tun: use sysconn instead of .Fd with Go 1.12

tun/tun.go

@@ -5,7 +5,10 @@
 
 package tun
 
-import "os"
+import (
+	"fmt"
+	"os"
+)
 
 type TUNEvent int
 
@@ -24,3 +27,15 @@ type TUNDevice interface {
 	Events() chan TUNEvent          // returns a constant channel of events related to the device
 	Close() error                   // stops the device and closes the event channel
 }
+
+func (tun *nativeTun) operateOnFd(fn func(fd uintptr)) {
+	sysconn, err := tun.tunFile.SyscallConn()
+	if err != nil {
+		tun.errors <- fmt.Errorf("unable to find sysconn for tunfile: %s", err.Error())
+		return
+	}
+	err = sysconn.Control(fn)
+	if err != nil {
+		tun.errors <- fmt.Errorf("unable to control sysconn for tunfile: %s", err.Error())
+	}
+}