mesh-controller/internal/link/calls.go

package link

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log"
	"regexp"
	"runtime/debug"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/nats-io/nats.go"
)

// A role's tool call, kept after it is answered (novox/hq issue 265).
//
// **A call that outlasts its caller must not end in silence.** A caller waits a bounded time (the
// console thirty seconds), and the bus lets a holder answer a request exactly once and only while the
// server still remembers it was asked (`allow_responses`). Before this, a push that ran longer than
// its caller waited did everything it was asked and its caller read "did not answer in time"; and a
// push whose first act sent the bus's own machine a changed user list had its answer refused
// outright — a broker reloading its users forgets every reply it was about to permit — so the
// answer went nowhere and the only trace was the client library's line on the controller's standard
// error. So every call is answered within AnswerWithin, with its whole answer when it has one by
// then and with "still running as call <id>" when it does not; and every call is kept here, with
// what came of it, including an answer the bus refused, so `calls` can say it.

// ErrHandingOver is a call refused because the controller answering it is being replaced: stopping, or
// unable to run its own build because the node-engine has moved it aside (novox/hq issue 289). Nothing
// was done, and the controller after it answers the same call — so the answer carries
// `"retry": RetryHandingOver`, and a caller asks once more.
var ErrHandingOver = errors.New("the controller is handing over to the next one; nothing was done, ask again")

// RetryHandingOver is the `retry` mark of an answer refused by ErrHandingOver.
const RetryHandingOver = "handing-over"

// AnswerWithin is how long a call runs before its caller is answered that it is still running. Well
// inside the shortest wait of a caller the mesh ships (the console's thirty seconds) and the bus's
// own window for an answer (broker.ResponseTTL), so the one answer a call has is never late for
// either. A variable so a test need not wait.
var AnswerWithin = 10 * time.Second

// KeptCalls is how many calls this process keeps in memory, newest first — to match a refusal to its
// call, and to answer at once; the bus keeps the last thousand (Durably). keptAnswer is the largest
// answer kept of a call whose caller was sent it. One its caller never had is kept whole.
const (
	KeptCalls  = 100
	keptAnswer = 64 << 10
)

// The states a kept call is in.
const (
	CallRunning  = "running"
	CallAnswered = "answered"
	// CallFinishedAfter is a call that finished after its caller was told it was still running: its
	// answer is here and nowhere else.
	CallFinishedAfter = "finished after its caller was answered"
	// CallAbandoned is a call whose controller stopped before it finished (novox/hq to-be 45 §6): a
	// controller starting finds it running under another and says so, rather than leaving it running
	// for ever in the record.
	CallAbandoned = "abandoned: the controller running it stopped before it finished"
)

// Call is one call of a role's tool, as `calls` shows it.
type Call struct {
	ID       string          `json:"call"`
	Seat     string          `json:"seat"`
	Verb     string          `json:"verb"`
	Args     json.RawMessage `json:"arguments,omitempty"`
	Started  time.Time       `json:"started"`
	Finished *time.Time      `json:"finished,omitempty"`
	State    string          `json:"state"`
	// Failed is the call's own answer being an error — an answer, not a timeout.
	Failed bool `json:"failed,omitempty"`
	// Answer is what the call answered, or would have: kept for a call whose caller did not get it.
	Answer json.RawMessage `json:"answer,omitempty"`
	// Refused is the bus refusing the answer this holder sent: the caller got nothing, and this is
	// the only place that says what it would have.
	Refused string `json:"answer refused by the bus,omitempty"`
	// Paged is an answer too large for one message, sent in parts to a caller that asked for them
	// (novox/hq issue 314): how large, and until when it is held.
	Paged string `json:"answer paged,omitempty"`
	// Caller is the bus principal that asked, read from the inbox its answer went to — every principal
	// is granted only its own (novox/hq to-be 45 §7: a hand act says who).
	Caller string `json:"caller,omitempty"`
	// Holder is the controller process that served it, so one starting can tell its own running
	// calls from those a stopped one left.
	Holder string `json:"holder,omitempty"`
	// Epoch is the controller lease epoch it was served under (novox/hq to-be 45 §6); zero for none.
	Epoch uint64 `json:"epoch,omitempty"`

	reply string // the subject the answer went to, which the bus names when it refuses it
}

