discovery: prevent double-close panic; fix IPv6 URLs & expiration guard

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I9f7b1a34065de77f4a8982b0f5feebc86a6a6964

internal/discovery/discovery.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"log/slog"
+	"net"
 	"sync"
 	"time"
 
@@ -20,6 +21,7 @@ type Discovery struct {
 	discovered       map[string]*discoveredPeer
 	mu               sync.RWMutex
 	stopCh           chan struct{}
+	stopOnce         sync.Once
 	waitGroup        sync.WaitGroup
 	onAddUpstream    func(url string, priority int)
 	onRemoveUpstream func(url string)
@@ -71,6 +73,7 @@ func (d *Discovery) Start(ctx context.Context) error {
 
 	if err := d.resolver.Browse(ctx, d.cfg.ServiceName, d.cfg.Domain, entries); err != nil {
 		close(entries)
+		d.stopOnce.Do(func() { close(d.stopCh) })
 		d.waitGroup.Wait()
 		return fmt.Errorf("browse services: %w", err)
 	}
@@ -85,7 +88,7 @@ func (d *Discovery) Start(ctx context.Context) error {
 
 // Stops the discovery process.
 func (d *Discovery) Stop() {
-	close(d.stopCh)
+	d.stopOnce.Do(func() { close(d.stopCh) })
 	d.waitGroup.Wait()
 }
 
@@ -122,8 +125,7 @@ func (d *Discovery) handleEntry(_ context.Context, entry *zeroconf.ServiceEntry)
 		addr = entry.AddrIPv6[0].String()
 	}
 
-	port := entry.Port
-	url := fmt.Sprintf("http://%s:%d", addr, port)
+	url := "http://" + net.JoinHostPort(addr, fmt.Sprintf("%d", entry.Port))
 	key := fmt.Sprintf("%s@%s", entry.Instance, entry.HostName)
 
 	d.mu.Lock()
@@ -184,6 +186,9 @@ func (d *Discovery) cleanupPeers() {
 	// TTL is the discovery response time; peers should re-announce periodically.
 	// Use 3x TTL as the expiration window.
 	expiration := d.cfg.DiscoveryTime.Duration * 3
+	if expiration == 0 {
+		expiration = time.Second
+	}
 
 	for key, peer := range d.discovered {
 		if now.Sub(peer.lastSeen) > expiration {