moonlight-relay-server/backend/internal/discovery/discovery.go

// Package discovery finds Apollo/Artemis hosts on the network via mDNS.
// Apollo advertises itself as _nvstream._tcp.local — the same service
// type that Moonlight scans for.
package discovery

import (
	"context"
	"log"
	"sync"
	"time"

	"github.com/grandcat/zeroconf"
)

const nvstreamService = "_nvstream._tcp"

// Host represents a discovered Apollo/Artemis streaming host.
type Host struct {
	Name    string   `json:"name"`
	IP      string   `json:"ip"`
	Port    int      `json:"port"`
	Online  bool     `json:"online"`
	SeenAt  time.Time `json:"seen_at"`
}

// Discoverer watches the local (Tailscale) network for Apollo hosts.
type Discoverer struct {
	mu    sync.RWMutex
	hosts map[string]*Host // keyed by name
}

// New creates a new Discoverer with an empty host list.
func New() *Discoverer {
	return &Discoverer{hosts: make(map[string]*Host)}
}

// Run starts mDNS browsing and blocks until ctx is cancelled.
// Discovered hosts are stored and retrievable via Hosts().
func (d *Discoverer) Run(ctx context.Context) {
	for {
		if err := d.browse(ctx); err != nil && ctx.Err() == nil {
			log.Printf("discovery: browse error: %v — retrying in 10s", err)
			select {
			case <-ctx.Done():
				return
			case <-time.After(10 * time.Second):
			}
		}
		if ctx.Err() != nil {
			return
		}
	}
}

func (d *Discoverer) browse(ctx context.Context) error {
	resolver, err := zeroconf.NewResolver(nil)
	if err != nil {
		return err
	}

	entries := make(chan *zeroconf.ServiceEntry)
	browseCtx, cancel := context.WithCancel(ctx)
	defer cancel()

	go func() {
		for entry := range entries {
			ip := ""
			if len(entry.AddrIPv4) > 0 {
				ip = entry.AddrIPv4[0].String()
			} else if len(entry.AddrIPv6) > 0 {
				ip = entry.AddrIPv6[0].String()
			}
			if ip == "" {
				continue
			}
			h := &Host{
				Name:   entry.ServiceInstanceName(),
				IP:     ip,
				Port:   entry.Port,
				Online: true,
				SeenAt: time.Now(),
			}
			d.upsert(h)
			log.Printf("discovery: found host %s at %s:%d", h.Name, h.IP, h.Port)
		}
	}()

	if err := resolver.Browse(browseCtx, nvstreamService, "local.", entries); err != nil {
		return err
	}
	<-browseCtx.Done()
	return nil
}

func (d *Discoverer) upsert(h *Host) {
	d.mu.Lock()
	defer d.mu.Unlock()
	d.hosts[h.Name] = h
}

// Hosts returns a snapshot of all known hosts.
func (d *Discoverer) Hosts() []*Host {
	d.mu.RLock()
	defer d.mu.RUnlock()

	// Mark hosts as offline if not seen in 30s
	cutoff := time.Now().Add(-30 * time.Second)
	out := make([]*Host, 0, len(d.hosts))
	for _, h := range d.hosts {
		copy := *h
		if h.SeenAt.Before(cutoff) {
			copy.Online = false
		}
		out = append(out, &copy)
	}
	return out
}

// AddManual adds a host by IP/port without mDNS (for hosts behind NAT or
// on subnets that don't support multicast).
func (d *Discoverer) AddManual(name, ip string, port int) {
	d.upsert(&Host{
		Name:   name,
		IP:     ip,
		Port:   port,
		Online: true,
		SeenAt: time.Now(),
	})
}