// CallKeeper keeps calls where the process serving them does not: the controller's bucket on the
// bus (novox/hq to-be 45 §6). A call is kept whole on every change — begun, finished, refused — so a
// controller replaced at any moment leaves the last word on each.
type CallKeeper interface {
	Keep(ctx context.Context, c Call) error
	// Kept is one call with its whole answer.
	Kept(ctx context.Context, id string) (Call, bool, error)
	// Recent is the kept calls, newest first, without their answers.
	Recent(ctx context.Context) ([]Call, error)
}

// CallLog keeps the latest calls a holder served.
type CallLog struct {
	mu    sync.Mutex
	calls []*Call // oldest first
	next  uint64
	now   func() time.Time
	// Follow is the tool that reads this log back, named in a running answer — set by a holder that
	// serves one (the controller's `calls`); without it, the answer points at the holder's journal.
	Follow string

	// keeper keeps every call beyond this process, when Durably was given one; writes go through
	// one goroutine, in order, so a call's last state is the one kept.
	keeper  CallKeeper
	holder  string
	writes  chan Call
	logger  *log.Logger
	lost    int // writes the keeper could not take, said once each
	keepErr error
	// epoch is the lease this process serves under (UnderLease): a call carries its epoch, and its
	// record is written only while the lease is held. Nil writes every record with no epoch.
	epoch func() (uint64, error)

	// pages are the answers too large for one message, held for their callers (pages.go).
	pages pageBook
}

// UnderLease makes every call carry the controller lease's epoch, and every write of a call's record
// pass the lease (novox/hq to-be 45 §6): a controller that lost the lease writes no finish over a call
// the next holder has marked abandoned — that mark is the record's last word.
func (l *CallLog) UnderLease(epoch func() (uint64, error)) {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.epoch = epoch
}

// Calls is this process's log: one holder process serves its seats on one connection.
var Calls = NewCallLog()

func NewCallLog() *CallLog { return &CallLog{now: time.Now} }

// keepTries is how many times one call's state is offered to the keeper before it is said lost: the
// bus reloading its user list refuses for a moment, and that is exactly when a push runs.
const keepTries = 5

// Durably keeps every call from now on with keeper as well as in memory, under this process's name,
// and marks running the calls a controller before this one left running: it stopped, so they cannot
// finish (novox/hq to-be 45 §6). Said, naming each.
func (l *CallLog) Durably(ctx context.Context, keeper CallKeeper, holder string, logger *log.Logger) error {
	l.mu.Lock()
	l.keeper, l.holder, l.logger = keeper, holder, logger
	if l.writes == nil {
		l.writes = make(chan Call, 256)
		go l.keepWrites()
	}
	l.mu.Unlock()
	kept, err := keeper.Recent(ctx)
	if err != nil {
		return fmt.Errorf("reading the calls kept on the bus: %w", err)
	}
	for _, c := range kept {
		if c.State != CallRunning || c.Holder == holder {
			continue
		}
		whole, found, err := keeper.Kept(ctx, c.ID)
		if err != nil || !found {
			whole = c
		}
		whole.State = CallAbandoned
		if err := keeper.Keep(ctx, whole); err != nil {
			return fmt.Errorf("marking %s abandoned: %w", c.ID, err)
		}
		if logger != nil {
			logger.Printf("%s (%s.%s, asked %s by %s) was running under %s (epoch %d), which stopped: marked "+
				"abandoned — it may have done part of what it was asked, and nothing will finish it", c.ID, c.Seat,
				c.Verb, c.Started.Format(time.RFC3339), orSomebody(c.Caller), orSomebody(c.Holder), c.Epoch)
		}
	}
	return nil
}

func orSomebody(s string) string {
	if s == "" {
		return "an unnamed caller"
	}
	return s
}

