net/tstun: merge in wgengine/tstun.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/tstun/fake.go

@@ -0,0 +1,56 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tstun
+
+import (
+	"io"
+	"os"
+
+	"github.com/tailscale/wireguard-go/tun"
+)
+
+type fakeTUN struct {
+	evchan    chan tun.Event
+	closechan chan struct{}
+}
+
+// NewFakeTUN returns a fake TUN device that does not depend on the
+// operating system or any special permissions.
+// It primarily exists for testing.
+func NewFakeTUN() tun.Device {
+	return &fakeTUN{
+		evchan:    make(chan tun.Event),
+		closechan: make(chan struct{}),
+	}
+}
+
+func (t *fakeTUN) File() *os.File {
+	panic("fakeTUN.File() called, which makes no sense")
+}
+
+func (t *fakeTUN) Close() error {
+	close(t.closechan)
+	close(t.evchan)
+	return nil
+}
+
+func (t *fakeTUN) Read(out []byte, offset int) (int, error) {
+	<-t.closechan
+	return 0, io.EOF
+}
+
+func (t *fakeTUN) Write(b []byte, n int) (int, error) {
+	select {
+	case <-t.closechan:
+		return 0, ErrClosed
+	default:
+	}
+	return len(b), nil
+}
+
+func (t *fakeTUN) Flush() error           { return nil }
+func (t *fakeTUN) MTU() (int, error)      { return 1500, nil }
+func (t *fakeTUN) Name() (string, error)  { return "FakeTUN", nil }
+func (t *fakeTUN) Events() chan tun.Event { return t.evchan }