device: allow compiling with Go 1.15

Until we depend on Go 1.16 (which isn't released yet), alias our own
variable to the private member of the net package. This will allow an
easy find replace to make this go away when we eventually switch to
1.16.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

conn/conn_linux.go

@@ -204,7 +204,7 @@ func (bind *nativeBind) ReceiveIPv6(buff []byte) (int, Endpoint, error) {
 
 	var end NativeEndpoint
 	if bind.sock6 == -1 {
-		return 0, nil, net.ErrClosed
+		return 0, nil, NetErrClosed
 	}
 	n, err := receive6(
 		bind.sock6,
@@ -220,7 +220,7 @@ func (bind *nativeBind) ReceiveIPv4(buff []byte) (int, Endpoint, error) {
 
 	var end NativeEndpoint
 	if bind.sock4 == -1 {
-		return 0, nil, net.ErrClosed
+		return 0, nil, NetErrClosed
 	}
 	n, err := receive4(
 		bind.sock4,
@@ -237,12 +237,12 @@ func (bind *nativeBind) Send(buff []byte, end Endpoint) error {
 	nend := end.(*NativeEndpoint)
 	if !nend.isV6 {
 		if bind.sock4 == -1 {
-			return net.ErrClosed
+			return NetErrClosed
 		}
 		return send4(bind.sock4, nend, buff)
 	} else {
 		if bind.sock6 == -1 {
-			return net.ErrClosed
+			return NetErrClosed
 		}
 		return send6(bind.sock6, nend, buff)
 	}

conn/net_err_closed.go

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2021 WireGuard LLC. All Rights Reserved.
+ */
+
+package conn
+
+import _ "unsafe"
+
+//TODO: replace this with net.ErrClosed for Go 1.16
+
+//go:linkname net.errClosed
+var NetErrClosed error

device/receive.go

@@ -112,7 +112,7 @@ func (device *Device) RoutineReceiveIncoming(IP int, bind conn.Bind) {
 
 		if err != nil {
 			device.PutMessageBuffer(buffer)
-			if errors.Is(err, net.ErrClosed) {
+			if errors.Is(err, conn.NetErrClosed) {
 				return
 			}
 			device.log.Error.Printf("Failed to receive packet: %v", err)

go.mod

@@ -1,6 +1,6 @@
 module golang.zx2c4.com/wireguard
 
-go 1.16
+go 1.15
 
 require (
 	golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad