chore(provider): use type assertion for port forwarders

internal/vpn/portforward.go

@@ -1,10 +1,27 @@
 package vpn
 
 import (
+	"context"
+	"errors"
+	"fmt"
+
 	"github.com/qdm12/gluetun/internal/configuration/settings"
 	"github.com/qdm12/gluetun/internal/portforward/service"
+	pfutils "github.com/qdm12/gluetun/internal/provider/utils"
 )
 
+func getPortForwarder(provider Provider, providers Providers, //nolint:ireturn
+	customPortForwarderName string) (portForwarder PortForwarder) {
+	if customPortForwarderName != "" {
+		provider = providers.Get(customPortForwarderName)
+	}
+	portForwarder, ok := provider.(PortForwarder)
+	if ok {
+		return portForwarder
+	}
+	return newNoPortForwarder(provider.Name())
+}
+
 func (l *Loop) startPortForwarding(data tunnelUpData) (err error) {
 	partialUpdate := service.Settings{
 		PortForwarder: data.portForwarder,
@@ -24,3 +41,28 @@ func (l *Loop) stopPortForwarding(vpnProvider string) (err error) {
 	}
 	return l.portForward.UpdateWith(partialUpdate)
 }
+
+type noPortForwarder struct {
+	providerName string
+}
+
+func newNoPortForwarder(providerName string) *noPortForwarder {
+	return &noPortForwarder{
+		providerName: providerName,
+	}
+}
+
+var ErrPortForwardingNotSupported = errors.New("custom port forwarding obtention is not supported")
+
+func (n *noPortForwarder) Name() string {
+	return n.providerName
+}
+
+func (n *noPortForwarder) PortForward(context.Context, pfutils.PortForwardObjects) (
+	port uint16, err error) {
+	return 0, fmt.Errorf("%w: for %s", ErrPortForwardingNotSupported, n.providerName)
+}
+
+func (n *noPortForwarder) KeepPortForward(context.Context, pfutils.PortForwardObjects) (err error) {
+	return fmt.Errorf("%w: for %s", ErrPortForwardingNotSupported, n.providerName)
+}