[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.go

@@ -1,12 +1,27 @@
+//go:build !ios && !android
+
 package networkmonitor
 
 import (
 	"context"
 	"errors"
+	"fmt"
+	"net/netip"
+	"runtime/debug"
 	"sync"
+	"time"
+
+	"github.com/cenkalti/backoff/v4"
+	log "github.com/sirupsen/logrus"
+
+	"github.com/netbirdio/netbird/client/internal/routemanager/systemops"
 )
 
-var ErrStopped = errors.New("monitor has been stopped")
+const (
+	debounceTime = 2 * time.Second
+)
+
+var checkChangeFn = checkChange
 
 // NetworkMonitor watches for changes in network configuration.
 type NetworkMonitor struct {
@@ -19,3 +34,99 @@ type NetworkMonitor struct {
 func New() *NetworkMonitor {
 	return &NetworkMonitor{}
 }
+
+// Listen begins monitoring network changes. When a change is detected, this function will return without error.
+func (nw *NetworkMonitor) Listen(ctx context.Context) (err error) {
+	nw.mu.Lock()
+	if nw.cancel != nil {
+		nw.mu.Unlock()
+		return errors.New("network monitor already started")
+	}
+
+	ctx, nw.cancel = context.WithCancel(ctx)
+	defer nw.cancel()
+	nw.wg.Add(1)
+	nw.mu.Unlock()
+
+	defer nw.wg.Done()
+
+	var nexthop4, nexthop6 systemops.Nexthop
+
+	operation := func() error {
+		var errv4, errv6 error
+		nexthop4, errv4 = systemops.GetNextHop(netip.IPv4Unspecified())
+		nexthop6, errv6 = systemops.GetNextHop(netip.IPv6Unspecified())
+
+		if errv4 != nil && errv6 != nil {
+			return errors.New("failed to get default next hops")
+		}
+
+		if errv4 == nil {
+			log.Debugf("Network monitor: IPv4 default route: %s, interface: %s", nexthop4.IP, nexthop4.Intf.Name)
+		}
+		if errv6 == nil {
+			log.Debugf("Network monitor: IPv6 default route: %s, interface: %s", nexthop6.IP, nexthop6.Intf.Name)
+		}
+
+		// continue if either route was found
+		return nil
+	}
+
+	expBackOff := backoff.WithContext(backoff.NewExponentialBackOff(), ctx)
+
+	if err := backoff.Retry(operation, expBackOff); err != nil {
+		return fmt.Errorf("failed to get default next hops: %w", err)
+	}
+
+	// recover in case sys ops panic
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("panic occurred: %v, stack trace: %s", r, debug.Stack())
+		}
+	}()
+
+	event := make(chan struct{}, 1)
+	go nw.checkChanges(ctx, event, nexthop4, nexthop6)
+
+	// debounce changes
+	timer := time.NewTimer(0)
+	timer.Stop()
+	for {
+		select {
+		case <-event:
+			timer.Reset(debounceTime)
+		case <-timer.C:
+			return nil
+		case <-ctx.Done():
+			timer.Stop()
+			return ctx.Err()
+		}
+	}
+}
+
+// Stop stops the network monitor.
+func (nw *NetworkMonitor) Stop() {
+	nw.mu.Lock()
+	defer nw.mu.Unlock()
+
+	if nw.cancel == nil {
+		return
+	}
+
+	nw.cancel()
+	nw.wg.Wait()
+}
+
+func (nw *NetworkMonitor) checkChanges(ctx context.Context, event chan struct{}, nexthop4 systemops.Nexthop, nexthop6 systemops.Nexthop) {
+	for {
+		if err := checkChangeFn(ctx, nexthop4, nexthop6); err != nil {
+			close(event)
+			return
+		}
+		// prevent blocking
+		select {
+		case event <- struct{}{}:
+		default:
+		}
+	}
+}