// keep queues one call's state for the keeper. Never blocks a call: a queue that is full is a keeper
// that is not taking writes, and that is said rather than waited on.
func (l *CallLog) keep(c Call) {
	if l.writes == nil {
		return
	}
	select {
	case l.writes <- c:
	default:
		l.lost++
		if l.logger != nil {
			l.logger.Printf("%s (%s.%s) is kept in memory only: the bus is not taking calls' records (%d not kept)",
				c.ID, c.Seat, c.Verb, l.lost)
		}
	}
}

func (l *CallLog) keepWrites() {
	for c := range l.writes {
		l.mu.Lock()
		gate := l.epoch
		l.mu.Unlock()
		if gate != nil {
			if _, err := gate(); err != nil {
				if l.logger != nil {
					l.logger.Printf("%s (%s.%s, %s) is not kept on the bus: %v — the controller holding the lease "+
						"marks it abandoned", c.ID, c.Seat, c.Verb, c.State, err)
				}
				continue
			}
		}
		var err error
		for try := 0; try < keepTries; try++ {
			ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
			err = l.keeper.Keep(ctx, c)
			cancel()
			if err == nil {
				break
			}
			time.Sleep(time.Duration(try+1) * time.Second)
		}
		if err != nil && l.logger != nil {
			l.logger.Printf("%s (%s.%s, %s) could not be kept on the bus after %d tries: %v — `calls` "+
				"answers it from memory until this controller stops", c.ID, c.Seat, c.Verb, c.State, keepTries, err)
		}
	}
}

// callerOf is the bus principal an answer goes to: every principal's inbox is `_INBOX.<its user>.`
// followed by the client's own random token, and a user may itself hold dots.
func callerOf(reply string) string {
	rest, ok := strings.CutPrefix(reply, "_INBOX.")
	if !ok {
		return ""
	}
	tokens := strings.Split(rest, ".")
	for i, t := range tokens {
		if i > 0 && isNUID(t) {
			return strings.Join(tokens[:i], ".")
		}
	}
	return ""
}

// isNUID is the client library's random inbox token: twenty-two letters and digits.
func isNUID(t string) bool {
	if len(t) != 22 {
		return false
	}
	for _, r := range t {
		if !(r >= '0' && r <= '9' || r >= 'a' && r <= 'z' || r >= 'A' && r <= 'Z') {
			return false
		}
	}
	return true
}

func (l *CallLog) begin(seat, verb string, args json.RawMessage, reply string) *Call {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.next++
	c := &Call{ID: "call-" + strconv.FormatInt(l.now().UnixNano(), 10) + "-" + strconv.FormatUint(l.next, 10),
		Seat: seat, Verb: verb, Args: args, Started: l.now(), State: CallRunning, reply: reply,
		Caller: callerOf(reply), Holder: l.holder}
	if l.epoch != nil {
		c.Epoch, _ = l.epoch() // zero when not held: its record is then not written either
	}
	l.calls = append(l.calls, c)
	if len(l.calls) > KeptCalls {
		l.calls = l.calls[len(l.calls)-KeptCalls:]
	}
	l.keep(*c)
	return c
}

