mesh-controller/internal/inventory/walkphases.go

package inventory

import (
	"context"
	"fmt"
	"sort"
	"strings"
	"time"
)

// Where a walk's time went (novox/hq ADR 0282 decision 6, issue 382): from its merge to every machine of its
// rest running its builds, phase by phase, worked out from the walk's own moments. Measurement only: nothing
// here decides anything about the walk.
//
// **A phase that cannot be measured is said unknown, never zero.** Where a moment is missing (a walk kept
// before it was recorded, a machine not yet reported), the phases around it are unknown, and the span between
// the moments on either side is counted as unknown time: the measured phases and the unknown time always add
// up to the total. A phase that did not happen (no window for a merge walked alone, no first machine for a
// module nobody runs) is said none, with no time.

// The phases, in the order a walk passes them (ADR 0282's table).
const (
	PhaseWindow   = "window"        // the merge to its batch's window closing
	PhaseQueued   = "queued"        // the window closed to the walk being cut: waiting behind another walk
	PhaseWord     = "word"          // the cut to the delivery's word, for a walk that waits for one
	PhaseBetween  = "between-tiers" // one tier's end to the next tier's ask
	PhaseBuild    = "build"         // a tier asked to its last module built
	PhaseSend     = "send-first"    // built to sent to the first machines
	PhaseBusy     = "busy-wait"     // sent first to a busy container there replaced, when one waited (ADR 0335)
	PhaseJudge    = "judgement"     // sent first to the first-node gate's last verdict: its readings
	PhaseRest     = "send-rest"     // judged to sent to the rest
	PhaseApply    = "apply"         // the last send to every machine of the rest reporting the build applied
	PhaseOpen     = "open"          // a walk still running: since its last moment
	PhaseMeasured = "measured"
	PhaseUnknown  = "unknown"
	PhaseNone     = "none"
)

// The classes of a walk (ADR 0282 decision 1) and their delivery budgets.
const (
	ClassCore = "core"
	ClassLeaf = "leaf"
)

// ApplySilentAfter is how long after a send to the rest a machine that has said nothing is left out of the
// walk's end, as a machine not heard from (ADR 0282 decision 1: a sleeping laptop is listed, not waited for).
const ApplySilentAfter = 15 * time.Minute

// WalkPhases is a walk's time, phase by phase.
type WalkPhases struct {
	Class string `json:"class,omitempty"`
	// From is the walk's earliest merge; End when its last phase ended: every machine of its rest reported the
	// build applied. Nil End while that is not known.
	From *time.Time `json:"from,omitempty"`
	End  *time.Time `json:"end,omitempty"`
	// TotalMS is End − From; zero while either is unknown, Total its words.
	TotalMS int64  `json:"total_ms,omitempty"`
	Total   string `json:"total,omitempty"`
	// UnknownMS is the time within the walk no phase could be measured over.
	UnknownMS int64       `json:"unknown_ms,omitempty"`
	Phases    []WalkPhase `json:"phases"`
	Silent    []string    `json:"silent,omitempty"`
	// Rest is each machine of the rest's own report of the send that carried the walk's build there (novox/hq ADR
	// 0282 decision 6, issue 582): when it was sent, when it said, and what.
	Rest   []RestReport `json:"rest,omitempty"`
	Said   string       `json:"said"`
	Merges []MergeStart `json:"merges,omitempty"`
}

// MergeStart is one merge the walk answers and when it was made: where that merge's delivery time starts.
type MergeStart struct {
	Repository string    `json:"repository"`
	Commit     string    `json:"commit"`
	Merged     time.Time `json:"merged"`
}

// WalkPhase is one phase of a walk.
type WalkPhase struct {
	Name string `json:"name"`
	// Tier is the tier a tier's phase belongs to; -1 for a phase of the walk.
	Tier   int        `json:"tier"`
	State  string     `json:"state"`
	Start  *time.Time `json:"start,omitempty"`
	End    *time.Time `json:"end,omitempty"`
	TookMS int64      `json:"took_ms,omitempty"`
	Took   string     `json:"took,omitempty"`
	Said   string     `json:"said,omitempty"`
}

// AppliedReport is a machine's first report, after a send, that it applied what it was sent.
type AppliedReport struct {
	At      time.Time
	Outcome string
	// Answered is, for a machine that waited for a busy container (novox/hq ADR 0335), its first report of the
	// send; Busy how long it then waited; Waiting that it still waits, At being its first report then.
	Answered time.Time
	Busy     time.Duration
	Waiting  bool
}

