[client] Fix engine restart (#3435)

- Refactor the network monitoring to handle one event and it after return
- In the engine restart cancel the upper layer context and the responsibility of the engine stop will be the upper layer
- Before triggering a restart, the engine checks whether the state is already down. This helps avoid unnecessary delayed network restart events.

client/internal/networkmonitor/monitor_test.go

@@ -0,0 +1,99 @@
+package networkmonitor
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/netbirdio/netbird/client/internal/routemanager/systemops"
+)
+
+type MocMultiEvent struct {
+	counter int
+}
+
+func (m *MocMultiEvent) checkChange(ctx context.Context, nexthopv4, nexthopv6 systemops.Nexthop) error {
+	if m.counter == 0 {
+		<-ctx.Done()
+		return ctx.Err()
+	}
+
+	time.Sleep(1 * time.Second)
+	m.counter--
+	return nil
+}
+
+func TestNetworkMonitor_Close(t *testing.T) {
+	checkChangeFn = func(ctx context.Context, nexthopv4, nexthopv6 systemops.Nexthop) error {
+		<-ctx.Done()
+		return ctx.Err()
+	}
+	nw := New()
+
+	var resErr error
+	done := make(chan struct{})
+	go func() {
+		resErr = nw.Listen(context.Background())
+		close(done)
+	}()
+
+	time.Sleep(1 * time.Second) // wait for the goroutine to start
+	nw.Stop()
+
+	<-done
+	if !errors.Is(resErr, context.Canceled) {
+		t.Errorf("unexpected error: %v", resErr)
+	}
+}
+
+func TestNetworkMonitor_Event(t *testing.T) {
+	checkChangeFn = func(ctx context.Context, nexthopv4, nexthopv6 systemops.Nexthop) error {
+		timeout, cancel := context.WithTimeout(ctx, 3*time.Second)
+		defer cancel()
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-timeout.Done():
+			return nil
+		}
+	}
+	nw := New()
+	defer nw.Stop()
+
+	var resErr error
+	done := make(chan struct{})
+	go func() {
+		resErr = nw.Listen(context.Background())
+		close(done)
+	}()
+
+	<-done
+	if !errors.Is(resErr, nil) {
+		t.Errorf("unexpected error: %v", nil)
+	}
+}
+
+func TestNetworkMonitor_MultiEvent(t *testing.T) {
+	eventsRepeated := 3
+	me := &MocMultiEvent{counter: eventsRepeated}
+	checkChangeFn = me.checkChange
+
+	nw := New()
+	defer nw.Stop()
+
+	done := make(chan struct{})
+	started := time.Now()
+	go func() {
+		if resErr := nw.Listen(context.Background()); resErr != nil {
+			t.Errorf("unexpected error: %v", resErr)
+		}
+		close(done)
+	}()
+
+	<-done
+	expectedResponseTime := time.Duration(eventsRepeated)*time.Second + debounceTime
+	if time.Since(started) < expectedResponseTime {
+		t.Errorf("unexpected duration: %v", time.Since(started))
+	}
+}