGs.
fn retain_history<T>(live: Vec<T>, mut terminal: Vec<(T, i64, u64)>, cap: usize) -> Vec<T> {
    // Newest first, so truncating to the cap keeps the most recent.
    terminal.sort_by(|a, b| b.1.cmp(&a.1).then(b.2.cmp(&a.2)));
    terminal.truncate(cap);
    let mut kept = live;
    kept.extend(terminal.into_iter().map(|(handle, _, _)| handle));
    kept
}

/// Truncate a node error to [`MAX_ERROR_LEN`] on a char boundary, appending `…`.
fn truncate_error(e: &str) -> String {
    if e.len() <= MAX_ERROR_LEN {
        return e.to_owned();
    }
    let cut = (0..=MAX_ERROR_LEN)
        .rev()
        .find(|i| e.is_char_boundary(*i))
        .unwrap_or(0);
    let mut msg = e[..cut].to_owned();
    msg.push('…');
    msg
}