// finish records a call's answer; answeredAlready is its caller having been told it was running.
func (l *CallLog) finish(c *Call, answer []byte, failed, answeredAlready bool) {
	l.mu.Lock()
	defer l.mu.Unlock()
	at := l.now()
	c.Finished = &at
	c.Failed = failed
	c.Answer = append(json.RawMessage(nil), answer...)
	if !answeredAlready && len(answer) > keptAnswer {
		// Its caller has it; kept only in case the bus refuses it, and a whole declaration a
		// hundred times over is memory nobody asked for.
		c.Answer, _ = json.Marshal(map[string]any{"not kept": fmt.Sprintf(
			"an answer of %d bytes, sent to its caller in full", len(answer))})
	}
	if answeredAlready {
		c.State = CallFinishedAfter
	} else {
		c.State = CallAnswered
	}
	if shownOnce[c.Seat+"."+c.Verb] {
		// **A secret shown once is never kept on the bus** (novox/hq ADR 0169): a join token is the
		// one-time right to become a machine of the mesh, and `calls` answers anyone who may call the
		// seat. Its caller was sent it; kept in this process's memory only when its caller has not
		// had it yet — told the call was still running — and then until a restart, never beyond.
		withheld, _ := json.Marshal(map[string]any{"not kept": "a secret shown once, to its caller"})
		onTheBus := *c
		onTheBus.Answer = withheld
		if !answeredAlready {
			c.Answer = withheld
		}
		l.keep(onTheBus)
		return
	}
	onTheBus := *c
	if c.Seat == CLISeat {
		// **A mesh-cli line's answer is its asker's alone** (ADR 0272): what a command at the controller's terminal
		// printed — a token, a secret's reference — and `calls` answers anyone who may call the seat. Kept in this
		// process's memory for the asker to follow, and never on the bus.
		onTheBus.Answer, _ = json.Marshal(map[string]any{"not kept": "a mesh-cli line's answer, its asker's alone: " +
			"kept in the memory of the controller that ran it, for the asker to follow"})
		l.keep(onTheBus)
		return
	}
	if len(onTheBus.Answer) > keptOnTheBusAtMost {
		// A record on the bus is one message too, and one larger than the bus carries is refused whole —
		// the call's state with it (novox/hq issue 314). The answer stays in this process's memory, and
		// `calls` with its id pages it from there.
		onTheBus.Answer, _ = json.Marshal(map[string]any{"not kept": fmt.Sprintf("an answer of %d bytes, more than "+
			"a record on the bus holds: kept in the memory of the controller that answered it", len(c.Answer))})
	}
	l.keep(onTheBus)
}

// keptOnTheBusAtMost is the largest answer a call's record on the bus carries: half of the bus's default
// message limit, leaving the rest of the record room.
const keptOnTheBusAtMost = 512 << 10

// shownOnce are the verbs whose answer is a secret shown once to its caller, as `<seat>.<verb>`.
var shownOnce = map[string]bool{"mesh-controller.token": true}

// Recent is the kept calls, newest first, as copies: this process's from memory, and, when they are
// kept durably, every other the bus holds — a call a controller before this one served included.
// Memory wins for a call in both, being the newer word on it. A bus that cannot be read is said in
// the error beside what memory holds, never answered as no calls.
// Running is every call this process is serving that has not finished, oldest first, without
// answers: what the watchdog of a call's bound (novox/hq to-be 45 S7) reads. This process's own,
// because a call another controller left running is said abandoned when this one starts.
func (l *CallLog) Running() []Call {
	l.mu.Lock()
	defer l.mu.Unlock()
	var out []Call
	for _, c := range l.calls {
		if c.State == CallRunning {
			running := *c
			running.Answer = nil
			out = append(out, running)
		}
	}
	return out
}

func (l *CallLog) Recent() ([]Call, error) {
	l.mu.Lock()
	out := make([]Call, 0, len(l.calls))
	seen := map[string]bool{}
	for i := len(l.calls) - 1; i >= 0; i-- {
		out = append(out, *l.calls[i])
		seen[l.calls[i].ID] = true
	}
	keeper := l.keeper
	l.mu.Unlock()
	if keeper == nil {
		return out, nil
	}
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	kept, err := keeper.Recent(ctx)
	if err != nil {
		return out, fmt.Errorf("the calls kept on the bus could not be read, so only this controller's own "+
			"are listed: %w", err)
	}
	for _, c := range kept {
		if !seen[c.ID] {
			out = append(out, c)
		}
	}
	sort.SliceStable(out, func(i, j int) bool { return out[i].Started.After(out[j].Started) })
	return out, nil
}

