mesh-host/internal/link/queue.go

package link

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"sync"
	"time"
)

// Queue is the node-engine's one apply queue (novox/hq to-be 45 §6, ADR 0227 rule 1).
//
// **One worker applies; everything else asks it to.** A declaration delivered over the link and the
// five-minute reconcile are two reasons to *enqueue* the same act, never two paths that apply. Before
// this they were two, each taking an apply lock, and every order between them was a fault met on the
// live mesh: the reconcile read what was kept before a delivery and applied it over the newer one
// (novox/hq issues 257, 261); the reconcile went first and its report about the older declaration
// reached the mesh after the delivery's (267). Each fix closed one order and left the class open.
//
// The queue holds at most one pending request, coalesced: whatever has been delivered and not yet
// taken, and whether a reconcile is due. When the worker starts it takes the **newest declaration held
// at that moment**, sets the rest aside — each reported as set aside, then settled unapplied — applies
// it once, and makes **one report**, naming the declaration's order and its own report sequence. A
// reconcile due while a delivery waits is that delivery's apply: applying the newest thing the mesh
// said is what reconciling is. Only with nothing delivered does a reconcile apply what was kept, read
// once its turn has come.
//
// **Reports leave in the order they are made**, because the worker makes and says them, one at a time.
// A report made while the link is down waits: an apply's in the unsaid store (issue 264), a
// reconcile's in one slot replaced by anything newer, and both are said, in that order, when the next
// link opens — before anything newly delivered is applied.
type Queue struct {
	// Membership is whose declarations these are: their signer, and the node the reports are from.
	Membership Membership
	// Apply applies a declaration the mesh signed.
	Apply Applier
	// Reconcile holds the machine to what it was last told, read once it is this act's turn. False
	// when there is nothing to hold it to, or the reconcile could not run; it says why itself.
	Reconcile Reconcile
	// News is whether a reconcile's report is worth saying unasked (novox/hq ADR 0100, ADR 0140). Nil
	// is never: a reconcile is otherwise silent.
	News func(Report) bool
	// Heard is told every account of the machine the mesh has taken — an apply's or a reconcile's,
	// not a refusal or a declaration set aside, which say nothing about the machine. Nil is allowed.
	Heard func(Report)
	// Unsaid keeps an apply's report until the mesh has taken it (issue 264). Nil keeps nothing.
	Unsaid Unsaid
	// Numbers keeps the report sequence and the refusals counted, across restarts and self-updates.
	// Nil counts in memory, which is only right where nothing outlives the process — a test.
	Numbers Numbers
	Say     Announce
	Timeout time.Duration

	once sync.Once
	wake chan struct{}

	mu   sync.Mutex
	idle *sync.Cond
	// delivering is a delivered declaration in hand: its report is owed on the link it came over, so
	// that link is not let go until it is said (detach). A reconcile in hand holds no link — its report
	// waits for the next one if this one goes.
	delivering bool
	waiting    []Declaration // delivered and not yet taken, in the order they arrived
	due        bool          // a reconcile asked for
	bus        Bus           // the link open now; nil while there is none
	unasked    *Report       // a reconcile's report not yet said, made while there was no link

	// saying is held while a report is made and said, so the order they leave in is the order they
	// are made in — whoever says them, the worker or a link opening.
	saying  sync.Mutex
	counted bool
	numbers numbered
}

// Reconcile is the reconcile's act, run by the queue's worker. See Queue.Reconcile.
type Reconcile func(ctx context.Context) (Report, bool)

// Numbers is where the queue keeps what it counts, so a successor goes on from where it stopped.
type Numbers interface {
	// Read is what was kept: zero for a node that never reported, an error when it cannot be read —
	// never zero for "could not tell".
	Read() (reportSequence, refusedOlder int64, err error)
	// Save keeps them, on disk when it returns.
	Save(reportSequence, refusedOlder int64) error
}

type numbered struct{ sequence, refused int64 }

func (q *Queue) init() {
	q.once.Do(func() {
		q.wake = make(chan struct{}, 1)
		q.idle = sync.NewCond(&q.mu)
		if q.Say == nil {
			q.Say = func(string) {}
		}
	})
}

