mesh-host/internal/link/handacts.go

package link

import (
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	stdio "git.novox.be/novox/mesh-sdk/go"
	"git.novox.be/novox/mesh-sdk/go/asker"
	"git.novox.be/novox/mesh-sdk/go/asks"
)

// The node seat node-engine's acts (novox/hq ADR 0563).
//
// A hand-over and an access act are the engine's own asks, performed only on the warrant (§3): a call performs nothing.
// The engine judges every value as it does for the controller's signed ask, then refuses in words or asks the operator
// under its own asker name (asks.EngineAsker) at the level approve, binding the digest of the verb, the node, every
// argument and the owner, group and mode it will set. A warrant is claimed once by compare-and-set in the engine's own
// state (asker.Client.Take), held to the ask it answers, and to the act bound and the disk as judged, judged again now;
// only then is the act performed and recorded with the warrant. A decline, an expiry or a changed disk says refused and
// changes nothing. One ask is open per path or access.
//
// The read-only verbs need no warrant (§2): judge-directories changes nothing; setuid-search is bounded here, one at a
// time and at most one started in an hour. Every act, with who asked as the runtime proved it, is kept in the engine's own
// declared state acts (ActsBucket, created by the grants part), under its own node's keys alone, read by the verb acts.

// The acts of the seat, as its verbs name them.
const (
	ActHandOver         = "hand-over"
	ActAccess           = "access-act"
	ActJudgeDirectories = "judge-directories"
	ActSetuidSearch     = "setuid-search"
	// ActComposition reads a module's composition beside what runs of it; read-only, no warrant (issue 774).
	ActComposition = "composition"
)

// The states of an act.
const (
	HandAsked    = "asked"
	HandDone     = "done"
	HandRefused  = "refused"
	HandAnswered = "answered"
)

// SearchEvery is how often a setuid search may be started on a node (ADR 0563 §2): one in any hour.
const SearchEvery = time.Hour

// askMargin is how long past an ask's expiry the router is given to say it ended, before the engine closes it itself.
const askMargin = 5 * time.Minute

// ActsBucket is the node-engine's declared state acts (its manifest's state, ADR 0563 §2, §3), the bucket the grants
// part creates (mesh-sdk buscompose EngineActsBucket): keys `<node>.act.<id>`, `<node>.open.<act>.<digest>` and
// `<node>.ask.<id>`, each written by this node's engine alone.
const ActsBucket = "mesh-host_acts"

// Judged is what the engine judged of one act before it asks: what it is about, what it will set (the owner, group and
// mode, in words), and what the disk holds now (Disk), compared again when the warrant arrives.
type Judged struct {
	// Subject is the path of a hand-over, or the access of an access act.
	Subject string
	// Path is the directory the act changes, as the engine states it.
	Path string
	// Sets is the owner, group and mode the act will set, in words; bound in the ask's digest.
	Sets string
	// Disk is what the engine read on disk when it judged: owner, group, mode and the file it is.
	Disk string
	// Words is what the operator is shown of it, without a path (ADR 0234 §6): the path is in the whole words alone.
	Words string
}

// HandAct is one act asked through the seat, as acts reads it.
type HandAct struct {
	ID      string    `json:"id"`
	Act     string    `json:"act"`
	Subject string    `json:"subject,omitempty"`
	Caller  string    `json:"caller"`
	Why     string    `json:"why,omitempty"`
	At      time.Time `json:"at"`
	State   string    `json:"state"`
	Said    string    `json:"said,omitempty"`
	Ended   time.Time `json:"ended,omitempty"`
	// Ask is the operator's ask of a hand-over or an access act; Sets and Disk what was judged when it was asked.
	Ask  string `json:"ask,omitempty"`
	Sets string `json:"sets,omitempty"`
	Disk string `json:"disk,omitempty"`
	Path string `json:"path,omitempty"`
	// Expires is when the ask ends unanswered.
	Expires time.Time `json:"expires,omitempty"`
	// By, Via, Proofs are the operator's warrant, recorded with the act (ADR 0563 §3).
	By     string   `json:"by,omitempty"`
	Via    string   `json:"via,omitempty"`
	Proofs []string `json:"proofs,omitempty"`
	// Auto says the engine asked it itself, on finding a library's root holding data outside its group (§4).
	Auto bool `json:"auto,omitempty"`
}

