The proxy is told its routes and the mesh on the bus, and serves internal names to the mesh only

The proxy answered every routed name to any request carrying it, so an
internal-only route would have been public under its internal name. Each
membership now carries what its module receives, from the same
composition as its received file, and every machine's private-network
address, the list the packet filter's "from the mesh" is. The proxy
follows its membership, serves internal names only to those machines and
itself, and keeps the file until the bus has spoken (novox/hq ADR 0167,
issue 191).

examples/route-proxy/bus.go

@@ -0,0 +1,132 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"log"
+	"os"
+	"strings"
+	"sync/atomic"
+	"time"
+
+	"github.com/nats-io/nats.go"
+
+	"github.com/novox/mesh-controller/internal/broker"
+)
+
+// What the mesh issued this proxy, read on the bus (novox/hq ADR 0160, ADR 0167).
+//
+// **The proxy is told, not left to work it out.** Its membership carries the routes it is given —
+// the same contributions its file is written from — and every machine's address on the private
+// network, which is who may be served an internal name. Read once at connect and followed, so a
+// route added or a machine joining reaches a running proxy without a restart.
+
+// credential is the bus account the mesh delivered as this module's own secret named broker.
+type credential struct {
+	URL         string `json:"url"`
+	Fingerprint string `json:"fingerprint"`
+	Node        string `json:"node"`
+	Module      string `json:"module"`
+	User        string `json:"user"`
+	Password    string `json:"password"`
+}
+
+// followMembership connects with the credential in path and applies every membership the mesh
+// issues this proxy. It retries the first connection for as long as it takes: a proxy that started
+// before the bus keeps serving the file, and takes the bus when it answers.
+func followMembership(path string, held *table, fromBus *atomic.Bool) {
+	for {
+		err := followOnce(path, held, fromBus)
+		if err == nil {
+			return
+		}
+		log.Printf("cannot follow this proxy's membership, serving the file meanwhile: %v", err)
+		time.Sleep(30 * time.Second)
+	}
+}
+
+func followOnce(path string, held *table, fromBus *atomic.Bool) error {
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		return err
+	}
+	var cred credential
+	if err := json.Unmarshal(raw, &cred); err != nil {
+		return fmt.Errorf("the broker credential is not one: %w", err)
+	}
+	if cred.Node == "" || cred.Module == "" {
+		return fmt.Errorf("the broker credential names no node or module, so it has no membership")
+	}
+
+	opts := []nats.Option{
+		nats.Name(cred.Node + "." + cred.Module),
+		nats.UserInfo(cred.User, cred.Password),
+		// Its own inbox, and nothing wider: every principal is granted `_INBOX.<its user>.>` alone.
+		nats.CustomInboxPrefix("_INBOX." + cred.User),
+		// The bus being restarted is an upgrade, not a reason to stop following.
+		nats.MaxReconnects(-1),
+	}
+	if strings.TrimSpace(cred.Fingerprint) != "" {
+		opts = append(opts, nats.Secure(broker.PinnedToFingerprint(cred.Fingerprint)))
+	}
+	conn, err := nats.Connect(cred.URL, opts...)
+	if err != nil {
+		return fmt.Errorf("connecting to the bus at %s: %w", broker.BareAddress(cred.URL), err)
+	}
+
+	subject := broker.MembershipSubject(cred.Node, cred.Module)
+	apply := func(body []byte) {
+		var issued broker.Membership
+		if err := json.Unmarshal(body, &issued); err != nil {
+			log.Printf("a membership arrived that is not one: %v", err)
+			return
+		}
+		if took := applyMembership(issued, held); took && !fromBus.Swap(true) {
+			log.Printf("routes now come from this proxy's membership on %s", subject)
+		}
+	}
+
+	// Followed first, read second: an issue landing between the two is applied, not missed.
+	if _, err := conn.Subscribe(subject, func(m *nats.Msg) { apply(m.Data) }); err != nil {
+		conn.Close()
+		return fmt.Errorf("cannot follow %s: %w", subject, err)
+	}
+	// The subject-addressed direct get: the one request this account may make of the stream.
+	got, err := conn.Request("$JS.API.DIRECT.GET."+broker.AssignmentsStream+"."+subject, nil, 5*time.Second)
+	switch {
+	case err != nil:
+		log.Printf("cannot read the membership issued on %s yet (%v); following it", subject, err)
+	case got.Header.Get("Status") != "" || len(got.Data) == 0:
+		log.Printf("no membership issued on %s yet; serving the file until one is", subject)
+	default:
+		apply(got.Data)
+	}
+	return nil
+}
+
+// applyMembership serves what a membership says, and says whether it said anything about routes.
+//
+// A membership with no routes in it is one from a controller older than ADR 0167, and the file stays
+// the source rather than every route being withdrawn because a field was absent.
+func applyMembership(issued broker.Membership, held *table) bool {
+	raw, carries := issued.Receives["route"]
+	if !carries {
+		return false
+	}
+	var contributions []contribution
+	if err := json.Unmarshal(raw, &contributions); err != nil {
+		log.Printf("the routes in this proxy's membership are not contributions, keeping what is served: %v", err)
+		return false
+	}
+	inside, err := sourcesOf(issued.Mesh)
+	if err != nil {
+		log.Printf("the mesh in this proxy's membership is unreadable, keeping what is served: %v", err)
+		return false
+	}
+	routes, public := routesOf(contributions)
+	held.set(routes, public)
+	held.setInside(inside)
+	log.Printf("serving %d route(s) from the membership, internal names to %d machine(s): %s",
+		len(routes), len(inside), strings.Join(held.names(), ", "))
+	return true
+}