// Deliver enqueues what the link delivered, in the order it arrived. It never waits for an apply.
func (q *Queue) Deliver(arrived ...Declaration) {
	q.init()
	q.mu.Lock()
	q.waiting = append(q.waiting, arrived...)
	q.mu.Unlock()
	q.signal()
}

// ReconcileDue enqueues a reconcile. One asked for while another is waiting is the same one.
func (q *Queue) ReconcileDue() {
	q.init()
	q.mu.Lock()
	q.due = true
	q.mu.Unlock()
	q.signal()
}

func (q *Queue) signal() {
	select {
	case q.wake <- struct{}{}:
	default:
		// One is already waiting to be read, and the worker takes everything pending when it does.
	}
}

// Run is the worker. It returns once the context ends and the act in hand is finished and said:
// asked to stop — the host standing aside for its successor — nothing further is applied.
func (q *Queue) Run(ctx context.Context) {
	q.init()
	for {
		for q.step(ctx) {
		}
		select {
		case <-ctx.Done():
			return
		case <-q.wake:
		}
	}
}

// step takes what is pending and does it, once. False when there was nothing to do, or the queue
// was asked to stop.
func (q *Queue) step(ctx context.Context) bool {
	q.mu.Lock()
	if ctx.Err() != nil || (len(q.waiting) == 0 && !q.due) {
		q.mu.Unlock()
		return false
	}
	batch, due := q.waiting, q.due
	q.waiting, q.due = nil, false
	q.delivering = len(batch) > 0
	q.mu.Unlock()
	defer func() {
		q.mu.Lock()
		q.delivering = false
		q.idle.Broadcast()
		q.mu.Unlock()
	}()

	if len(batch) == 0 {
		q.reconcile(ctx)
		return true
	}

	latest, aside := pick(batch)
	for _, old := range aside {
		if d := declaredIn(old.Body()); d != "" && d == declaredIn(latest.Body()) {
			// The same declaration again — delivered twice while the worker was busy. Nothing is set
			// aside: it is about to be applied.
			_ = old.Handled()
			continue
		}
		q.Say(fmt.Sprintf("set aside declaration %s (%s): a newer one arrived with it",
			short(declaredIn(old.Body())), orderOf(old.Body()).Words()))
		q.tell(ctx, Report{Declared: declaredIn(old.Body()), Order: orderOf(old.Body()),
			Superseded: declaredIn(latest.Body())}, setAside, old)
	}

	report := handleBody(ctx, q.Membership, latest.Body(), q.Apply)
	switch {
	case report.OlderThan != nil:
		// **Refused, counted and said** (rule 2): one line here, the count on this and every later
		// report, and the report itself naming what was refused and what is held.
		total := q.countRefusal()
		q.Say(fmt.Sprintf("refused declaration %s (%s): this node applied %s, from a newer lease "+
			"holder — %d refused as older so far", short(report.Declared), report.Order.Words(),
			report.OlderThan.Words(), total))
	case report.Refused != "":
		q.Say("refused a declaration: " + report.Refused)
	case len(report.Failed) > 0:
		q.Say(fmt.Sprintf("applied %d and failed: %v%s",
			len(report.Applied), report.Failed, heldNote(report.Held)))
	default:
		q.Say(fmt.Sprintf("applied %d resource(s)%s", len(report.Applied), heldNote(report.Held)))
	}
	if due && report.Refused != "" {
		// Nothing was applied, so the reconcile this stood in for has not happened. It runs next.
		q.mu.Lock()
		q.due = true
		q.mu.Unlock()
	}
	// A refusal as older is not kept to be said again: what the mesh waits for from this node is the
	// account of the last apply, and that is still what is kept.
	kind := applied
	if report.OlderThan != nil {
		kind = refusedOlder
	}
	q.tell(ctx, report, kind, latest)
	return true
}

func (q *Queue) reconcile(ctx context.Context) {
	if q.Reconcile == nil {
		return
	}
	report, ok := q.Reconcile(ctx)
	if !ok || q.News == nil || !q.News(report) {
		return
	}
	q.tell(ctx, report, unasked)
}

