Hold a peer offer or answer that arrives before the handshaker starts listening

client/internal/peer/handshaker.go

@@ -81,14 +81,19 @@ type Handshaker struct {
 
 func NewHandshaker(log *log.Entry, config ConnConfig, signaler *Signaler, ice *WorkerICE, relay *WorkerRelay, metricsStages *MetricsStages) *Handshaker {
 	h := &Handshaker{
-		log:            log,
-		config:         config,
-		signaler:       signaler,
-		ice:            ice,
-		relay:          relay,
-		metricsStages:  metricsStages,
-		remoteOffersCh: make(chan OfferAnswer),
-		remoteAnswerCh: make(chan OfferAnswer),
+		log:           log,
+		config:        config,
+		signaler:      signaler,
+		ice:           ice,
+		relay:         relay,
+		metricsStages: metricsStages,
+		// Buffered by one so an offer or answer that arrives between Open launching
+		// the Listen goroutine and it reaching its receive is held rather than
+		// dropped. A peer activated by an incoming signal receives the remote's
+		// message in that window; an unbuffered channel skips it as "receiver not
+		// ready", and the connection cannot proceed until the remote re-sends.
+		remoteOffersCh: make(chan OfferAnswer, 1),
+		remoteAnswerCh: make(chan OfferAnswer, 1),
 	}
 	// assume remote supports ICE until we learn otherwise from received offers
 	h.remoteICESupported.Store(ice != nil)

client/internal/peer/handshaker_test.go

@@ -0,0 +1,69 @@
+package peer
+
+import (
+	"testing"
+	"time"
+
+	log "github.com/sirupsen/logrus"
+	"golang.zx2c4.com/wireguard/wgctrl/wgtypes"
+
+	signal "github.com/netbirdio/netbird/shared/signal/client"
+	sProto "github.com/netbirdio/netbird/shared/signal/proto"
+)
+
+func newTestHandshaker(t *testing.T) *Handshaker {
+	t.Helper()
+
+	localKey, err := wgtypes.GeneratePrivateKey()
+	if err != nil {
+		t.Fatalf("generate local key: %v", err)
+	}
+	remoteKey, err := wgtypes.GeneratePrivateKey()
+	if err != nil {
+		t.Fatalf("generate remote key: %v", err)
+	}
+
+	signaler := NewSignaler(&signal.MockClient{
+		ReadyFunc: func() bool { return true },
+		SendFunc:  func(*sProto.Message) error { return nil },
+	}, localKey)
+
+	cfg := ConnConfig{
+		Key:      remoteKey.PublicKey().String(),
+		LocalKey: localKey.PublicKey().String(),
+	}
+
+	return NewHandshaker(log.WithField("test", t.Name()), cfg, signaler, nil, nil, nil)
+}
+
+// TestHandshakerHoldsSignalArrivingBeforeListen covers the case where a peer is
+// activated by an incoming signal: the remote's offer/answer arrives in the same
+// step that opens the connection, before the Listen loop starts reading. The
+// message must be held rather than dropped, or the connection cannot proceed until
+// the remote re-sends. This is the path taken when an eager peer connects to a
+// lazily-managed one.
+//
+// The answer path is used because its Listen branch dispatches to the same
+// listeners as the offer path without also sending an answer, so it exercises the
+// buffered-channel behavior (which covers both channels) without needing a relay.
+func TestHandshakerHoldsSignalArrivingBeforeListen(t *testing.T) {
+	h := newTestHandshaker(t)
+
+	processed := make(chan struct{}, 4)
+	h.AddRelayListener(func(*OfferAnswer) { processed <- struct{}{} })
+
+	// Delivered before Listen is reading, exactly as when the peer is woken by the
+	// remote's signal and the message is delivered right after Open.
+	h.OnRemoteAnswer(OfferAnswer{
+		WgListenPort:   51820,
+		IceCredentials: IceCredentials{UFrag: "ufrag", Pwd: "pwd"},
+	})
+
+	go h.Listen(t.Context())
+
+	select {
+	case <-processed:
+	case <-time.After(2 * time.Second):
+		t.Fatal("signal that arrived before Listen was ready was dropped")
+	}
+}