// inMemory is one call this process served and still holds, whole.
func (l *CallLog) inMemory(id string) (Call, bool) {
	l.mu.Lock()
	defer l.mu.Unlock()
	for _, c := range l.calls {
		if c.ID == id {
			return *c, true
		}
	}
	return Call{}, false
}

// Shown is one kept call as `calls` shows it: whole, but for a mesh-cli line's answer, which is its asker's alone
// and followed by it (ADR 0272) — `calls` answers anyone who may call the seat.
func (l *CallLog) Shown(id string) (Call, bool, error) {
	c, found, err := l.Get(id)
	if found && c.Seat == CLISeat {
		c.Answer, _ = json.Marshal(map[string]any{"not shown": "a mesh-cli line's answer is its asker's alone: " +
			"mesh-cli follows it"})
	}
	return c, found, err
}

// Get is one kept call: from memory, or from the bus when this process did not serve it.
func (l *CallLog) Get(id string) (Call, bool, error) {
	l.mu.Lock()
	for _, c := range l.calls {
		if c.ID == id {
			found := *c
			l.mu.Unlock()
			return found, true, nil
		}
	}
	keeper := l.keeper
	l.mu.Unlock()
	if keeper == nil {
		return Call{}, false, nil
	}
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	return keeper.Kept(ctx, id)
}

// IsDurable says whether calls outlive this process.
func (l *CallLog) IsDurable() bool {
	l.mu.Lock()
	defer l.mu.Unlock()
	return l.keeper != nil
}

// kept is what a call's arguments are kept as: each argument by name, a value only when it is short
// and not one that carries settings or a secret — `calls` answers anyone who may call the seat.
func kept(args json.RawMessage) json.RawMessage {
	var given map[string]any
	if json.Unmarshal(args, &given) != nil {
		return nil
	}
	out := map[string]any{}
	for k, v := range given {
		s, isString := v.(string)
		switch {
		case k == "values" || k == "secret":
			out[k] = "(given, not kept)"
		case k == "line":
			// A mesh-cli line (ADR 0272): its command word, never the rest, which may carry settings.
			if words, ok := v.([]any); ok && len(words) > 0 {
				out[k] = []any{words[0], "(the rest given, not kept)"}
			}
		case isString && len(s) <= 120:
			out[k] = s
		case isString:
			out[k] = s[:120] + "…"
		default:
			out[k] = v
		}
	}
	body, _ := json.Marshal(out)
	return body
}

// refusedPublish is the bus's words for a publish it refused, with the subject.
var refusedPublish = regexp.MustCompile(`Permissions Violation for Publish to "([^"]+)"`)

// Refusal records the bus refusing an answer, from the error the client library hands the
// connection's error handler. It reports whether the error was the refusal of a kept call's answer.
func (l *CallLog) Refusal(err error, logger *log.Logger) bool {
	if err == nil {
		return false
	}
	m := refusedPublish.FindStringSubmatch(err.Error())
	if m == nil {
		return false
	}
	l.mu.Lock()
	var hit *Call
	for i := len(l.calls) - 1; i >= 0; i-- {
		if c := l.calls[i]; c.reply != "" && c.reply == m[1] {
			hit = c
			break
		}
	}
	if hit == nil {
		l.mu.Unlock()
		return false
	}
	at := l.now()
	hit.Refused = fmt.Sprintf("%s: %s", at.Format(time.RFC3339), err)
	id, verb, took := hit.ID, hit.Seat+"."+hit.Verb, at.Sub(hit.Started).Round(time.Second)
	l.keep(*hit)
	l.mu.Unlock()
	if logger != nil {
		// Said in the mesh's words, beside the library's own line: which call, and where its answer is.
		logger.Printf("the bus refused the answer to %s (%s, asked %s ago): its caller got no answer. "+
			"What it answered is kept under %s. A broker reloading its user list while a call runs "+
			"forgets that it may be answered", id, verb, took, id)
	}
	return true
}

