[client] Manage the IP forwarding sysctl setting in global way (#3270)

Add new package ipfwdstate that implements reference counting for IP forwarding
state management. This allows multiple usage to safely request IP forwarding
without interfering with each other.

client/internal/routemanager/ipfwdstate/ipfwdstate.go

@@ -0,0 +1,51 @@
+package ipfwdstate
+
+import (
+	"fmt"
+
+	log "github.com/sirupsen/logrus"
+
+	"github.com/netbirdio/netbird/client/internal/routemanager/systemops"
+)
+
+// IPForwardingState is a struct that keeps track of the IP forwarding state.
+// todo: read initial state of the IP forwarding from the system and reset the state based on it
+type IPForwardingState struct {
+	enabledCounter int
+}
+
+func NewIPForwardingState() *IPForwardingState {
+	return &IPForwardingState{}
+}
+
+func (f *IPForwardingState) RequestForwarding() error {
+	if f.enabledCounter != 0 {
+		f.enabledCounter++
+		return nil
+	}
+
+	if err := systemops.EnableIPForwarding(); err != nil {
+		return fmt.Errorf("failed to enable IP forwarding with sysctl: %w", err)
+	}
+	f.enabledCounter = 1
+	log.Info("IP forwarding enabled")
+
+	return nil
+}
+
+func (f *IPForwardingState) ReleaseForwarding() error {
+	if f.enabledCounter == 0 {
+		return nil
+	}
+
+	if f.enabledCounter > 1 {
+		f.enabledCounter--
+		return nil
+	}
+
+	// if failed to disable IP forwarding we anyway decrement the counter
+	f.enabledCounter = 0
+
+	// todo call systemops.DisableIPForwarding()
+	return nil
+}

client/internal/routemanager/server_nonandroid.go

@@ -13,7 +13,6 @@ import (
 	firewall "github.com/netbirdio/netbird/client/firewall/manager"
 	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/internal/peer"
-	"github.com/netbirdio/netbird/client/internal/routemanager/systemops"
 	"github.com/netbirdio/netbird/route"
 )
 
@@ -70,13 +69,6 @@ func (m *serverRouter) updateRoutes(routesMap map[route.ID]*route.Route) error {
 		m.routes[id] = newRoute
 	}
 
-	if len(m.routes) > 0 {
-		err := systemops.EnableIPForwarding()
-		if err != nil {
-			return err
-		}
-	}
-
 	return nil
 }