// AppliedLookup answers a machine's report of the send the walk made to it: send is what the walk kept of it, and
// walkSent the walk's own record of its send to the rest; false when it has not reported.
type AppliedLookup func(node string, send SentDeclaration, walkSent time.Time) (AppliedReport, bool)

// RestReport is one machine of the rest and its own report of a send of the walk (novox/hq issue 582).
type RestReport struct {
	Node    string   `json:"node"`
	Modules []string `json:"modules"`
	// Sent is when that machine was sent, by its own send time where the walk kept it; else the walk's record of
	// its send to the rest, and SentSaid says so.
	Sent     time.Time  `json:"sent"`
	SentSaid string     `json:"sent_said,omitempty"`
	Sequence int64      `json:"sequence,omitempty"`
	Reported *time.Time `json:"reported,omitempty"`
	Took     string     `json:"took,omitempty"`
	Outcome  string     `json:"outcome,omitempty"`
	// Busy is how long it waited for a busy container before it ran the build (novox/hq ADR 0335): inside Took,
	// never read as the machine being silent.
	Busy string `json:"busy,omitempty"`
	// State is applied, failed, waiting (not yet heard, within ApplySilentAfter), busy (it answered and waits for
	// a busy container: never silent) or silent (not heard within it: listed, never moving the walk's end).
	State string `json:"state"`
	busy  time.Duration
}

// The states of a machine of the rest.
const (
	RestApplied = "applied"
	RestFailed  = "failed"
	RestWaiting = "waiting"
	RestBusy    = "busy"
	RestSilent  = "silent"
)

// point is one moment of the walk, ending the phase named.
type point struct {
	phase string
	tier  int
	at    *time.Time
	none  bool   // the phase did not happen
	said  string // why it is none, or unknown
}

// Phases is the walk's time phase by phase, at now; applied answers the rest's reports (nil: none read).
func (p Plan) WalkPhases(now time.Time, applied AppliedLookup) *WalkPhases {
	if p.Release != nil || p.Batch() {
		return nil
	}
	w := &WalkPhases{}
	if p.Times != nil {
		w.Class = p.Times.Class
	}
	from := p.firstMerge(w)
	if from != nil {
		f := from.Truncate(time.Millisecond)
		from = &f
	}
	if from == nil {
		w.Said = "when its merge was made is not kept: its delivery time is unknown"
	} else {
		w.From = from
	}
	points := p.points(now, applied, w)
	// Walk the moments: each known moment after a known one is a measured phase; a missing moment makes the
	// phases up to the next known one unknown, and their span unknown time.
	last := from
	var pending []int
	for _, pt := range points {
		if pt.none {
			w.Phases = append(w.Phases, WalkPhase{Name: pt.phase, Tier: pt.tier, State: PhaseNone, Said: pt.said})
			continue
		}
