mesh-controller/internal/link/receive_nats.go

package link

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"strings"
	"time"

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

	"github.com/novox/mesh-controller/internal/broker"
)

// The consume side on the bus being built.
//
// The shape is the AMQP one's, because the seam made them comparable: one loop, one message at a
// time, and the same window deciding. What differs is where a held message lives — and that is the
// whole point of the move. On the bus the mesh has, holding one means keeping an unacknowledged
// delivery in this process, bounded by the prefetch and lost if the controller stops. Here it is a
// `nak` with a delay: the message stays the server's, the controller keeps nothing but the moment
// it first could not take it, and a controller that restarts mid-window has nothing to lose.

// natsInbound consumes what nodes and modules say over NATS.
type natsInbound struct {
	js *broker.JetStream
	// follows is the kinds asked for beyond what nodes say (Also). The events those are are the
	// only ones the controller subscribes, and only when something is listening.
	follows map[string]bool
	// since is when the controller first could not take a message, by that message's place in its
	// stream.
	//
	// **A timestamp, not a message.** This is the whole difference the move buys: the AMQP side
	// keeps the delivery, and this keeps eight bytes saying when the window opened. A controller
	// that restarts loses these and starts the window again, which is correct — it is holding
	// nothing, and the messages are all still on the server.
	since map[uint64]time.Time
}

// Nats is the consume side of the bus being built.
func Nats(js *broker.JetStream) Inbound {
	return &natsInbound{js: js, follows: map[string]bool{}, since: map[uint64]time.Time{}}
}

// Also records one more kind to subscribe. Nothing is subscribed here: the controller's consumer on
// the events stream carries both of these as filters, so it is created once, in Receive, with
// whatever was asked for — and not at all when nothing was.
func (n *natsInbound) Also(kind string) error {
	switch kind {
	case KindModuleMoved, KindCatchUp:
		n.follows[kind] = true
		return nil
	default:
		return fmt.Errorf("nothing subscribes %s separately on this bus", kind)
	}
}

func (n *natsInbound) Close() {}

// Receive consumes until the context ends.
//
// Three subscriptions, and each is a channel the one loop selects on. **Channels rather than
// callbacks**: the library would run a handler on its own goroutine, and the window's bookkeeping —
// which message is held, and since when — is read and written without a lock because the AMQP loop
// never had two. A second goroutine would make that wrong in a way no test would catch.
func (n *natsInbound) Receive(ctx context.Context, act func(context.Context, Control)) error {
	if err := broker.AssertMeshConsumers(n.js); err != nil {
		return err
	}
	js, conn := n.js.Context(), n.js.Conn()

	// What nodes say, off the CONTROL stream. Bound to the durable the controller asserted rather
	// than creating one here: the consumer is an object with a configuration — ack policy, ack
	// wait, redelivery — and a client that creates its own would be a second opinion about it.
	control := make(chan *nats.Msg, Prefetch)
	said, err := js.ChanSubscribe("", control, nats.Bind("CONTROL", broker.ControllerName))
	if err != nil {
		return fmt.Errorf("subscribing to what nodes say: %w", err)
	}
	defer func() { _ = said.Unsubscribe() }()

	// Heartbeats, on core NATS and off any stream (design 25 §3). Their own subscription because
	// they are their own guarantee: a lost one is the next one.
	beats := make(chan *nats.Msg, Prefetch)
	alive, err := conn.ChanSubscribe(AliveSubjects, beats)
	if err != nil {
		return fmt.Errorf("subscribing to heartbeats: %w", err)
	}
	defer func() { _ = alive.Unsubscribe() }()

	// The events the controller follows, when something is listening for them.
	var events chan *nats.Msg
	if len(n.follows) > 0 {
		events = make(chan *nats.Msg, Prefetch)
		followed, err := js.ChanSubscribe("", events, nats.Bind("EVENTS", broker.ControllerName))
		if err != nil {
			return fmt.Errorf("subscribing to what the catalogue says: %w", err)
		}
		defer func() { _ = followed.Unsubscribe() }()
	}

	// A connection that dropped is said, not discovered. A controller whose bus connection is gone
	// is a mesh where nothing can be told anything.
	gone := make(chan error, 1)
	conn.SetDisconnectErrHandler(func(_ *nats.Conn, err error) {
		select {
		case gone <- err:
		default:
		}
	})

	for {
		select {
		case <-ctx.Done():
			return nil
		case err := <-gone:
			return fmt.Errorf("the bus connection dropped: %w", err)
		case msg := <-beats:
			n.deliver(ctx, act, msg, false)
		case msg := <-events:
			n.deliver(ctx, act, msg, true)
		case msg, ok := <-control:
			if !ok {
				return errors.New("the bus stopped delivering")
			}
			n.deliver(ctx, act, msg, true)
		}
	}
}