// RootVerdict is the engine's answer to root-free (ADR 0563 §6), in the controller's words.
type RootVerdict struct {
	Machine string    `json:"machine"`
	Free    bool      `json:"free"`
	Why     string    `json:"why"`
	Judged  time.Time `json:"judged"`
}

// HandActs is the engine's side of the seat. Every func is the engine's own judgement or act; nil refuses.
type HandActs struct {
	Node string
	// State is where the acts and the asks are kept: this node's keys of ActsBucket (BusState), each key one act.
	State ActStore
	// Asker asks the operator as asks.EngineAsker(Node), with State as its store.
	Asker *asker.Client
	// JudgeHandOver judges a hand-over's path, refusing in words; HandOver records it on the warrant.
	JudgeHandOver func(path string) (Judged, error)
	HandOver      HandOverFunc
	// JudgeAccess judges an access act on an access, refusing in words; AccessAct runs it on the warrant.
	JudgeAccess func(act, access string) (Judged, error)
	AccessAct   AccessActFunc
	// JudgeDirectories, SetuidSearch and Searching are the read-only verbs' and whether a search runs now.
	JudgeDirectories JudgeDirectoriesFunc
	SetuidSearch     SetuidSearchFunc
	Searching        func() bool
	// RootFree is this machine's root-free verdict now.
	RootFree func() RootVerdict
	// Composition reads one module's composition beside what runs of it, read-only (the verb composition, issue 774):
	// the latest of its want's build or the one whose digest is given, and with diff the resources that differ.
	Composition func(module, digest string, diff bool) (any, error)
	// Say says one of the seat's events, asked, done or refused; nil says nothing.
	Say func(event string, body any)
	Now func() time.Time
	// Nonce keeps two asks of one second apart; nil is random.
	Nonce func() string

	mu sync.Mutex
}

func (h *HandActs) now() time.Time {
	if h.Now != nil {
		return h.Now()
	}
	return time.Now()
}

func (h *HandActs) nonce() string {
	if h.Nonce != nil {
		return h.Nonce()
	}
	var b [2]byte
	_, _ = rand.Read(b[:])
	return hex.EncodeToString(b[:])
}

func (h *HandActs) say(event string, a HandAct) {
	if h.Say != nil {
		h.Say(event, map[string]any{"act": a.Act, "subject": a.Subject, "caller": a.Caller, "ask": a.Ask,
			"state": a.State, "said": a.Said, "node": h.Node})
	}
}

// Call is the seat's one entry: a verb, the caller as the runtime proved it, and its arguments.
func (h *HandActs) Call(c EngineCall) (any, error) {
	arg := func(k string) string { s, _ := c.Args[k].(string); return strings.TrimSpace(s) }
	switch c.Verb {
	case ActHandOver:
		return h.AskHandOver(c.Caller, arg("path"), arg("why"), false)
	case ActAccess:
		return h.AskAccess(c.Caller, arg("act"), arg("access"), arg("why"), false)
	case ActJudgeDirectories:
		return h.Judge(c.Caller)
	case ActSetuidSearch:
		return h.Search(c.Caller, arg("why"))
	case "root-free":
		if h.RootFree == nil {
			return nil, errors.New("this engine judges no machine root-free")
		}
		return map[string]any{"machines": []RootVerdict{h.RootFree()}}, nil
	case ActComposition:
		if h.Composition == nil {
			return nil, errors.New("this engine reads no composition yet: its node agent has not started")
		}
		diff := arg("diff") == "true"
		if b, ok := c.Args["diff"].(bool); ok {
			diff = b
		}
		return h.Composition(arg("module"), arg("digest"), diff)
	case "acts":
		if id := arg("id"); id != "" {
			return h.Act(id)
		}
		limit, _ := strconv.Atoi(arg("limit"))
		if f, ok := c.Args["limit"].(float64); ok {
			limit = int(f)
		}
		return h.Acts(limit)
	}
	return nil, fmt.Errorf("the node seat %s serves no verb %q", EngineSeat, c.Verb)
}

// ---- the record --------------------------------------------------------------------------------------------------