// What a report is, for what tell does with it besides saying it.
type kindOf int

const (
	// applied is an apply's account: kept until said (issue 264), and fresher than any reconcile's
	// report still waiting.
	applied kindOf = iota
	// setAside is a declaration a newer one took the place of, unapplied.
	setAside
	// refusedOlder is a declaration refused as older than what was applied. Not kept: what the mesh
	// waits for from this node is still the account of the last apply.
	refusedOlder
	// unasked is a reconcile's report. Not said — no link, or the bus refused it — the newest waits
	// for the next link, and anything the worker says before then replaces it.
	unasked
)

// tell makes a report this node's — its node, its report sequence, the refusals counted — and says it
// on the link open now. Kept first, when it is an apply's, so a report lost between here and the bus
// is said by whichever host links next. Each declaration it settles is settled after it is said,
// either way. True when the broker took it.
func (q *Queue) tell(ctx context.Context, r Report, kind kindOf, settle ...Declaration) bool {
	q.saying.Lock()
	defer q.saying.Unlock()

	r = q.stamp(r)
	keep := kind == applied
	if keep {
		// What this says supersedes any reconcile's report still waiting to be said.
		q.unasked = nil
		if q.Unsaid != nil {
			if err := q.Unsaid.Keep(r); err != nil {
				q.Say("cannot keep this apply's report until it is said: " + err.Error())
			}
		}
	}
	q.mu.Lock()
	bus := q.bus
	q.mu.Unlock()

	published := false
	if bus != nil {
		// **Said even when the link is being let go** (issue 264): not cancelled with it, still
		// bounded by the timeout, and the link is not closed until this returns (detach).
		published = publishReport(context.WithoutCancel(ctx), bus, q.Membership, r, q.Say, q.Timeout)
	}
	if !published && kind == unasked {
		q.unasked = &r
	}
	if published {
		if keep && q.Unsaid != nil {
			if err := q.Unsaid.Said(r.Declared); err != nil {
				q.Say("said this apply's report, and cannot forget it: " + err.Error())
			}
		}
		if q.Heard != nil && (kind == applied || kind == unasked) && r.Refused == "" {
			q.Heard(r)
		}
	}
	// Settled after the report is published. A node that dies between applying and reporting
	// leaves the declaration with the mesh and applies it again on return, which is safe because
	// applying is reconciliation — it converges rather than repeating.
	for _, d := range settle {
		_ = d.Handled()
	}
	return published
}

// stamp gives a report its node, the next report sequence and the refusals counted. Called holding
// saying.
func (q *Queue) stamp(r Report) Report {
	q.load()
	q.numbers.sequence++
	q.keepNumbers()
	r.Node = q.Membership.Node
	r.ReportSequence = q.numbers.sequence
	r.RefusedOlder = q.numbers.refused
	return r
}

func (q *Queue) countRefusal() int64 {
	q.saying.Lock()
	defer q.saying.Unlock()
	q.load()
	q.numbers.refused++
	q.keepNumbers()
	return q.numbers.refused
}

// load reads what was counted, once. **Unreadable is said, and the sequence goes on from above
// anything it can have reached** — the time in milliseconds — rather than from one, which would make
// every report this host says next older than what it already said, and refused.
func (q *Queue) load() {
	if q.counted {
		return
	}
	q.counted = true
	if q.Numbers == nil {
		return
	}
	sequence, refused, err := q.Numbers.Read()
	if err != nil {
		floor := time.Now().UnixMilli()
		q.Say(fmt.Sprintf("cannot read this node's report sequence: %v — going on from %d, above any "+
			"it can have reached, and counting refusals again from none", err, floor))
		q.numbers = numbered{sequence: floor}
		return
	}
	q.numbers = numbered{sequence: sequence, refused: refused}
}

func (q *Queue) keepNumbers() {
	if q.Numbers == nil {
		return
	}
	if err := q.Numbers.Save(q.numbers.sequence, q.numbers.refused); err != nil {
		// Said, and not fatal: the report is still worth saying. What is lost is that a successor
		// could number one again.
		q.Say("cannot keep this node's report sequence: " + err.Error())
	}
}