		ph := WalkPhase{Name: pt.phase, Tier: pt.tier, Said: pt.said}
		if pt.at == nil {
			ph.State = PhaseUnknown
			w.Phases = append(w.Phases, ph)
			pending = append(pending, len(w.Phases)-1)
			continue
		}
		at := pt.at.Truncate(time.Millisecond)
		ph.End = &at
		if last != nil && at.Before(*last) {
			// **Out of order** (issue 382): this moment was recorded before the one before it — a module's first
			// send kept before its build was, two clocks, a late record. Neither phase can be measured honestly:
			// the one before it is said unknown and its time joins the unknown time, and this one is said unknown
			// with no time. The walk goes on from the later moment, so nothing is negative and the sum holds.
			for i := len(w.Phases) - 1; i >= 0; i-- {
				prev := &w.Phases[i]
				if prev.End == nil {
					continue // a phase that did not happen, or is unknown with no moment: the one before carries last
				}
				if prev.State == PhaseMeasured {
					w.UnknownMS += prev.TookMS
				}
				prev.State, prev.Start, prev.TookMS, prev.Took = PhaseUnknown, nil, 0, ""
				prev.Said = "out of order: the phase after it ended first"
				break
			}
			ph.State, ph.Said = PhaseUnknown, "out of order: it ended before the phase before it"
			w.Phases = append(w.Phases, ph)
			pending = nil
			continue
		}
		if last != nil && len(pending) == 0 {
			start := *last
			ph.State, ph.Start = PhaseMeasured, &start
			ph.TookMS = at.Sub(start).Milliseconds()
			ph.Took = words(at.Sub(start))
		} else {
			ph.State = PhaseUnknown
			if last != nil {
				w.UnknownMS += at.Sub(*last).Milliseconds()
			}
		}
		w.Phases = append(w.Phases, ph)
		pending = nil
		last = &at
	}
	if len(pending) > 0 {
		// The walk's last moments are unknown: it has no end.
		if w.Said == "" {
			w.Said = "its end is unknown: " + w.Phases[pending[0]].Name + " " + orNot(w.Phases[pending[0]].Said)
		}
		return w
	}
	if last == nil || from == nil {
		return w
	}
	if p.Open() {
		since := *last
		w.Phases = append(w.Phases, WalkPhase{Name: PhaseOpen, Tier: -1, State: PhaseOpen, Start: &since,
			TookMS: now.Sub(since).Milliseconds(), Took: words(now.Sub(since)), Said: "the walk is " + p.State})
		w.Said = "open: " + p.State + ", " + words(now.Sub(*from)) + " since its merge"
		return w
	}
	if p.State != PlanDone {
		w.Said = "ended " + p.State + ": no delivery time"
		return w
	}
	end := *last
	w.End = &end
	w.TotalMS = end.Sub(*from).Milliseconds()
	w.Total = words(end.Sub(*from))
	if w.Said == "" {
		if w.UnknownMS > 0 {
			w.Said = fmt.Sprintf("%s from its merge to running everywhere, %s of it unknown", w.Total,
				words(time.Duration(w.UnknownMS)*time.Millisecond))
		} else {
			w.Said = w.Total + " from its merge to running everywhere"
		}
	}
	return w
}

