mesh-host/internal/link/hearing_nats.go

package link

import (
	"context"
	"errors"
	"fmt"
	"strings"
	"time"

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

// The host's link on the bus being built.
//
// Two things a host does here that it cannot do on the other bus, and one it must not try.
//
// **It declares nothing.** On the bus the mesh has, a host declares its own queue on connecting,
// because a queue that is not there means a node that hears nothing. Here the object it reads
// through is a durable consumer, and a host's account reaches no part of the JetStream API — by
// design, because the controller is the only writer of consumer definitions (design 25 §3). So the
// host **binds** to a consumer the controller made when this node enrolled, and a missing one is
// said as what it is rather than quietly created with whatever configuration this client happens to
// default to.
//
// **It gets order for free, and keeps the drain anyway.** The declaration subject is last-per-subject
// (design 29 §4), so a node that was away receives exactly the current declaration rather than a
// queue of superseded ones, and the stream's sequence orders them definitively — the wire-level
// answer to novox/hq issue 107. What the drain in run.go still answers is the live case: three
// pushes to a *connected* node are three deliveries whatever the stream later retains.

// EnrolSubject is where a joining machine asks. One subject for every node, because a machine
// enrolling has no name the mesh has agreed to yet — which is why its authority to publish here is
// the whole of what its enrolment user may do.
const EnrolSubject = "mesh.control.enrol"

// DeclareSubject is where this node's declaration lands. Its own, and no other node's: a host's
// account subscribes exactly this and the subject is the authority on which node a declaration is
// for.
func DeclareSubject(node string) string { return "mesh.node." + node + ".declare" }

// AskReportSubject is where the mesh asks this node to say again what it last applied (novox/hq to-be 45
// §6): its own, on core NATS and off any stream. The answer is an ordinary report on its report subject,
// the one thing it already says — nobody's inbox is answered.
func AskReportSubject(node string) string { return "mesh.node." + node + ".ask.report" }

// natsURL is a bus address as the client wants it. A membership records host and port, because that
// is what genesis sealed into it and what the other transport takes; the scheme is this transport's
// own business.
func natsURL(address string) string {
	if strings.Contains(address, "://") {
		return address
	}
	return "nats://" + address
}

// natsLink is this node's connection as a JetStream subscription.
type natsLink struct {
	conn    *nats.Conn
	js      nats.JetStreamContext
	sub     *nats.Subscription
	asking  *nats.Subscription
	asked   chan struct{}
	node    string
	arrived chan Declaration
	lost    chan error
}

func dialNats(ctx context.Context, m Membership, timeout time.Duration) (Link, error) {
	// Pinned exactly as the other transport is, and for once the Go client makes that easy: it
	// takes a *tls.Config, so the same PinnedConfig with the same VerifyPeerCertificate does the
	// work. **The constraint recorded against the tool runtime does not apply here** — that client
	// takes PEM strings with no verify hook, which is why the bus's certificate must carry a name
	// matching the address *modules* dial it by. A host checks the fingerprint and nothing else.
	config, err := PinnedConfig(m.Fingerprint)
	if err != nil {
		return nil, err
	}
	opts := []nats.Option{
		nats.Secure(config),
		// The mesh names a machine's bus user "node.<name>" (the controller's principal scheme), and
		// the server refused the bare name the first time a machine dialled it: "authentication
		// error - User". The same string the mesh composed into the user list, or nothing connects.
		nats.UserInfo("node."+m.Node, m.Password),
		// Replies to what this client asks the server arrive on its inbox, and the mesh grants a
		// machine exactly its own: the same prefix the user list was composed with.
		nats.CustomInboxPrefix("_INBOX.node." + m.Node),
		nats.Name("mesh-host/" + m.Node),
		nats.Timeout(timeout),
		// A node that has silently lost its route notices, rather than holding a connection the
		// server forgot about and believing it is still in the mesh.
		nats.PingInterval(10 * time.Second),
		nats.MaxPingsOutstanding(2),
		// Reconnection is the caller's: Hold already decides when to try again and how long to
		// wait, and a client quietly reconnecting underneath it would make that reasoning a
		// duplicate of the library's.
		nats.NoReconnect(),
	}

