device, tun: rearrange code and fix device tests

Signed-off-by: Matt Layher <mdlayher@gmail.com>

device/tun_test.go

@@ -0,0 +1,56 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2017-2019 WireGuard LLC. All Rights Reserved.
+ */
+
+package device
+
+import (
+	"errors"
+	"os"
+
+	"golang.zx2c4.com/wireguard/tun"
+)
+
+// newDummyTUN creates a dummy TUN device with the specified name.
+func newDummyTUN(name string) tun.TUNDevice {
+	return &dummyTUN{
+		name:    name,
+		packets: make(chan []byte, 100),
+		events:  make(chan tun.TUNEvent, 10),
+	}
+}
+
+// A dummyTUN is a tun.TUNDevice which is used in unit tests.
+type dummyTUN struct {
+	name    string
+	mtu     int
+	packets chan []byte
+	events  chan tun.TUNEvent
+}
+
+func (d *dummyTUN) Events() chan tun.TUNEvent { return d.events }
+func (*dummyTUN) File() *os.File              { return nil }
+func (*dummyTUN) Flush() error                { return nil }
+func (d *dummyTUN) MTU() (int, error)         { return d.mtu, nil }
+func (d *dummyTUN) Name() (string, error)     { return d.name, nil }
+
+func (d *dummyTUN) Close() error {
+	close(d.events)
+	close(d.packets)
+	return nil
+}
+
+func (d *dummyTUN) Read(b []byte, offset int) (int, error) {
+	buf, ok := <-d.packets
+	if !ok {
+		return 0, errors.New("device closed")
+	}
+	copy(b[offset:], buf)
+	return len(buf), nil
+}
+
+func (d *dummyTUN) Write(b []byte, offset int) (int, error) {
+	d.packets <- b[offset:]
+	return len(b), nil
+}