TUN inbound: Enhance Darwin interface support (#5598)

* Proxy: TUN: Enhance Darwin interface support.
- reduce number of actions done to create/configure the interface in the system
- assign synthetic static link-local ipv4/ipv6 addresses to the interface, that are required by the OS for the routing to work
- make tun_darwin_endpoint be implemented significantly more similar to tun_windows_enpoint, preparing them for potential unification

* Proxy: TUN: Unify Darwin/Windows endpoint, which are now extremely similar, into one GVisorEndpoint.
Making darwin/windows tun implement GVisorDevice with simple readpacket/writepacket methods that GVisorEndpoint untilise

proxy/tun/stack_gvisor_endpoint.go

@@ -0,0 +1,155 @@
+package tun
+
+import (
+	"context"
+	"errors"
+
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+)
+
+var ErrQueueEmpty = errors.New("queue is empty")
+
+type GVisorDevice interface {
+	WritePacket(packet *stack.PacketBuffer) tcpip.Error
+	ReadPacket() (byte, *stack.PacketBuffer, error)
+	Wait()
+}
+
+// LinkEndpoint implements GVisor stack.LinkEndpoint
+var _ stack.LinkEndpoint = (*LinkEndpoint)(nil)
+
+type LinkEndpoint struct {
+	deviceMTU        uint32
+	device           GVisorDevice
+	dispatcherCancel context.CancelFunc
+}
+
+func (e *LinkEndpoint) MTU() uint32 {
+	return e.deviceMTU
+}
+
+func (e *LinkEndpoint) SetMTU(_ uint32) {
+	// not Implemented, as it is not expected GVisor will be asking tun device to be modified
+}
+
+func (e *LinkEndpoint) MaxHeaderLength() uint16 {
+	return 0
+}
+
+func (e *LinkEndpoint) LinkAddress() tcpip.LinkAddress {
+	return ""
+}
+
+func (e *LinkEndpoint) SetLinkAddress(_ tcpip.LinkAddress) {
+	// not Implemented, as it is not expected GVisor will be asking tun device to be modified
+}
+
+func (e *LinkEndpoint) Capabilities() stack.LinkEndpointCapabilities {
+	return stack.CapabilityRXChecksumOffload
+}
+
+func (e *LinkEndpoint) Attach(dispatcher stack.NetworkDispatcher) {
+	if e.dispatcherCancel != nil {
+		e.dispatcherCancel()
+		e.dispatcherCancel = nil
+	}
+
+	if dispatcher != nil {
+		ctx, cancel := context.WithCancel(context.Background())
+		go e.dispatchLoop(ctx, dispatcher)
+		e.dispatcherCancel = cancel
+	}
+}
+
+func (e *LinkEndpoint) IsAttached() bool {
+	return e.dispatcherCancel != nil
+}
+
+func (e *LinkEndpoint) Wait() {
+
+}
+
+func (e *LinkEndpoint) ARPHardwareType() header.ARPHardwareType {
+	return header.ARPHardwareNone
+}
+
+func (e *LinkEndpoint) AddHeader(buffer *stack.PacketBuffer) {
+	// tun interface doesn't have link layer header, it will be added by the OS
+}
+
+func (e *LinkEndpoint) ParseHeader(ptr *stack.PacketBuffer) bool {
+	return true
+}
+
+func (e *LinkEndpoint) Close() {
+	if e.dispatcherCancel != nil {
+		e.dispatcherCancel()
+		e.dispatcherCancel = nil
+	}
+}
+
+func (e *LinkEndpoint) SetOnCloseAction(_ func()) {
+
+}
+
+func (e *LinkEndpoint) WritePackets(packetBufferList stack.PacketBufferList) (int, tcpip.Error) {
+	var n int
+	var err tcpip.Error
+
+	for _, packetBuffer := range packetBufferList.AsSlice() {
+		err = e.device.WritePacket(packetBuffer)
+		if err != nil {
+			return n, &tcpip.ErrAborted{}
+		}
+		n++
+	}
+
+	return n, nil
+}
+
+func (e *LinkEndpoint) dispatchLoop(ctx context.Context, dispatcher stack.NetworkDispatcher) {
+	var networkProtocolNumber tcpip.NetworkProtocolNumber
+	var version byte
+	var packet *stack.PacketBuffer
+	var err error
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		default:
+			version, packet, err = e.device.ReadPacket()
+			// on "queue empty", ask device to yield slightly and continue
+			if errors.Is(err, ErrQueueEmpty) {
+				e.device.Wait()
+				continue
+			}
+			// stop dispatcher loop on any other interface failure
+			if err != nil {
+				e.Attach(nil)
+				return
+			}
+
+			// extract network protocol number from the packet first byte
+			// (which is returned separately, since it is so incredibly hard to extract one byte from
+			// stack.PacketBuffer without additional memory allocation and full copying it back and forth)
+			switch version {
+			case 4:
+				networkProtocolNumber = header.IPv4ProtocolNumber
+			case 6:
+				networkProtocolNumber = header.IPv6ProtocolNumber
+			default:
+				// discard unknown network protocol packet
+				packet.DecRef()
+				continue
+			}
+
+			// dispatch the buffer to the stack
+			dispatcher.DeliverNetworkPacket(networkProtocolNumber, packet)
+			// signal the buffer that it can be released
+			packet.DecRef()
+		}
+	}
+}