add gvisor based service library (#965)

* add service/ library

overlay/user.go

@@ -0,0 +1,63 @@
+package overlay
+
+import (
+	"io"
+	"net"
+
+	"github.com/sirupsen/logrus"
+	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/iputil"
+)
+
+func NewUserDeviceFromConfig(c *config.C, l *logrus.Logger, tunCidr *net.IPNet, routines int) (Device, error) {
+	return NewUserDevice(tunCidr)
+}
+
+func NewUserDevice(tunCidr *net.IPNet) (Device, error) {
+	// these pipes guarantee each write/read will match 1:1
+	or, ow := io.Pipe()
+	ir, iw := io.Pipe()
+	return &UserDevice{
+		tunCidr:        tunCidr,
+		outboundReader: or,
+		outboundWriter: ow,
+		inboundReader:  ir,
+		inboundWriter:  iw,
+	}, nil
+}
+
+type UserDevice struct {
+	tunCidr *net.IPNet
+
+	outboundReader *io.PipeReader
+	outboundWriter *io.PipeWriter
+
+	inboundReader *io.PipeReader
+	inboundWriter *io.PipeWriter
+}
+
+func (d *UserDevice) Activate() error {
+	return nil
+}
+func (d *UserDevice) Cidr() *net.IPNet                      { return d.tunCidr }
+func (d *UserDevice) Name() string                          { return "faketun0" }
+func (d *UserDevice) RouteFor(ip iputil.VpnIp) iputil.VpnIp { return ip }
+func (d *UserDevice) NewMultiQueueReader() (io.ReadWriteCloser, error) {
+	return d, nil
+}
+
+func (d *UserDevice) Pipe() (*io.PipeReader, *io.PipeWriter) {
+	return d.inboundReader, d.outboundWriter
+}
+
+func (d *UserDevice) Read(p []byte) (n int, err error) {
+	return d.outboundReader.Read(p)
+}
+func (d *UserDevice) Write(p []byte) (n int, err error) {
+	return d.inboundWriter.Write(p)
+}
+func (d *UserDevice) Close() error {
+	d.inboundWriter.Close()
+	d.outboundWriter.Close()
+	return nil
+}