// firstMerge is when the walk's earliest merge was made, and every merge's start kept on w.
func (p Plan) firstMerge(w *WalkPhases) *time.Time {
	var first *time.Time
	if p.Delivery != nil {
		for _, m := range p.Delivery.Merges {
			if m.Merged.IsZero() {
				continue
			}
			w.Merges = append(w.Merges, MergeStart{Repository: m.Repository, Commit: m.Commit, Merged: m.Merged})
			if first == nil || m.Merged.Before(*first) {
				at := m.Merged
				first = &at
			}
		}
	}
	if first == nil {
		for _, c := range p.Carried() {
			if c.Merged.IsZero() {
				continue
			}
			w.Merges = append(w.Merges, MergeStart{Repository: c.Repository, Commit: c.Commit, Merged: c.Merged})
			if first == nil || c.Merged.Before(*first) {
				at := c.Merged
				first = &at
			}
		}
	}
	return first
}

// points are the walk's moments in order.
func (p Plan) points(now time.Time, applied AppliedLookup, w *WalkPhases) []point {
	var out []point
	// The window and the wait behind another walk.
	cut := p.Created
	if p.Times != nil && p.Times.Cut != nil {
		cut = *p.Times.Cut
	}
	switch {
	case p.Times == nil:
		out = append(out, point{phase: PhaseWindow, tier: -1, said: "not kept for a walk made before ADR 0282"},
			point{phase: PhaseQueued, tier: -1, at: &cut, said: "the window and the wait behind another walk together"})
	case p.Times.WindowClosed == nil:
		out = append(out, point{phase: PhaseWindow, tier: -1, none: true, said: "no window: walked on its own"},
			point{phase: PhaseQueued, tier: -1, at: &cut})
	default:
		closed := *p.Times.WindowClosed
		out = append(out, point{phase: PhaseWindow, tier: -1, at: &closed}, point{phase: PhaseQueued, tier: -1, at: &cut})
	}
	if p.Delivery != nil && p.Delivery.Awaits != "" {
		out = append(out, point{phase: PhaseWord, tier: -1, at: p.Delivery.Go, said: "waiting for " + p.Delivery.Awaits + "'s word"})
	} else {
		out = append(out, point{phase: PhaseWord, tier: -1, none: true, said: "waits for no word"})
	}
	var lastSends []restSend
	for t, tier := range p.Tiers {
		if t > p.Tier || (t == p.Tier && p.Tier < len(p.Tiers) && !askedAnyOf(p, tier)) {
			break
		}
		var asked, built, first, judged, rest, busyEnded *time.Time
		allBuilt, anyFirst, allJudged, allRest := true, false, true, true
		// A busy container on a first machine holds the judging until it is replaced (novox/hq ADR 0335 §4): a
		// phase of its own, only in a tier where one waited, counted in the delivery time and never exempted.
		anyBusy, allBusyEnded, busyOn := false, true, ""
		for _, m := range tier {
			s := p.Modules[m]
			if s == nil {
				allBuilt, allRest = false, false
				continue
			}
			asked = earliest(asked, s.AskedAt)
			if s.BuiltAt == nil {
				if s.State != "deleted" {
					allBuilt = false
				}
			} else {
				built = latest(built, s.BuiltAt)
			}
			if s.FirstAt != nil {
				anyFirst = true
				first = earliest(first, s.FirstAt)
				if s.Gate != nil && s.Gate.Busy != nil {
					anyBusy = true
					if s.Gate.Busy.Ended == nil {
						allBusyEnded, busyOn = false, s.Gate.Busy.Machine
					} else {
						busyEnded = latest(busyEnded, s.Gate.Busy.Ended)
					}
				}
				if s.Gate != nil {
					if s.Gate.JudgedAt == nil {
						allJudged = false
					} else {
						judged = latest(judged, s.Gate.JudgedAt)
					}
				}
			}
			if s.SentAt == nil {
				if s.State != "deleted" {
					allRest = false
				}
			} else {
				rest = latest(rest, s.SentAt)
				for node, what := range s.Rest {
					lastSends = append(lastSends, restSend{module: m, node: node, walkSent: *s.SentAt, what: what})
				}
			}
		}
		if t > 0 {
			out = append(out, point{phase: PhaseBetween, tier: t, at: asked})
		}
		if !allBuilt {
			built = nil
		}
		out = append(out, point{phase: PhaseBuild, tier: t, at: built})
		if anyFirst {
			if !allJudged {
				judged = nil
			}
			out = append(out, point{phase: PhaseSend, tier: t, at: first})
			if anyBusy {
				said := ""
				if !allBusyEnded {
					busyEnded, said = nil, "waits for a busy container on "+busyOn
				}
				out = append(out, point{phase: PhaseBusy, tier: t, at: busyEnded, said: said})
			}
			out = append(out, point{phase: PhaseJudge, tier: t, at: judged})
		} else {
			out = append(out, point{phase: PhaseSend, tier: t, none: true, said: "no first machine: nothing to judge"},
				point{phase: PhaseJudge, tier: t, none: true, said: "no first machine: nothing to judge"})
		}
		if !allRest {
			rest = nil
		}
		out = append(out, point{phase: PhaseRest, tier: t, at: rest})
	}
