[client, relay-server] Feature/relay notification (#4083)

- Clients now subscribe to peer status changes.
- The server manages and maintains these subscriptions.
- Replaced raw string peer IDs with a custom peer ID type for better type safety and clarity.

relay/client/peer_subscription_test.go

@@ -0,0 +1,99 @@
+package client
+
+import (
+	"bytes"
+	"context"
+	"testing"
+	"time"
+
+	"github.com/netbirdio/netbird/relay/messages"
+
+	"github.com/sirupsen/logrus"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+type mockRelayedConn struct {
+}
+
+func (m *mockRelayedConn) Write(p []byte) (n int, err error) {
+	return len(p), nil
+}
+
+func TestWaitToBeOnlineAndSubscribe_Success(t *testing.T) {
+	peerID := messages.HashID("peer1")
+	mockConn := &mockRelayedConn{}
+	logger := logrus.New()
+	logger.SetOutput(&bytes.Buffer{}) // discard log output
+	sub := NewPeersStateSubscription(logrus.NewEntry(logger), mockConn, nil)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	// Launch wait in background
+	go func() {
+		time.Sleep(100 * time.Millisecond)
+		sub.OnPeersOnline([]messages.PeerID{peerID})
+	}()
+
+	err := sub.WaitToBeOnlineAndSubscribe(ctx, peerID)
+	assert.NoError(t, err)
+}
+
+func TestWaitToBeOnlineAndSubscribe_Timeout(t *testing.T) {
+	peerID := messages.HashID("peer2")
+	mockConn := &mockRelayedConn{}
+	logger := logrus.New()
+	logger.SetOutput(&bytes.Buffer{})
+	sub := NewPeersStateSubscription(logrus.NewEntry(logger), mockConn, nil)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	defer cancel()
+
+	err := sub.WaitToBeOnlineAndSubscribe(ctx, peerID)
+	assert.Error(t, err)
+	assert.Equal(t, context.DeadlineExceeded, err)
+}
+
+func TestWaitToBeOnlineAndSubscribe_Duplicate(t *testing.T) {
+	peerID := messages.HashID("peer3")
+	mockConn := &mockRelayedConn{}
+	logger := logrus.New()
+	logger.SetOutput(&bytes.Buffer{})
+	sub := NewPeersStateSubscription(logrus.NewEntry(logger), mockConn, nil)
+
+	ctx := context.Background()
+	go func() {
+		_ = sub.WaitToBeOnlineAndSubscribe(ctx, peerID)
+
+	}()
+	time.Sleep(100 * time.Millisecond)
+	err := sub.WaitToBeOnlineAndSubscribe(ctx, peerID)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "already waiting")
+}
+
+func TestUnsubscribeStateChange(t *testing.T) {
+	peerID := messages.HashID("peer4")
+	mockConn := &mockRelayedConn{}
+	logger := logrus.New()
+	logger.SetOutput(&bytes.Buffer{})
+	sub := NewPeersStateSubscription(logrus.NewEntry(logger), mockConn, nil)
+
+	doneChan := make(chan struct{})
+	go func() {
+		_ = sub.WaitToBeOnlineAndSubscribe(context.Background(), peerID)
+		close(doneChan)
+	}()
+	time.Sleep(100 * time.Millisecond)
+
+	err := sub.UnsubscribeStateChange([]messages.PeerID{peerID})
+	assert.NoError(t, err)
+
+	select {
+	case <-doneChan:
+	case <-time.After(200 * time.Millisecond):
+		// Expected timeout, meaning the subscription was successfully unsubscribed
+		t.Errorf("timeout")
+	}
+}