	conn, err := nats.Connect(natsURL(m.Broker), opts...)
	if err != nil {
		if errors.Is(err, ErrWrongCertificate) {
			return nil, err
		}
		return nil, fmt.Errorf("cannot reach the bus at %s: %w", m.Broker, err)
	}
	js, err := conn.JetStream()
	if err != nil {
		conn.Close()
		return nil, fmt.Errorf("the bus at %s has no JetStream: %w", m.Broker, err)
	}

	l := &natsLink{
		conn: conn, js: js, node: m.Node,
		arrived: make(chan Declaration, drainDepth),
		lost:    make(chan error, 1),
		asked:   make(chan struct{}, 1),
	}

	// Bound to the consumer the controller made for this node, named after the node because that is
	// what the node's own ack grant allows (`$JS.ACK.NODES.<node>.>`).
	feed := make(chan *nats.Msg, drainDepth)
	// The subject as well as the binding: the client checks what is asked for against the
	// consumer's own filter, and an empty subject is refused rather than taken to mean "whatever
	// that consumer delivers".
	sub, err := js.ChanSubscribe(DeclareSubject(m.Node), feed, nats.Bind("NODES", m.Node))
	if err != nil {
		conn.Close()
		return nil, fmt.Errorf(
			"this node cannot read its declarations: %w. The mesh creates that when a node enrols, "+
				"and a host may not create one itself — so this is the mesh's to answer, not this "+
				"machine's", err)
	}
	l.sub = sub

	l.hearAsks()

	conn.SetDisconnectErrHandler(func(_ *nats.Conn, err error) {
		select {
		case l.lost <- fmt.Errorf("the link dropped: %w", err):
		default:
		}
	})
	conn.SetClosedHandler(func(*nats.Conn) {
		select {
		case l.lost <- errors.New("the link closed"):
		default:
		}
	})

	go func() {
		defer close(l.arrived)
		for {
			select {
			case <-ctx.Done():
				return
			case msg, ok := <-feed:
				if !ok {
					select {
					case l.lost <- errors.New("the bus stopped delivering"):
					default:
					}
					return
				}
				select {
				case l.arrived <- natsDeclaration{msg}:
				case <-ctx.Done():
					return
				}
			}
		}
	}()

	return l, nil
}

// hearAsks listens for the mesh asking what this node last applied (the `report` verb). One question
// waiting is enough: a second asked before the first is taken is the same question. A bus whose user
// list is older than the grant refuses the subscription asynchronously — said by the client, and the
// mesh then sends again instead of asking; nothing here depends on being asked.
func (l *natsLink) hearAsks() {
	if l.asked == nil {
		l.asked = make(chan struct{}, 1)
	}
	asking, err := l.conn.Subscribe(AskReportSubject(l.node), func(*nats.Msg) {
		select {
		case l.asked <- struct{}{}:
		default:
		}
	})
	if err == nil {
		l.asking = asking
	}
}

func (l *natsLink) Declarations() <-chan Declaration { return l.arrived }
func (l *natsLink) AskedToReport() <-chan struct{}   { return l.asked }
func (l *natsLink) Lost() <-chan error               { return l.lost }

func (l *natsLink) Close() {
	if l.sub != nil {
		_ = l.sub.Unsubscribe()
	}
	if l.asking != nil {
		_ = l.asking.Unsubscribe()
	}
	if l.conn != nil {
		l.conn.Close()
	}
}

func (l *natsLink) Report(ctx context.Context, node string, body []byte) error {
	return OverNATS{Conn: l.conn, JS: l.js}.Report(ctx, node, body)
}

func (l *natsLink) Alive(ctx context.Context, node string, body []byte) error {
	return OverNATS{Conn: l.conn, JS: l.js}.Alive(ctx, node, body)
}

// natsDeclaration is one declaration off the NODES stream.
type natsDeclaration struct{ msg *nats.Msg }

func (d natsDeclaration) Body() []byte { return d.msg.Data }

// Handled acknowledges it. The ack goes to this node's own ack subject, which is the one thing
// besides its reports a node's account may publish.
func (d natsDeclaration) Handled() error { return d.msg.Ack() }