	if p.State != PlanDone {
		return out
	}
	// Every machine of the rest running the build: its first report after the send, applied.
	if p.Times == nil {
		return append(out, point{phase: PhaseApply, tier: -1, said: "the rest's sends are not kept for a walk made before ADR 0282"})
	}
	if len(lastSends) == 0 {
		return append(out, point{phase: PhaseApply, tier: -1, none: true,
			said: "no machine beyond the first: the first-node gate's readings were its run"})
	}
	if applied == nil {
		return append(out, point{phase: PhaseApply, tier: -1, said: "the rest's reports were not read"})
	}
	// Each machine of the rest, by its own report of the send that carried the build there (novox/hq issue 582):
	// never a report of a send made after it, which another walk's send may be.
	reports := map[string]*RestReport{}
	var order []string
	for _, s := range lastSends {
		at, said := s.walkSent, ""
		if !s.what.At.IsZero() {
			at = s.what.At
		} else {
			said = "the walk's record of its send to the rest: this machine's own send time was not kept"
		}
		key := s.node + "\x00" + s.what.Digest + "\x00" + at.Format(time.RFC3339Nano)
		if r := reports[key]; r != nil {
			r.Modules = appendOnce(r.Modules, s.module)
			continue
		}
		rr := &RestReport{Node: s.node, Modules: []string{s.module}, Sent: at, SentSaid: said, Sequence: s.what.Sequence}
		if r, ok := applied(s.node, s.what, s.walkSent); ok {
			got := r.At
			rr.Reported, rr.Outcome, rr.Took = &got, r.Outcome, words(r.At.Sub(at))
			if r.Busy > 0 {
				rr.Busy = words(r.Busy)
				rr.busy = r.Busy
			}
			if r.Waiting {
				rr.Reported, rr.Took, rr.State = nil, "", RestBusy
			}
		}
		reports[key] = rr
		order = append(order, key)
	}
	// A machine sent twice by the walk (one tier's rest, then the next's) that said nothing of the first send has
	// said it of the second, which carries the first's builds too: both are this walk's.
	for _, k := range order {
		rr := reports[k]
		if rr.Reported != nil {
			continue
		}
		for _, l := range order {
			later := reports[l]
			if later.Node == rr.Node && later.Sent.After(rr.Sent) && later.Reported != nil &&
				(rr.Reported == nil || later.Reported.Before(*rr.Reported)) {
				got := *later.Reported
				rr.Reported, rr.Outcome, rr.Took, rr.State = &got, later.Outcome, words(got.Sub(rr.Sent)), ""
			}
		}
	}
	var end *time.Time
	var waiting, failed, busy []string
	for _, k := range order {
		rr := reports[k]
		switch {
		case rr.State == RestBusy:
			// It answered and waits for a busy container: never silent, and the walk's end waits for it.
			busy = appendOnce(busy, rr.Node)
		case rr.Reported == nil && now.Sub(rr.Sent) > ApplySilentAfter,
			rr.Reported != nil && rr.Reported.Sub(rr.Sent)-rr.busy > ApplySilentAfter:
			rr.State = RestSilent
			w.Silent = appendOnce(w.Silent, rr.Node)
		case rr.Reported == nil:
			rr.State = RestWaiting
			waiting = appendOnce(waiting, rr.Node)
		case rr.Outcome != OutcomeApplied:
			rr.State = RestFailed
			failed = appendOnce(failed, rr.Node+" ("+rr.Outcome+")")
		default:
			rr.State = RestApplied
			end = latest(end, rr.Reported)
		}
		sort.Strings(rr.Modules)
		w.Rest = append(w.Rest, *rr)
	}
	sort.SliceStable(w.Rest, func(a, b int) bool {
		if w.Rest[a].Node != w.Rest[b].Node {
			return w.Rest[a].Node < w.Rest[b].Node
		}
		return w.Rest[a].Sent.Before(w.Rest[b].Sent)
	})
	switch {
	case len(failed) > 0:
		return append(out, point{phase: PhaseApply, tier: -1, said: "not applied on " + strings.Join(failed, ", ")})
	case len(waiting) > 0 || len(busy) > 0:
		var said []string
		if len(waiting) > 0 {
			said = append(said, "waiting for "+strings.Join(waiting, ", ")+" to report it applied")
		}
		if len(busy) > 0 {
			said = append(said, "waits for a busy container on "+strings.Join(busy, ", "))
		}
		return append(out, point{phase: PhaseApply, tier: -1, said: strings.Join(said, "; ")})
	case end == nil:
		return append(out, point{phase: PhaseApply, tier: -1, said: "no machine of the rest heard from: " +
			strings.Join(w.Silent, ", ")})
	}
	// A machine may have reported before the last tier ended: the walk ends at whichever is later.
	for i := len(out) - 1; i >= 0; i-- {
		if out[i].at != nil {
			if out[i].at.After(*end) {
				e := *out[i].at
				end = &e
			}
			break
		}
	}
	said := ""
	if len(w.Silent) > 0 {
		sort.Strings(w.Silent)
		said = "not waited for, not heard from: " + strings.Join(w.Silent, ", ")
	}
	return append(out, point{phase: PhaseApply, tier: -1, at: end, said: said})
}