// WatchRefusals chains the connection's error handler so a refused answer is recorded against its
// call. The handler the dialler set — the library's default, which prints — still runs after it.
func (l *CallLog) WatchRefusals(conn *nats.Conn, logger *log.Logger) {
	if conn == nil {
		return
	}
	watched.Lock()
	defer watched.Unlock()
	if watched.conns == nil {
		watched.conns = map[*nats.Conn]bool{}
	}
	if watched.conns[conn] {
		return
	}
	watched.conns[conn] = true
	before := conn.ErrorHandler()
	conn.SetErrorHandler(func(c *nats.Conn, s *nats.Subscription, err error) {
		if errors.Is(err, nats.ErrPermissionViolation) {
			l.Refusal(err, logger)
		}
		if before != nil {
			before(c, s, err)
		}
	})
}

var watched struct {
	sync.Mutex
	conns map[*nats.Conn]bool
}

type callIDKey struct{}

// WithCallID is ctx carrying the call it serves; CallIDIn reads it back, empty outside one.
func WithCallID(ctx context.Context, id string) context.Context {
	return context.WithValue(ctx, callIDKey{}, id)
}

// CallIDIn is the call ctx serves, or empty.
func CallIDIn(ctx context.Context) string {
	id, _ := ctx.Value(callIDKey{}).(string)
	return id
}

// answerNow is how a handler asks for its caller to be answered before it goes on (Acknowledge).
type answerNowKey struct{}

// Acknowledge answers the call's caller now that it is running, rather than after AnswerWithin. For
// a handler about to do what makes its answer unsendable: a push sends the bus's own machine first,
// and a broker reloading its user list forgets every answer it was about to permit. Without effect
// outside a served call, and after the first time.
func Acknowledge(ctx context.Context) {
	if f, ok := ctx.Value(answerNowKey{}).(func()); ok {
		f()
	}
}

// running is the interim answer: what a caller reads when the call has not finished.
func running(c *Call, within time.Duration, acknowledged bool, follow string) []byte {
	why := fmt.Sprintf("it has not finished in %s", within)
	if acknowledged {
		why = "it answers before it starts, because what it does can leave the bus unable to carry a later answer"
	}
	next := "its holder's journal says how it ended."
	if follow != "" {
		next = fmt.Sprintf("%s with call %q says what came of it.", follow, c.ID)
	}
	said := fmt.Sprintf("%s.%s is running as %s — %s. This is not a failure, and it may already have "+
		"done what was asked: %s", c.Seat, c.Verb, c.ID, why, next)
	body, _ := json.Marshal(map[string]any{"result": map[string]any{
		"running": true, "call": c.ID, "started": c.Started, "output": said,
	}})
	return body
}

// serveCall runs one call and answers it once: in full when the handler returns within AnswerWithin
// and has not acknowledged, and otherwise that it is running — its answer then kept, and logged.
func (l *CallLog) serveCall(seat, verb string, args json.RawMessage, reply string, handle ToolHandler,
	respond func([]byte) error, logger *log.Logger) {
	l.serveCallWithin(seat, verb, args, reply, handle, respond, 0, logger)
}