// ActStore is the engine's own state, this node's keys alone (BusState prefixes them with the node): each act one key,
// written by compare-and-set on its revision, an open ask one key made by a create.
type ActStore interface {
	Get(key string) (*stdio.StateEntry, error)
	Create(key string, value any) (uint64, error)
	Update(key string, value any, revision uint64) (uint64, error)
	Delete(key string) error
	// Keys are the keys under prefix.
	Keys(prefix string) ([]string, error)
}

func actKey(id string) string { return "act." + id }

// openKey is the one key an open ask of act on subject holds: made by a create, so a second ask of it is refused.
func openKey(act, subject string) string {
	sum := sha256.Sum256([]byte(subject))
	return "open." + act + "." + hex.EncodeToString(sum[:12])
}

// read is every act kept, oldest first.
func (h *HandActs) read() ([]HandAct, error) {
	keys, err := h.State.Keys("act.")
	if err != nil {
		return nil, fmt.Errorf("the engine's acts cannot be listed: %w", err)
	}
	var out []HandAct
	for _, k := range keys {
		a, _, err := h.readOne(k)
		if err != nil {
			return nil, err
		}
		if a != nil {
			out = append(out, *a)
		}
	}
	sort.SliceStable(out, func(i, j int) bool { return out[i].At.Before(out[j].At) })
	return out, nil
}

func (h *HandActs) readOne(key string) (*HandAct, uint64, error) {
	e, err := h.State.Get(key)
	if err != nil || e == nil {
		return nil, 0, err
	}
	var a HandAct
	if err := json.Unmarshal(e.Value, &a); err != nil {
		return nil, 0, fmt.Errorf("the engine's act %s cannot be read: %w", key, err)
	}
	return &a, e.Revision, nil
}

// judgedKey keeps the node's judgements and refused searches, the last keptJudged of them: any caller may ask them, so
// they roll in one key and never add one (ADR 0563 §2: a wrong caller gains nothing, and fills no bucket).
const (
	judgedKey  = "judged"
	keptJudged = 50
)

// roll adds a to the rolling key by compare-and-set, the oldest falling out.
func (h *HandActs) roll(a HandAct) error {
	for range 5 {
		e, err := h.State.Get(judgedKey)
		if err != nil {
			return err
		}
		var kept []HandAct
		if e != nil {
			if err := json.Unmarshal(e.Value, &kept); err != nil {
				kept = nil // an unreadable roll is started again, never a reason to refuse
			}
		}
		kept = append(kept, a)
		if len(kept) > keptJudged {
			kept = kept[len(kept)-keptJudged:]
		}
		if e == nil {
			_, err = h.State.Create(judgedKey, kept)
		} else {
			_, err = h.State.Update(judgedKey, kept, e.Revision)
		}
		if !errors.Is(err, stdio.ErrChanged) {
			return err
		}
	}
	return errors.New("the engine's judgements kept changing under its write")
}

func (h *HandActs) rolled() ([]HandAct, error) {
	e, err := h.State.Get(judgedKey)
	if err != nil || e == nil {
		return nil, err
	}
	var kept []HandAct
	if err := json.Unmarshal(e.Value, &kept); err != nil {
		return nil, fmt.Errorf("the engine's judgements cannot be read: %w", err)
	}
	return kept, nil
}

func (h *HandActs) record(a HandAct) error {
	_, err := h.State.Create(actKey(a.ID), a)
	return err
}

// end marks the open act of an ask ended, once, by compare-and-set on its revision, and frees its open key: false when
// it was not open.
func (h *HandActs) end(ask string, set func(*HandAct)) (HandAct, bool, error) {
	for range 5 {
		a, rev, err := h.readOne(actKey(ask))
		if err != nil || a == nil || a.State != HandAsked {
			return HandAct{}, false, err
		}
		set(a)
		a.Ended = h.now().UTC()
		if _, err := h.State.Update(actKey(ask), *a, rev); errors.Is(err, stdio.ErrChanged) {
			continue
		} else if err != nil {
			return HandAct{}, false, err
		}
		_ = h.State.Delete(openKey(a.Act, a.Subject))
		return *a, true, nil
	}
	return HandAct{}, false, errors.New("the engine's act " + ask + " kept changing under its write")
}

// Act is the verb acts with an id: that one act.
func (h *HandActs) Act(id string) (any, error) {
	a, _, err := h.readOne(actKey(id))
	if err != nil {
		return nil, err
	}
	if a == nil {
		return nil, fmt.Errorf("this engine keeps no act %q", id)
	}
	return map[string]any{"node": h.Node, "acts": []HandAct{*a}}, nil
}

