"discovery: mDNS/DNS-SD peer discovery; dynamic upstream management"

Allows ncro instances to discover each other dynamically without static
configuration and enables a peer-to-peer style (hello funny cube) mesh
where nodes share cached builds locally instead of all hitting upstream
caches.

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I7d723876c6816cb6aaaf3fe14cb24a426a6a6964

cmd/ncro/root.go

@@ -18,6 +18,7 @@ import (
 	"github.com/spf13/viper"
 	"notashelf.dev/ncro/internal/cache"
 	"notashelf.dev/ncro/internal/config"
+	"notashelf.dev/ncro/internal/discovery"
 	"notashelf.dev/ncro/internal/mesh"
 	"notashelf.dev/ncro/internal/metrics"
 	"notashelf.dev/ncro/internal/prober"
@@ -132,6 +133,26 @@ func runServer(_ *cobra.Command, _ []string) error {
 	probeDone := make(chan struct{})
 	go p.RunProbeLoop(30*time.Second, probeDone)
 
+	// Setup mDNS discovery if enabled
+	var discoveryMgr *discovery.Discovery
+	if cfg.Discovery.Enabled {
+		discoveryMgr, err = discovery.New(cfg.Discovery, p)
+		if err != nil {
+			return fmt.Errorf("create discovery manager: %w", err)
+		}
+		discoveryMgr.SetCallbacks(
+			func(url string, priority int) {
+				slog.Info("adding discovered upstream", "url", url)
+				p.AddUpstream(url, priority)
+			},
+			func(url string) {
+				slog.Info("removing discovered upstream", "url", url)
+				p.RemoveUpstream(url)
+			},
+		)
+		slog.Info("mDNS discovery enabled", "service", cfg.Discovery.ServiceName)
+	}
+
 	r := router.New(db, p, cfg.Cache.TTL.Duration, 5*time.Second, cfg.Cache.NegativeTTL.Duration)
 	for _, u := range cfg.Upstreams {
 		if u.PublicKey != "" {
@@ -174,6 +195,22 @@ func runServer(_ *cobra.Command, _ []string) error {
 		slog.Info("mesh enabled", "addr", cfg.Mesh.BindAddr, "peers", len(cfg.Mesh.Peers))
 	}
 
+	// Start mDNS discovery in background
+	discoveryDone := make(chan struct{})
+	if discoveryMgr != nil {
+		go func() {
+			ctx, cancel := context.WithCancel(context.Background())
+			defer cancel()
+			if err := discoveryMgr.Start(ctx); err != nil {
+				slog.Error("discovery error", "error", err)
+			}
+		}()
+		go func() {
+			<-discoveryDone
+			discoveryMgr.Stop()
+		}()
+	}
+
 	srv := &http.Server{
 		Addr:         cfg.Server.Listen,
 		Handler:      server.New(r, p, db, cfg.Upstreams, cfg.Server.CachePriority),
@@ -206,6 +243,7 @@ func runServer(_ *cobra.Command, _ []string) error {
 	if gossipDone != nil {
 		close(gossipDone)
 	}
+	close(discoveryDone)
 
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()