// serveCallWithin is serveCall on a bus that carries at most limit bytes in one message: an answer
// larger than that is paged (pages.go), and one the bus would not carry anyway is said to its caller
// and to `calls` — never dropped with only a line in the journal (novox/hq issue 314). A limit of 0 is
// one not known.
func (l *CallLog) serveCallWithin(seat, verb string, args json.RawMessage, reply string, handle ToolHandler,
	respond func([]byte) error, limit int64, logger *log.Logger) {
	ctx, cancel := context.WithTimeout(context.Background(), HandlerTimeout)
	defer cancel()
	if len(args) == 0 {
		args = json.RawMessage(`{}`)
	}
	c := l.begin(seat, verb, kept(args), reply)
	// Who asked travels with the call, so an act it does by hand says so (novox/hq to-be 45 §7).
	ctx = context.WithValue(ctx, callerKey{}, c.Caller)
	// And which call it is, for what the handler says of it in the journal.
	ctx = WithCallID(ctx, c.ID)
	acknowledged := make(chan struct{})
	var once sync.Once
	ctx = context.WithValue(ctx, answerNowKey{}, func() { once.Do(func() { close(acknowledged) }) })

	type outcome struct {
		body   []byte
		failed bool
	}
	done := make(chan outcome, 1)
	go func() {
		var body []byte
		// **A handler that panics is an answer, not a dead controller.** A verb answered in the serving
		// process (novox/hq issue 327: conditions, hand-acts, queue, dead-letters) runs on this goroutine,
		// where a panic would take every other call and the controller with it.
		defer func() {
			if p := recover(); p != nil {
				if logger != nil {
					logger.Printf("%s.%s: call %s panicked: %v\n%s", seat, verb, c.ID, p, debug.Stack())
				}
				body, _ = json.Marshal(map[string]any{"error": fmt.Sprintf("%s failed inside the controller and "+
					"answered nothing: %v. It is in the controller's log", verb, p)})
				done <- outcome{body, true}
			}
		}()
		result, err := handle(ctx, args)
		failed := err != nil
		if errors.Is(err, ErrHandingOver) {
			// Marked as well as said, so a caller asks again without reading the words (issue 289).
			body, _ = json.Marshal(map[string]any{"error": err.Error(), "retry": RetryHandingOver})
		} else if err != nil {
			body, _ = json.Marshal(map[string]any{"error": err.Error()})
		} else if body, err = json.Marshal(map[string]any{"result": result}); err != nil {
			failed = true
			body, _ = json.Marshal(map[string]any{"error": "the answer could not be written as JSON: " + err.Error()})
		}
		done <- outcome{body, failed}
	}()

	say := func(body []byte) {
		if limit > 0 && int64(len(body)) > limit {
			if logger != nil {
				logger.Printf("%s.%s: the answer to %s is %d bytes, more than the bus carries in one message (%d): "+
					"paged", seat, verb, c.ID, len(body), limit)
			}
			body = l.page(c, body, limit)
		}
		err := respond(body)
		if err == nil {
			return
		}
		if logger != nil {
			logger.Printf("%s.%s: could not answer %s: %v", seat, verb, c.ID, err)
		}
		l.unsent(c, err)
		if errors.Is(err, nats.ErrMaxPayload) {
			// The limit was not known, or was not the bus's: its caller is told, in a message that fits.
			if again := respond(cannotCarry(seat, verb, c.ID, len(body), err)); again != nil && logger != nil {
				logger.Printf("%s.%s: could not say to the caller of %s why it has no answer: %v", seat, verb, c.ID, again)
			}
		}
	}
	timer := time.NewTimer(AnswerWithin)
	defer timer.Stop()
	select {
	case o := <-done:
		l.finish(c, o.body, o.failed, false)
		say(o.body)
		return
	case <-acknowledged:
		say(running(c, AnswerWithin, true, l.Follow))
	case <-timer.C:
		say(running(c, AnswerWithin, false, l.Follow))
	}
	o := <-done
	l.finish(c, o.body, o.failed, true)
	if logger != nil {
		how := "and it succeeded"
		if o.failed {
			how = "and it answered an error"
		}
		logger.Printf("%s (%s.%s) finished after %s, after its caller was told it was running, %s — its answer is kept under %s", c.ID,
			seat, verb, time.Since(c.Started).Round(time.Second), how, c.ID)
	}
}