// Acts is the verb acts: every act kept, newest first, at most limit when given.
func (h *HandActs) Acts(limit int) (any, error) {
	acts, err := h.read()
	if err != nil {
		return nil, err
	}
	judged, err := h.rolled()
	if err != nil {
		return nil, err
	}
	acts = append(acts, judged...)
	sort.SliceStable(acts, func(i, j int) bool { return acts[i].At.Before(acts[j].At) })
	out := make([]HandAct, 0, len(acts))
	for i := len(acts) - 1; i >= 0; i-- {
		out = append(out, acts[i])
	}
	if limit > 0 && len(out) > limit {
		out = out[:limit]
	}
	return map[string]any{"node": h.Node, "acts": out}, nil
}

// ---- the read-only verbs (§2) -------------------------------------------------------------------------------------

// Judge is judge-directories: the engine's judgement, read-only, recorded with its caller.
func (h *HandActs) Judge(caller string) (any, error) {
	if h.JudgeDirectories == nil {
		return nil, errors.New("this engine judges no directory")
	}
	said, err := h.JudgeDirectories(caller)
	a := HandAct{ID: h.id(ActJudgeDirectories), Act: ActJudgeDirectories, Caller: caller, At: h.now().UTC(), State: HandAnswered}
	if err != nil {
		a.State, a.Said = HandRefused, err.Error()
	}
	if rerr := h.roll(a); rerr != nil && err == nil {
		return nil, rerr
	}
	if err != nil {
		return nil, err
	}
	return map[string]any{"node": h.Node, "said": said}, nil
}

// Search is setuid-search: refused while one runs and when one was started within the hour, naming when the next may
// start; otherwise the last search is thrown away and a full one starts, recorded with its caller.
func (h *HandActs) Search(caller, why string) (any, error) {
	if h.SetuidSearch == nil {
		return nil, errors.New("this engine keeps no search for setuid programs")
	}
	h.mu.Lock()
	defer h.mu.Unlock()
	now := h.now().UTC()
	if h.Searching != nil && h.Searching() {
		return nil, errors.New("a search for setuid programs runs on this machine now; no second is started while it does")
	}
	// The last start is a key of its own, claimed by compare-and-set before the search starts: no number of other acts,
	// nor the record of acts, moves it (ADR 0563 §2).
	if err := h.claimSearch(now, caller); err != nil {
		return nil, err
	}
	said, err := h.SetuidSearch(caller)
	a := HandAct{ID: h.id(ActSetuidSearch), Act: ActSetuidSearch, Caller: caller, Why: why, At: now, State: HandDone, Said: said}
	if err != nil {
		a.State, a.Said = HandRefused, err.Error()
	}
	// A search started is bounded to one an hour and kept as an act of its own; a refused one, which any caller may
	// cause, rolls with the judgements.
	record := h.record
	if err != nil {
		record = h.roll
	}
	if rerr := record(a); rerr != nil {
		return nil, rerr
	}
	if err != nil {
		return nil, err
	}
	return map[string]any{"node": h.Node, "said": said}, nil
}

// searchKey keeps the last search's start on this node, and libKey each library's own take ask: keys of their own, which
// no other act's key moves, kept for the bucket's age (ninety days, the node-engine's manifest).
const searchKey = "search.last"

func libKey(access string) string {
	sum := sha256.Sum256([]byte(access))
	return "lib." + hex.EncodeToString(sum[:12]) + ".asked"
}

// started is the last search's start as kept, and who asked it.
type started struct {
	At time.Time `json:"at"`
	By string    `json:"by"`
}

// claimSearch refuses a search within SearchEvery of the last start, naming when the next may start, and otherwise
// claims this one by compare-and-set: a second claim at the same moment loses.
func (h *HandActs) claimSearch(now time.Time, caller string) error {
	e, err := h.State.Get(searchKey)
	if err != nil {
		return err
	}
	var rev uint64
	if e != nil {
		var last started
		if err := json.Unmarshal(e.Value, &last); err != nil {
			return fmt.Errorf("the last search's start cannot be read, so none is started: %w", err)
		}
		if now.Sub(last.At) < SearchEvery {
			return fmt.Errorf("a search was started at %s by %s, and one is started in any hour: the next may start "+
				"at %s", last.At.Format(time.RFC3339), last.By, last.At.Add(SearchEvery).Format(time.RFC3339))
		}
		rev = e.Revision
	}
	claim := started{At: now, By: caller}
	if rev == 0 {
		_, err = h.State.Create(searchKey, claim)
	} else {
		_, err = h.State.Update(searchKey, claim, rev)
	}
	if errors.Is(err, stdio.ErrChanged) {
		return errors.New("another search was started at this moment; no second is started")
	}
	return err
}