// deliver names one message and hands it to the loop, or drops it where the mesh has no name for
// its subject — which cannot happen through a filter the controller wrote, and is said rather than
// ignored for exactly that reason.
func (n *natsInbound) deliver(ctx context.Context, act func(context.Context, Control),
	msg *nats.Msg, streamed bool) {

	kind, known := kindOfSubject(msg.Subject)
	if !known {
		if streamed {
			_ = msg.Term()
		}
		return
	}
	m := &natsControl{kind: kind, msg: msg, on: n}
	if streamed {
		// A message with no metadata is not from a stream, whatever it was delivered on, and the
		// window has nothing to hold it by. Said by leaving the sequence at zero.
		if meta, err := msg.Metadata(); err == nil {
			m.seq = meta.Sequence.Stream
			m.delivered = meta.NumDelivered
		}
	}
	act(ctx, m)
}

// kindOfSubject is how this transport's addressing becomes what the mesh calls a message.
//
// By subject, which is the only thing the server enforces: a body claiming to be a report does not
// make it one, and on this bus the subject an account may publish *is* its authority (design 29
// §2). The mirror of the routing-key table on the bus the mesh has.
func kindOfSubject(subject string) (string, bool) {
	switch subject {
	case EnrolSubject:
		return KindEnrolment, true
	case BuiltSubject:
		return KindBuilt, true
	}
	if node, rest, ok := strings.Cut(strings.TrimPrefix(subject, "mesh.control."), "."); ok &&
		node != "" && !strings.Contains(node, ".") {
		switch rest {
		case "report":
			return KindReport, true
		case "alive":
			return KindHeartbeat, true
		}
	}
	switch subject {
	case broker.ControllerFollows[0]:
		return KindModuleMoved, true
	case broker.ControllerFollows[1]:
		return KindCatchUp, true
	}
	return "", false
}

// natsControl is one message from the bus being built, as the controller reads it.
type natsControl struct {
	kind string
	msg  *nats.Msg
	on   *natsInbound
	// seq is this message's place in its stream; zero for a core message, which has none and
	// cannot be held.
	seq uint64
	// delivered is how many times the server has handed this message over, this time included.
	delivered uint64
}

func (m *natsControl) Kind() string { return m.kind }
func (m *natsControl) Body() []byte { return m.msg.Data }

// Redelivered is what the server counted, not what the controller remembers. Which is the answer to
// a question the AMQP side could only guess at across a restart: an enrolment redelivered because
// the controller stopped mid-answer reads as redelivered to the controller that comes back.
func (m *natsControl) Redelivered() bool { return m.delivered > 1 }

func (m *natsControl) HeldFor() time.Duration {
	if m.seq == 0 {
		return 0
	}
	first, held := m.on.since[m.seq]
	if !held {
		return 0
	}
	return time.Since(first)
}

// About is nothing here, and that is the point.
//
// Setting a held message aside when a newer one about the same thing arrives is what a controller
// holding deliveries in memory can do. A naked message belongs to the server and comes back
// whatever happened meanwhile, so the question "is this the past?" is answered by what the message
// says instead — the digest of the declaration a report is about (window.go, design 25 §3).
func (m *natsControl) About(string) {}

// Answer publishes to the reply subject the request carries **in its payload**.
//
// Not `Respond`, and not the message's reply field: a message a JetStream consumer delivers has had
// that field claimed for the consumer's own ack address, so answering it would send the reply to
// `$JS.ACK.CONTROL.controller.…` and the enrolling node would wait out its timeout. Verified
// against a running server (design 25 §2), which is why it is a field of the request and this reads
// it from there.
func (m *natsControl) Answer(ctx context.Context, body []byte) error {
	var addressed replyAddressed
	if err := json.Unmarshal(m.msg.Data, &addressed); err != nil {
		return fmt.Errorf("that request cannot be read, so its reply address cannot be: %w", err)
	}
	if addressed.ReplyTo == "" {
		return errors.New("that request named no reply subject in its payload, so nothing can be " +
			"told the answer")
	}
	return m.on.js.Conn().PublishMsg(&nats.Msg{Subject: addressed.ReplyTo, Data: body})
}

func (m *natsControl) Took() error {
	m.forget()
	if m.seq == 0 {
		// Core NATS: nothing is keeping it, so there is nothing to settle.
		return nil
	}
	return m.msg.Ack(nats.Context(context.Background()))
}

// Drop terminates the delivery: understood, and the server is told not to send it again. Different
// from an ack only in the server's own accounting, which is where somebody asking "what happened to
// that message" will look.
func (m *natsControl) Drop() error {
	m.forget()
	if m.seq == 0 {
		return nil
	}
	return m.msg.Term()
}

// Hold hands the message back with a delay, and remembers when the window opened.
func (m *natsControl) Hold(after time.Duration) error {
	if m.seq == 0 {
		return errors.New("a message that is not in a stream cannot be held: nothing is keeping it")
	}
	if _, already := m.on.since[m.seq]; !already {
		m.on.since[m.seq] = time.Now()
	}
	return m.msg.NakWithDelay(after)
}

func (m *natsControl) forget() {
	if m.on != nil && m.seq != 0 {
		delete(m.on.since, m.seq)
	}
}

// replyAddressed is the one field every message that expects an answer carries.
type replyAddressed struct {
	ReplyTo string `json:"reply_to,omitempty"`
}