type restSend struct {
	module, node string
	walkSent     time.Time
	what         SentDeclaration
}

func askedAnyOf(p Plan, tier []string) bool {
	for _, m := range tier {
		if s := p.Modules[m]; s != nil && s.AskedAt != nil {
			return true
		}
	}
	return false
}

func earliest(a, b *time.Time) *time.Time {
	if b == nil {
		return a
	}
	if a == nil || b.Before(*a) {
		t := *b
		return &t
	}
	return a
}

func latest(a, b *time.Time) *time.Time {
	if b == nil {
		return a
	}
	if a == nil || b.After(*a) {
		t := *b
		return &t
	}
	return a
}

func appendOnce(to []string, s string) []string {
	for _, x := range to {
		if x == s {
			return to
		}
	}
	return append(to, s)
}

func orNot(s string) string {
	if s == "" {
		return "is not known"
	}
	return "(" + s + ")"
}

// words is a duration as a person reads it.
func words(d time.Duration) string {
	if d < 0 {
		return "-" + words(-d)
	}
	return d.Round(100 * time.Millisecond).String()
}

// FirstAppliedAfter is a machine's report of the send a walk made to it: the controller's `apply` durations, which
// measure every send to its first report (to-be 45 Phase 0), each row starting at the machine's own send time and
// naming the declaration it was about. send is what the walk kept of that send; walkSent its own record of the
// send to the rest, made after every machine of the rest was sent (novox/hq issue 582). FirstReportOf chooses.
func (i *Inventory) FirstAppliedAfter(ctx context.Context, node string, send SentDeclaration, walkSent time.Time) (AppliedReport, bool, error) {
	from, to := applyRowsBetween(send, walkSent)
	rows, err := i.store.Pool().Query(ctx,
		`select kind, ref, started, took_ms, detail from duration
		  where subject = $2 and ((kind = $1 and started >= $3 and started < $4) or (kind = $5 and started >= $3))
		  order by started`,
		DurationApply, node, from, to, DurationBusy)
	if err != nil {
		return AppliedReport{}, false, err
	}
	defer rows.Close()
	var ds []Duration
	for rows.Next() {
		d := Duration{Subject: node, Node: node}
		var ms int64
		if err := rows.Scan(&d.Kind, &d.Ref, &d.Started, &ms, &d.Detail); err != nil {
			return AppliedReport{}, false, err
		}
		d.Took = time.Duration(ms) * time.Millisecond
		ds = append(ds, d)
	}
	if err := rows.Err(); err != nil {
		return AppliedReport{}, false, err
	}
	r, ok := FirstReportOf(ds, send, walkSent)
	return r, ok, nil
}

// applyRowsBetween is the span of a machine's `apply` durations that may hold its report of a walk's send.
func applyRowsBetween(send SentDeclaration, walkSent time.Time) (time.Time, time.Time) {
	switch {
	case !send.At.IsZero():
		return send.At.Add(-time.Millisecond), send.At.Add(time.Millisecond)
	case send.Digest != "":
		return walkSent.Add(-restSendBound), walkSent.Add(ApplySilentAfter)
	}
	return walkSent.Add(-appliedSlack), walkSent.Add(ApplySilentAfter)
}

// FirstReportOf picks, from a machine's `apply` durations ordered by their start, its report of the send a walk made
// to it (novox/hq issue 582):
//   - with the machine's own send time kept (every walk since issue 582): the duration of that very send, and of the
//     declaration sent where the walk kept its digest. A report of a later send is never this walk's: a send made
//     later, by another walk, would end this walk at that walk's send.
//   - with only the declaration's digest kept (a walk kept before): the first duration about that declaration
//     within restSendBound before the walk's record of its send, the machine sent first in the rest included.
//   - with neither (a walk kept before ADR 0282's rest): the first duration started no earlier than appliedSlack
//     before the walk's record of its send, as it was read before.
//
// A send past ApplySilentAfter is never read: its machine slept, and is listed silent (ADR 0282 decision 1).
//
// **A first report that said a container waits because it is busy** (novox/hq ADR 0335) is the machine's answer,
// and not yet the build running: its report is the end of its busy wait, the `busy-wait` duration of the same
// send, with the wait said apart (Busy). While none is kept the machine is still waiting (Waiting), never silent.
func FirstReportOf(ds []Duration, send SentDeclaration, walkSent time.Time) (AppliedReport, bool) {
	from, to := applyRowsBetween(send, walkSent)
	for _, d := range ds {
		if d.Kind == DurationBusy || d.Started.Before(from) || !d.Started.Before(to) {
			continue
		}
		if send.Digest != "" && !strings.HasPrefix(d.Ref, send.Digest+"@") {
			continue
		}
		answered := d.Started.Add(d.Took).UTC()
		if d.Detail != ApplyBusy {
			return AppliedReport{At: answered, Outcome: d.Detail}, true
		}
		for _, b := range ds {
			if b.Kind == DurationBusy && b.Ref == d.Ref {
				return AppliedReport{At: b.Started.Add(b.Took).UTC(), Outcome: OutcomeApplied, Answered: answered,
					Busy: b.Took}, true
			}
		}
		return AppliedReport{At: answered, Outcome: OutcomeApplied, Answered: answered, Waiting: true}, true
	}
	return AppliedReport{}, false
}

// appliedSlack is how much earlier than a walk's record of its send the machine's own record of it may be, for a
// walk that kept neither the machine's send time nor what it sent.
const appliedSlack = 2 * time.Second

// restSendBound is how long before a walk's record of its send to the rest a machine of the rest may have been
// sent, for a walk that kept what it sent and not when: the send to the rest goes machine by machine, and took
// 19 to 28 seconds on 2026-10-12 (novox/hq issue 582).
const restSendBound = 5 * time.Minute