// ---- the asks (§3, §4) --------------------------------------------------------------------------------------------

func (h *HandActs) id(act string) string {
	return act + "-" + h.now().UTC().Format("20060102t150405") + "-" + h.nonce()
}

// bound is the act an ask's yes binds: the verb, the node, every argument and what it will set (ADR 0563 §3).
func (h *HandActs) bound(a HandAct) asks.Act {
	act := asks.Act{"verb": a.Act, "node": h.Node, "arg.subject": a.Subject, "arg.why": a.Why, "sets": a.Sets}
	if a.Path != "" {
		act["path"] = a.Path
	}
	return act
}

// AskHandOver is hand-over: judged, refused in words or asked of the operator. auto is the engine asking by itself.
func (h *HandActs) AskHandOver(caller, path, why string, auto bool) (any, error) {
	if h.JudgeHandOver == nil || h.HandOver == nil {
		return nil, errors.New("this engine takes no hand-over")
	}
	j, err := h.JudgeHandOver(path)
	if err != nil {
		return nil, err
	}
	return h.ask(ActHandOver, caller, why, auto, j)
}

// AskAccess is access-act: the act take on an access, judged on the library's home, refused or asked of the operator.
func (h *HandActs) AskAccess(caller, act, access, why string, auto bool) (any, error) {
	if h.JudgeAccess == nil || h.AccessAct == nil {
		return nil, errors.New("this engine runs no act on an accessed folder")
	}
	if act != ActTake {
		return nil, fmt.Errorf("the act asked is %q, and the one act on an access is %q", act, ActTake)
	}
	j, err := h.JudgeAccess(act, access)
	if err != nil {
		return nil, err
	}
	return h.ask(ActAccess, caller, why, auto, j)
}

// AskedFor says whether an act on subject was ever asked by the engine itself: a library's take is asked by itself once
// (ADR 0563 §4), and again only by hand.
func (h *HandActs) AskedFor(act, subject string) bool {
	if act == ActAccess {
		e, err := h.State.Get(libKey(subject))
		return err != nil || e != nil // unread is never a reason to ask again
	}
	acts, err := h.read()
	if err != nil {
		return true
	}
	for _, a := range acts {
		if a.Act == act && a.Subject == subject && (a.Auto || a.State == HandAsked) {
			return true
		}
	}
	return false
}