// attach is a link opening. **What the last apply did, if the mesh never heard it**, is said first,
// exactly as it was made; then a reconcile's report that waited; and only then may the worker say
// anything on it — so a report said again can never land after, and read as newer than, a later one.
func (q *Queue) attach(ctx context.Context, bus Bus) {
	q.init()
	q.saying.Lock()
	defer q.saying.Unlock()
	reporting := context.WithoutCancel(ctx)
	q.load()

	said := true
	if q.Unsaid != nil {
		switch kept, ok, err := q.Unsaid.Pending(); {
		case err != nil:
			q.Say("cannot read the report this node kept unsaid: " + err.Error())
		case ok:
			q.Say("saying again what the last apply did: its report never reached the mesh")
			if said = publishReport(reporting, bus, q.Membership, kept, q.Say, q.Timeout); said {
				if err := q.Unsaid.Said(kept.Declared); err != nil {
					q.Say("said the kept report, and cannot forget it: " + err.Error())
				}
				if q.Heard != nil && kept.Refused == "" {
					q.Heard(kept)
				}
			}
		}
	}
	if said && q.unasked != nil {
		if publishReport(reporting, bus, q.Membership, *q.unasked, q.Say, q.Timeout) {
			if q.Heard != nil {
				q.Heard(*q.unasked)
			}
			q.unasked = nil
		}
	}

	q.mu.Lock()
	q.bus = bus
	q.mu.Unlock()
}

// detach is the link closing: it waits for a delivery in hand to be applied and said — an apply that
// stood aside for its successor is reported on this link before it is let go (issue 264) — and then
// nothing more is said on it. A reconcile in hand is not waited for: a link that was dropped or roused
// is opened again at once, and the reconcile's report, if it is news, waits for it.
func (q *Queue) detach(bus Bus) {
	q.init()
	q.mu.Lock()
	for q.delivering {
		q.idle.Wait()
	}
	q.mu.Unlock()

	q.saying.Lock()
	defer q.saying.Unlock()
	q.mu.Lock()
	if q.bus == bus {
		q.bus = nil
	}
	q.mu.Unlock()
}

// pick is the newest of what was delivered and the rest, in arrival order, to set aside: by order
// when the declarations claim one, by arrival when they do not (Order.Supersedes).
func pick(batch []Declaration) (Declaration, []Declaration) {
	latest := batch[0]
	var aside []Declaration
	for _, next := range batch[1:] {
		if orderOf(next.Body()).Supersedes(orderOf(latest.Body())) {
			aside = append(aside, latest)
			latest = next
			continue
		}
		aside = append(aside, next)
	}
	return latest, aside
}

// orderOf is the order a signed declaration claims, or none when it claims none or cannot be read.
// Read from the envelope alone; the signature is verified when the winner is applied, and a forged
// message that lied about its order would only set aside real ones — which are reported as set aside,
// and the next push sends the current one again.
func orderOf(body []byte) Order {
	var signed Signed
	if err := json.Unmarshal(body, &signed); err != nil {
		return Order{}
	}
	var o Order
	if err := json.Unmarshal(signed.Declaration, &o); err != nil || o.Epoch < 0 || o.Sequence < 0 {
		return Order{}
	}
	return o
}

// declaredIn names a signed declaration as the mesh does — the digest of the declaration's bytes, the
// same the apply's report carries — for a report about one that was not applied. Empty if the message
// is not one: a forged or garbled message is refused when its turn comes; here it is only named.
func declaredIn(body []byte) string {
	var signed Signed
	if err := json.Unmarshal(body, &signed); err != nil || len(signed.Declaration) == 0 {
		return ""
	}
	sum := sha256.Sum256(signed.Declaration)
	return hex.EncodeToString(sum[:])
}