disco: add a result cache for peer ready check

disco/udp/peerkeeper.go

@@ -12,6 +12,11 @@ import (
 	"github.com/sigcn/pg/disco"
 )
 
+type cache[T any] struct {
+	result T
+	time   time.Time
+}
+
 type peerkeeper struct {
 	udpConn    atomic.Pointer[net.UDPConn]
 	peerID     disco.PeerID
@@ -23,6 +28,9 @@ type peerkeeper struct {
 	keepaliveInterval time.Duration
 
 	statesMutex sync.RWMutex
+
+	// caches
+	readyCache atomic.Value
 }
 
 func (peer *peerkeeper) heartbeat(addr *net.UDPAddr) {
@@ -55,16 +63,35 @@ func (peer *peerkeeper) healthcheck() {
 	}
 }
 
-// ready when peer context has at least one active udp address
+// ready when has at least one active udp address
 func (peer *peerkeeper) ready() bool {
-	peer.statesMutex.RLock()
-	defer peer.statesMutex.RUnlock()
-	for _, state := range peer.states {
-		if time.Since(state.LastActiveTime) <= peer.keepaliveInterval+2*time.Second {
-			return true
+	now := time.Now()
+	doRealCheck := func() bool {
+		peer.statesMutex.RLock()
+		defer peer.statesMutex.RUnlock()
+		for _, state := range peer.states {
+			if now.Sub(state.LastActiveTime) <= peer.keepaliveInterval+2*time.Second {
+				return true
+			}
+		}
+		return false
+	}
+
+	// perform real-time checks within the first 20 seconds after creation,
+	if now.Sub(peer.createTime) < 20*time.Second {
+		return doRealCheck()
+	}
+
+	// then cache for one heartbeat interval thereafter
+	if value := peer.readyCache.Load(); value != nil {
+		cache := value.(cache[bool])
+		if now.Sub(cache.time) < peer.keepaliveInterval {
+			return cache.result
 		}
 	}
-	return false
+	result := doRealCheck()
+	peer.readyCache.Store(cache[bool]{result: result, time: now})
+	return result
 }
 
 // selectPeerUDP select one of the multiple UDP addresses discovered by the peer
@@ -92,7 +119,7 @@ func (peer *peerkeeper) selectPeerUDP() *PeerState {
 func (peer *peerkeeper) writeUDP(p []byte) (int, error) {
 	if peerState := peer.selectPeerUDP(); p != nil {
 		slog.Log(context.Background(), -3, "[UDP] WriteTo", "peer", peer.peerID, "addr", peerState.Addr)
-		if time.Since(peerState.LastActiveTime) > peer.keepaliveInterval+200*time.Millisecond {
+		if time.Since(peerState.LastActiveTime) > peer.keepaliveInterval+time.Second {
 			peer.udpConn.Load().WriteTo(p, peerState.Addr)
 			return 0, ErrUDPConnInactive
 		}