wireguard-go data race in Peer.SendBuffer()

[Ben Burkert]

Greetings,

The Go race detector is reporting a data race in my application due to
what appears to be concurrent calls to Peer.SendHandshakeInitiation().
It seems that it is unsafe to call golang.org/x/sys/unix.SendmsgN()
concurrently due to the sockaddr() implementation which mutates
memory: https://github.com/golang/sys/blob/bd437916bb0eb726b873ee8e9b2dcf212d32e2fd/unix/syscall_aix.go#L53-L55

I believe this data race can be fixed by switching to an exclusive
lock in the Peer.SendBuffer() method:
https://github.com/WireGuard/wireguard-go/blob/f2ea85e9f9921bc29a7dcb2007212b153540c01a/device/peer.go#L144-L145

If it helps I can mail a patch to this effect. I've copied the output
of the race detector below.

Best,
-Ben

==================
==================
WARNING: DATA RACE
Write at 0x00c0011f2740 by goroutine 27:
  golang.org/x/sys/unix.(*SockaddrInet4).sockaddr()
      /go/pkg/mod/golang.org/x/sys at v0.0.0-20191105231009-c1f44814a5cd/unix/syscall_linux.go:384
+0x114
  golang.org/x/sys/unix.SendmsgN()
      /go/pkg/mod/golang.org/x/sys at v0.0.0-20191105231009-c1f44814a5cd/unix/syscall_linux.go:1304
+0x288
  golang.zx2c4.com/wireguard/device.send4()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/conn_linux.go:485
+0x11f
  golang.zx2c4.com/wireguard/device.(*nativeBind).Send()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/conn_linux.go:268
+0x1d6
  golang.zx2c4.com/wireguard/device.(*Peer).SendBuffer()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/peer.go:151
+0x285
  golang.zx2c4.com/wireguard/device.(*Peer).SendHandshakeInitiation()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/send.go:163
+0x692
  golang.zx2c4.com/wireguard/device.(*Device).RoutineReadFromTUN()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/send.go:318
+0x4b8

Previous write at 0x00c0011f2740 by goroutine 386:
  golang.org/x/sys/unix.(*SockaddrInet4).sockaddr()
      /go/pkg/mod/golang.org/x/sys at v0.0.0-20191105231009-c1f44814a5cd/unix/syscall_linux.go:384
+0x114
  golang.org/x/sys/unix.SendmsgN()
      /go/pkg/mod/golang.org/x/sys at v0.0.0-20191105231009-c1f44814a5cd/unix/syscall_linux.go:1304
+0x288
  golang.zx2c4.com/wireguard/device.send4()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/conn_linux.go:485
+0x11f
  golang.zx2c4.com/wireguard/device.(*nativeBind).Send()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/conn_linux.go:268
+0x1d6
  golang.zx2c4.com/wireguard/device.(*Peer).SendBuffer()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/peer.go:151
+0x285
  golang.zx2c4.com/wireguard/device.(*Peer).SendHandshakeInitiation()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/send.go:163
+0x692
  golang.zx2c4.com/wireguard/device.expiredRetransmitHandshake()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/timers.go:110
+0x40c
  golang.zx2c4.com/wireguard/device.(*Peer).NewTimer.func1()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/timers.go:42
+0xd8

Goroutine 27 (running) created at:
  golang.zx2c4.com/wireguard/device.NewDevice()
      /go/pkg/mod/golang.zx2c4.com/wireguard at v0.0.20191012/device/device.go:322
+0x5e8
  main.main()
      /go/src/x/main.go:102 +0x58e

Goroutine 386 (finished) created at:
  time.goFunc()
      /usr/local/go/src/time/sleep.go:168 +0x51
==================