func (h *HandActs) ask(act, caller, why string, auto bool, j Judged) (any, error) {
	if h.Asker == nil {
		return nil, errors.New("this engine reaches no operator to ask: nothing was asked and nothing changed")
	}
	h.mu.Lock()
	defer h.mu.Unlock()
	now := h.now().UTC()
	a := HandAct{ID: h.id(act), Act: act, Subject: j.Subject, Path: j.Path, Caller: caller, Why: why, At: now,
		State: HandAsked, Sets: j.Sets, Disk: j.Disk, Expires: now.Add(asks.ApproveLasts - time.Minute), Auto: auto}
	a.Ask = a.ID
	// One ask open per path or access: its open key made by a create, which a second ask cannot make.
	if _, err := h.State.Create(openKey(act, j.Subject), map[string]string{"ask": a.Ask}); errors.Is(err, stdio.ErrChanged) {
		held := "another"
		if e, _ := h.State.Get(openKey(act, j.Subject)); e != nil {
			var o struct {
				Ask string `json:"ask"`
			}
			if json.Unmarshal(e.Value, &o) == nil && o.Ask != "" {
				held = o.Ask
			}
		}
		return nil, fmt.Errorf("%s of %s is asked already: the ask %s waits for the operator's answer", act, j.Subject, held)
	} else if err != nil {
		return nil, err
	}
	if err := h.record(a); err != nil {
		_ = h.State.Delete(openKey(act, j.Subject))
		return nil, err
	}
	bind, err := asks.ActDigest(h.bound(a))
	if err != nil {
		return nil, err
	}
	headline, does, stays := "Hand a directory on "+h.Node+" to the mesh?", "The mesh gives it "+j.Sets+".",
		"The directory stays as it was found."
	if act == ActAccess {
		headline, does, stays = "Keep the library "+strings.TrimPrefix(j.Subject, "library:")+" on "+h.Node+"?",
			"The library's group, group write and setgid are set on everything under its root.",
			"The library waits and nothing under its root changes."
	}
	if len([]rune(headline)) > asks.HeadlineLength {
		headline = string([]rune(headline)[:asks.HeadlineLength])
	}
	asked := asks.Ask{ID: a.Ask, Headline: headline, Who: asks.Operator,
		Explanation: j.Words + " Your yes sets " + j.Sets + "; nothing changes until you answer.",
		Whole:       j.Words + " (" + j.Path + ") Your yes sets " + j.Sets + "; nothing changes until you answer.",
		Details:     "Asked by " + caller + " on " + h.Node + orWhy(why),
		Options: []asks.Option{{ID: "yes", Label: "Yes", Does: does, Level: asks.Approve, Binds: bind},
			{ID: "no", Label: "No, leave it", Does: stays, Level: asks.Acknowledge}},
		Expires: a.Expires, OnExpiry: stays, About: EngineSeat + "-" + h.Node + "-" + act}
	if err := h.Asker.Ask(asked); err != nil {
		_, _, _ = h.end(a.Ask, func(x *HandAct) { x.State, x.Said = HandRefused, "the operator could not be asked: "+err.Error() })
		return nil, fmt.Errorf("the operator could not be asked, and nothing changed: %w", err)
	}
	// Once asked, a library's own take ask is kept's own take ask: once, in a key of its own (ADR 0563 §4).
	if auto && act == ActAccess {
		_, _ = h.State.Create(libKey(j.Subject), map[string]any{"ask": a.Ask, "at": now})
	}
	h.say(HandAsked, a)
	return map[string]any{"asked": a.Ask, "node": h.Node, "says": "the operator is asked on their channel; only " +
		"their yes changes anything. acts shows where it stands."}, nil
}

func orWhy(why string) string {
	if why == "" {
		return ""
	}
	return ": " + why
}

// Warranted is the act on the operator's answer, handed once by asker.Client.Take after it held the warrant to the
// ask: a no is recorded refused; a yes is held to the act the option bound and the disk as judged, judged again now,
// and only then performed and recorded done with the warrant.
func (h *HandActs) Warranted(o asks.Option, w asks.Warrant) error {
	a, _, err := h.readOne(actKey(w.Ask))
	if err != nil {
		return err
	}
	if a == nil || a.State != HandAsked {
		return fmt.Errorf("the warrant answers the ask %s, which is not open here: nothing is done", w.Ask)
	}
	refuse := func(why string) error {
		ended, ok, err := h.end(w.Ask, func(x *HandAct) { x.State, x.Said = HandRefused, why; warrantOn(x, w) })
		if err == nil && ok {
			h.say(HandRefused, ended)
		}
		return err
	}
	if o.ID != "yes" {
		return refuse("the operator said no")
	}
	var j Judged
	switch a.Act {
	case ActHandOver:
		j, err = h.JudgeHandOver(a.Subject)
	case ActAccess:
		j, err = h.JudgeAccess(ActTake, a.Subject)
	default:
		err = fmt.Errorf("the act %s is not performed on a warrant", a.Act)
	}
	if err != nil {
		return refuse("judged again on the operator's yes, it is refused: " + err.Error())
	}
	now := *a
	now.Sets, now.Path = j.Sets, j.Path
	if err := o.Performs(h.bound(now)); err != nil {
		return refuse("what the yes bound is not what would be done now: " + err.Error())
	}
	if j.Disk != a.Disk {
		return refuse("the disk changed since the operator was asked (" + a.Disk + ", now " + j.Disk + "): ask again")
	}
	// Recorded with the way it came (ADR 0563 §5): the operator's yes to this engine's ask, not a terminal's line.
	by := operatorOf(w) + ", to " + EngineSeat + "'s ask " + w.Ask
	var said string
	switch a.Act {
	case ActHandOver:
		said, err = h.HandOver(a.Subject, by)
	case ActAccess:
		said, err = h.AccessAct(AccessAct{Act: ActTake, Access: a.Subject, Path: j.Path, Warrant: w.Ask, By: by})
	}
	if err != nil {
		return refuse(err.Error())
	}
	ended, ok, err := h.end(w.Ask, func(x *HandAct) { x.State, x.Said = HandDone, said; warrantOn(x, w) })
	if err == nil && ok {
		h.say(HandDone, ended)
	}
	return err
}

