intra/tunnel: seperate out tun fd updates from mtu

intra/netstack/fdbased.go

@@ -53,8 +53,8 @@ var _ FdSwapper = (*linkFdSwap)(nil)
 const invalidfd int = -1
 
 type FdSwapper interface {
-	// Swap closes existing FDs; uses new fd and mtu.
-	Swap(fd, mtu int) error
+	// Swap closes existing FDs; uses new fd.
+	Swap(fd int) error
 	// Dispose closes all existing FDs.
 	Dispose() error
 }
@@ -212,7 +212,9 @@ func NewFdbasedInjectableEndpoint(opts *Options) (SeamlessEndpoint, error) {
 		return nil, fmt.Errorf("len(opts.FDs) = %d, expected 1", len(opts.FDs))
 	}
 
-	if err := e.Swap(opts.FDs[0], int(opts.MTU)); err != nil {
+	e.SetMTU(opts.MTU)
+
+	if err := e.Swap(opts.FDs[0]); err != nil {
 		return nil, err
 	}
 
@@ -252,17 +254,15 @@ func (e *endpoint) Dispose() (err error) {
 }
 
 // Implements Swapper.
-func (e *endpoint) Swap(fd, mtu int) (err error) {
-	e.SetMTU(uint32(mtu))
-
+func (e *endpoint) Swap(fd int) (err error) {
 	if err = unix.SetNonblock(fd, true); err != nil {
 		clos(fd)
 		return fmt.Errorf("unix.SetNonblock(%v) failed: %v", fd, err)
 	}
 
 	prevfd := e.fds.Swap(fd) // commence WritePackets() on fd
 
-	log.D("ns: swapping tun... fd: %d => %d, mtu: %d", prevfd, fd, mtu)
+	log.D("ns: swapping tun... fd: %d => %d", prevfd, fd)
 
 	e.Lock()
 	defer e.Unlock()
@@ -277,7 +277,8 @@ func (e *endpoint) Swap(fd, mtu int) (err error) {
 	if err == nil && hasDispatcher { // attached?
 		go e.dispatchLoop(e.inboundDispatcher)
 	} else {
-		log.W("ns: tun(%d => %d): Swap: no dispatcher? %t for new fd; err %v", prevfd, fd, !hasDispatcher, err)
+		log.W("ns: tun(%d => %d): Swap: no dispatcher? %t for new fd; err %v",
+			prevfd, fd, !hasDispatcher, err)
 	}
 	return
 }

intra/tunnel.go

@@ -209,7 +209,8 @@ func (t *rtunnel) SetLinkAndRoutes(fd, mtu, engine int) error {
 			}
 		})
 	}()
-	return t.Tunnel.SetLink(fd, mtu) // route is always dual-stack
+	t.Tunnel.SetMTU(int32(mtu))
+	return t.Tunnel.SetLink(fd) // route is always dual-stack
 }
 
 func (t *rtunnel) internalCtx() context.Context {

tunnel/tunnel.go

@@ -57,8 +57,10 @@ type Tunnel interface {
 	Write(data []byte) (int, error)
 	// Close connections
 	CloseConns(activecsv string) (closedcsv string)
-	// Creates a new link using fd (tun device) and mtu.
-	SetLink(fd, mtu int) error
+	// Creates a new link using fd (tun device).
+	SetLink(fd int) error
+	// Sets the MTU.
+	SetMTU(mtu int32)
 	// Unsets existing link and closes the fd (tun device).
 	Unlink() error
 	// Creates the link and updates the routes
@@ -244,7 +246,8 @@ func (t *gtunnel) SetPcap(fp string) error {
 func (t *gtunnel) SetLinkAndRoutes(fd, mtu, engine int) (err error) {
 	// route is always dual-stack (settings.IP46); never changed
 	log.I("tun: requested route (%s); unchanged", settings.L3(engine))
-	return t.SetLink(fd, mtu)
+	t.SetMTU(int32(mtu))
+	return t.SetLink(fd)
 }
 
 func (t *gtunnel) Unlink() error {
@@ -253,7 +256,7 @@ func (t *gtunnel) Unlink() error {
 	return t.ep.Dispose()
 }
 
-func (t *gtunnel) SetLink(fd, mtu int) error {
+func (t *gtunnel) SetLink(fd int) error {
 	defer core.Recover(core.Exit11, "g.SetLink")
 
 	dupfd, err := dup(fd) // tunnel will own dupfd
@@ -262,12 +265,17 @@ func (t *gtunnel) SetLink(fd, mtu int) error {
 		return err
 	}
 
-	err = t.ep.Swap(dupfd, mtu) // swap fd and mtu
+	err = t.ep.Swap(dupfd) // swap fd and mtu
 
-	log.I("tun: new link; fd(%d), mtu(%d); err? %v", dupfd, mtu, err)
+	log.I("tun: new link, fd(%d); err? %v", dupfd, err)
 	return err
 }
 
+func (t *gtunnel) SetMTU(mtu int32) {
+	t.ep.SetMTU(uint32(mtu))
+	log.I("tun: new mtu; %d", mtu)
+}
+
 func (t *gtunnel) SetRoute(engine int) error {
 	defer core.Recover(core.Exit11, "g.SetRoute")