[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/check_change_bsd.go

@@ -0,0 +1,99 @@
+//go:build (darwin && !ios) || dragonfly || freebsd || netbsd || openbsd
+
+package networkmonitor
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"syscall"
+	"unsafe"
+
+	log "github.com/sirupsen/logrus"
+	"golang.org/x/net/route"
+	"golang.org/x/sys/unix"
+
+	"github.com/netbirdio/netbird/client/internal/routemanager/systemops"
+)
+
+func checkChange(ctx context.Context, nexthopv4, nexthopv6 systemops.Nexthop) error {
+	fd, err := unix.Socket(syscall.AF_ROUTE, syscall.SOCK_RAW, syscall.AF_UNSPEC)
+	if err != nil {
+		return fmt.Errorf("failed to open routing socket: %v", err)
+	}
+	defer func() {
+		err := unix.Close(fd)
+		if err != nil && !errors.Is(err, unix.EBADF) {
+			log.Warnf("Network monitor: failed to close routing socket: %v", err)
+		}
+	}()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+			buf := make([]byte, 2048)
+			n, err := unix.Read(fd, buf)
+			if err != nil {
+				if !errors.Is(err, unix.EBADF) && !errors.Is(err, unix.EINVAL) {
+					log.Warnf("Network monitor: failed to read from routing socket: %v", err)
+				}
+				continue
+			}
+			if n < unix.SizeofRtMsghdr {
+				log.Debugf("Network monitor: read from routing socket returned less than expected: %d bytes", n)
+				continue
+			}
+
+			msg := (*unix.RtMsghdr)(unsafe.Pointer(&buf[0]))
+
+			switch msg.Type {
+			// handle route changes
+			case unix.RTM_ADD, syscall.RTM_DELETE:
+				route, err := parseRouteMessage(buf[:n])
+				if err != nil {
+					log.Debugf("Network monitor: error parsing routing message: %v", err)
+					continue
+				}
+
+				if route.Dst.Bits() != 0 {
+					continue
+				}
+
+				intf := "<nil>"
+				if route.Interface != nil {
+					intf = route.Interface.Name
+				}
+				switch msg.Type {
+				case unix.RTM_ADD:
+					log.Infof("Network monitor: default route changed: via %s, interface %s", route.Gw, intf)
+					return nil
+				case unix.RTM_DELETE:
+					if nexthopv4.Intf != nil && route.Gw.Compare(nexthopv4.IP) == 0 || nexthopv6.Intf != nil && route.Gw.Compare(nexthopv6.IP) == 0 {
+						log.Infof("Network monitor: default route removed: via %s, interface %s", route.Gw, intf)
+						return nil
+					}
+				}
+			}
+		}
+	}
+}
+
+func parseRouteMessage(buf []byte) (*systemops.Route, error) {
+	msgs, err := route.ParseRIB(route.RIBTypeRoute, buf)
+	if err != nil {
+		return nil, fmt.Errorf("parse RIB: %v", err)
+	}
+
+	if len(msgs) != 1 {
+		return nil, fmt.Errorf("unexpected RIB message msgs: %v", msgs)
+	}
+
+	msg, ok := msgs[0].(*route.RouteMessage)
+	if !ok {
+		return nil, fmt.Errorf("unexpected RIB message type: %T", msgs[0])
+	}
+
+	return systemops.MsgToRoute(msg)
+}