func warrantOn(a *HandAct, w asks.Warrant) {
	a.Proofs = w.Proofs
	if w.By != nil {
		a.By, a.Via = w.By.Who, w.By.Kind
		if w.By.Verified != "" {
			a.Via += " (" + w.By.Verified + ")"
		}
	}
}

func operatorOf(w asks.Warrant) string {
	if w.By == nil {
		return "the operator"
	}
	return "the " + w.By.Who + " via " + w.By.Kind
}

// Heard is a warrant heard on the engine's decided subject: taken once (asker.Client.Take), and an ask that ended with
// no choice recorded refused.
func (h *HandActs) Heard(w asks.Warrant) {
	if h.Asker == nil {
		return
	}
	// A warrant the ask kept here does not hold to (another digest, another option, too late) ends nothing: it is no
	// answer to the ask, which stays open for the operator's.
	if handled, _ := h.Asker.Take(w, h.Warranted); handled == asker.Recorded {
		why := "the ask ended " + string(w.Outcome)
		if w.Words != "" {
			why += ": " + w.Words
		}
		if ended, ok, _ := h.end(w.Ask, func(x *HandAct) { x.State, x.Said = HandRefused, why; warrantOn(x, w) }); ok {
			h.say(HandRefused, ended)
		}
	}
}

// Reconcile catches up what the engine missed while it was away: a warrant the router holds for an open ask is taken,
// and an ask past its expiry and a margin is closed refused, expired.
func (h *HandActs) Reconcile() {
	if h.Asker == nil {
		return
	}
	acts, err := h.read()
	if err != nil {
		return
	}
	for _, a := range acts {
		if a.State != HandAsked || a.Ask == "" {
			continue
		}
		if _, w, err := h.Asker.Where(a.Ask); err == nil && w != nil {
			h.Heard(*w)
			continue
		}
		if !a.Expires.IsZero() && h.now().After(a.Expires.Add(askMargin)) {
			if ended, ok, _ := h.end(a.Ask, func(x *HandAct) { x.State, x.Said = HandRefused, "the ask expired unanswered" }); ok {
				h.say(HandRefused, ended)
			}
		}
	}
}

// MemState is an ActStore in memory: an engine's acts while no bus is reached in a test.
type MemState struct {
	mu   sync.Mutex
	keys map[string]stdio.StateEntry
	top  uint64
}

func (m *MemState) Get(key string) (*stdio.StateEntry, error) {
	m.mu.Lock()
	defer m.mu.Unlock()
	if e, ok := m.keys[key]; ok {
		return &e, nil
	}
	return nil, nil
}

func (m *MemState) Create(key string, value any) (uint64, error) { return m.put(key, value, 0, true) }

func (m *MemState) Update(key string, value any, revision uint64) (uint64, error) {
	return m.put(key, value, revision, false)
}

func (m *MemState) put(key string, value any, revision uint64, create bool) (uint64, error) {
	raw, err := json.Marshal(value)
	if err != nil {
		return 0, err
	}
	m.mu.Lock()
	defer m.mu.Unlock()
	was, kept := m.keys[key]
	if (create && kept) || (!create && (!kept || was.Revision != revision)) {
		return 0, stdio.ErrChanged
	}
	if m.keys == nil {
		m.keys = map[string]stdio.StateEntry{}
	}
	m.top++
	m.keys[key] = stdio.StateEntry{Key: key, Value: raw, Revision: m.top}
	return m.top, nil
}

func (m *MemState) Delete(key string) error {
	m.mu.Lock()
	defer m.mu.Unlock()
	delete(m.keys, key)
	return nil
}

func (m *MemState) Keys(prefix string) ([]string, error) {
	m.mu.Lock()
	defer m.mu.Unlock()
	var out []string
	for k := range m.keys {
		if strings.HasPrefix(k, prefix) {
			out = append(out, k)
		}
	}
	sort.Strings(out)
	return out, nil
}