mesh-host/internal/link/asking_nats.go

package link

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"fmt"
	"time"

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

// The enrolment conversation on the bus being built.
//
// **The reply address is the whole of what changes**, and it changes for a reason the transport
// forces rather than a preference. Core NATS request/reply puts the caller's inbox in the message's
// reply field and a plain responder answers it — but this request goes into a stream, and a message
// a JetStream consumer delivers has had that field claimed for the consumer's own ack address. So by
// the time the controller reads the request, the transport's reply field names where the
// *controller* must acknowledge. Verified against a running server (design 25 §2).
//
// The address therefore travels as a field of the request, and this subscribes it before publishing:
// a node that published first could miss an answer to a question nobody was listening for.

// enrolInbox is where a node enrolling waits.
//
// Under `_INBOX.enrol.<node>.`, which is exactly what its enrolment user may subscribe and no
// wider — so an answer sealed to one machine cannot be read by another enrolling beside it. The
// random tail is this attempt's own: a reply left over from an attempt that timed out is not the
// answer to this question, which is what the correlation id does on the other transport.
func enrolInbox(node string) (string, error) {
	tail := make([]byte, 8)
	if _, err := rand.Read(tail); err != nil {
		return "", fmt.Errorf("cannot make a reply address: %w", err)
	}
	return "_INBOX.enrol." + node + "." + hex.EncodeToString(tail), nil
}

type natsAsking struct {
	conn    *nats.Conn
	js      nats.JetStreamContext
	inbox   string
	answers *nats.Subscription
	lost    chan error
}

func presentNats(_ context.Context, to Approach, node, secret string,
	timeout time.Duration) (Asking, error) {

	config, err := PinnedConfig(to.Fingerprint)
	if err != nil {
		return nil, err
	}

	inbox, err := enrolInbox(node)
	if err != nil {
		return nil, err
	}

	lost := make(chan error, 1)
	// The user is this token's own — `enrol.<node>`, which may publish the enrolment subject and
	// subscribe its own inbox and nothing else (design 25 §6). The secret is its password, the same
	// string the request claims, so the server proves somebody holds the token and the request
	// proves the same thing to the controller without it having to ask the server who connected.
	conn, err := nats.Connect(natsURL(to.Address),
		nats.Secure(config),
		nats.UserInfo("enrol."+node, secret),
		nats.Name("mesh-host/enrol/"+node),
		nats.Timeout(timeout),
		nats.NoReconnect(),
		nats.DisconnectErrHandler(func(_ *nats.Conn, err error) {
			select {
			case lost <- fmt.Errorf("the bus closed the connection: %w", err):
			default:
			}
		}),
	)
	if err != nil {
		if errors.Is(err, ErrWrongCertificate) {
			return nil, err
		}
		// Not quoted back with the credential: the secret is one-time and still a secret.
		return nil, fmt.Errorf("cannot reach the bus at %s as %s: %w", to.Address, node, err)
	}
	js, err := conn.JetStream()
	if err != nil {
		conn.Close()
		return nil, fmt.Errorf("the bus at %s has no JetStream: %w", to.Address, err)
	}

	// Subscribed before anything is published, so an answer cannot arrive before there is anywhere
	// for it to land.
	answers, err := conn.SubscribeSync(inbox)
	if err != nil {
		conn.Close()
		return nil, fmt.Errorf("this node cannot listen for the mesh's answer: %w", err)
	}
	if err := conn.Flush(); err != nil {
		conn.Close()
		return nil, fmt.Errorf("this node's reply address did not reach the bus: %w", err)
	}

	return &natsAsking{conn: conn, js: js, inbox: inbox, answers: answers, lost: lost}, nil
}

func (a *natsAsking) Close() {
	if a.answers != nil {
		_ = a.answers.Unsubscribe()
	}
	if a.conn != nil {
		a.conn.Close()
	}
}

// Ask publishes the request with this attempt's reply address written into it, and waits there.
func (a *natsAsking) Ask(ctx context.Context, request []byte, wait time.Duration) ([]byte, error) {
	addressed, err := withReplyTo(request, a.inbox)
	if err != nil {
		return nil, err
	}

	publish, cancel := context.WithTimeout(ctx, wait)
	defer cancel()
	// Into the stream and awaited: an enrolment the bus never accepted must fail here rather than be
	// assumed, because the node has nothing else to go on.
	if _, err := a.js.Publish(EnrolSubject, addressed, nats.Context(publish)); err != nil {
		return nil, fmt.Errorf("cannot ask the mesh to enrol this node: %w", err)
	}

	// Waited for rather than assumed. A published message that nothing answers means the controller
	// is not running, and a node that carried on regardless would believe it had joined a mesh that
	// has never heard of it.
	//
	// **The wait may legitimately be several store-window cycles long**: the controller naks the
	// request with a delay while its store is restarting, and the node is waiting on the other side
	// of that — which is exactly the combination that would have delivered the answer to a caller
	// who had given up, had the address travelled in the transport's field.
	answered, cancelAnswer := context.WithTimeout(ctx, wait)
	defer cancelAnswer()
	for {
		msg, err := a.answers.NextMsgWithContext(answered)
		switch {
		case err == nil:
			return msg.Data, nil
		case errors.Is(err, context.DeadlineExceeded):
			select {
			case reason := <-a.lost:
				return nil, reason
			default:
			}
			return nil, fmt.Errorf(
				"the bus accepted this node's connection and nothing answered within %s. The mesh's "+
					"bus is running and its controller is not", wait)
		case errors.Is(err, context.Canceled):
			return nil, ctx.Err()
		default:
			select {
			case reason := <-a.lost:
				return nil, reason
			default:
			}
			return nil, fmt.Errorf("waiting for the mesh's answer: %w", err)
		}
	}
}