tun: make NativeTun.Close well behaved, not crash on double close

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tun/tun_darwin.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"net"
 	"os"
+	"sync"
 	"syscall"
 	"time"
 	"unsafe"
@@ -26,6 +27,7 @@ type NativeTun struct {
 	events      chan Event
 	errors      chan error
 	routeSocket int
+	closeOnce   sync.Once
 }
 
 func retryInterfaceByIndex(index int) (iface *net.Interface, err error) {
@@ -256,14 +258,16 @@ func (tun *NativeTun) Flush() error {
 }
 
 func (tun *NativeTun) Close() error {
-	var err2 error
+	var err1, err2 error
-	err1 := tun.tunFile.Close()
+	tun.closeOnce.Do(func() {
+		err1 = tun.tunFile.Close()
 		if tun.routeSocket != -1 {
 			unix.Shutdown(tun.routeSocket, unix.SHUT_RDWR)
 			err2 = unix.Close(tun.routeSocket)
 		} else if tun.events != nil {
 			close(tun.events)
 		}
+	})
 	if err1 != nil {
 		return err1
 	}

tun/tun_freebsd.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"net"
 	"os"
+	"sync"
 	"syscall"
 	"unsafe"
 
@@ -82,6 +83,7 @@ type NativeTun struct {
 	events      chan Event
 	errors      chan error
 	routeSocket int
+	closeOnce   sync.Once
 }
 
 func (tun *NativeTun) routineRouteListener(tunIfindex int) {
@@ -472,9 +474,10 @@ func (tun *NativeTun) Flush() error {
 }
 
 func (tun *NativeTun) Close() error {
-	var err3 error
+	var err1, err2, err3 error
-	err1 := tun.tunFile.Close()
+	tun.closeOnce.Do(func() {
-	err2 := tunDestroy(tun.name)
+		err1 = tun.tunFile.Close()
+		err2 = tunDestroy(tun.name)
 		if tun.routeSocket != -1 {
 			unix.Shutdown(tun.routeSocket, unix.SHUT_RDWR)
 			err3 = unix.Close(tun.routeSocket)
@@ -482,6 +485,7 @@ func (tun *NativeTun) Close() error {
 		} else if tun.events != nil {
 			close(tun.events)
 		}
+	})
 	if err1 != nil {
 		return err1
 	}

tun/tun_linux.go

@@ -39,6 +39,8 @@ type NativeTun struct {
 	hackListenerClosed      sync.Mutex
 	statusListenersShutdown chan struct{}
 
+	closeOnce sync.Once
+
 	nameOnce  sync.Once // guards calling initNameCache, which sets following fields
 	nameCache string    // name of interface
 	nameErr   error
@@ -372,7 +374,8 @@ func (tun *NativeTun) Events() chan Event {
 }
 
 func (tun *NativeTun) Close() error {
-	var err1 error
+	var err1, err2 error
+	tun.closeOnce.Do(func() {
 		if tun.statusListenersShutdown != nil {
 			close(tun.statusListenersShutdown)
 			if tun.netlinkCancel != nil {
@@ -381,8 +384,8 @@ func (tun *NativeTun) Close() error {
 		} else if tun.events != nil {
 			close(tun.events)
 		}
-	err2 := tun.tunFile.Close()
+		err2 = tun.tunFile.Close()
-
+	})
 	if err1 != nil {
 		return err1
 	}

tun/tun_openbsd.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"net"
 	"os"
+	"sync"
 	"syscall"
 	"unsafe"
 
@@ -32,6 +33,7 @@ type NativeTun struct {
 	events      chan Event
 	errors      chan error
 	routeSocket int
+	closeOnce   sync.Once
 }
 
 func (tun *NativeTun) routineRouteListener(tunIfindex int) {
@@ -245,8 +247,9 @@ func (tun *NativeTun) Flush() error {
 }
 
 func (tun *NativeTun) Close() error {
-	var err2 error
+	var err1, err2 error
-	err1 := tun.tunFile.Close()
+	tun.closeOnce.Do(func() {
+		err1 = tun.tunFile.Close()
 		if tun.routeSocket != -1 {
 			unix.Shutdown(tun.routeSocket, unix.SHUT_RDWR)
 			err2 = unix.Close(tun.routeSocket)
@@ -254,6 +257,7 @@ func (tun *NativeTun) Close() error {
 		} else if tun.events != nil {
 			close(tun.events)
 		}
+	})
 	if err1 != nil {
 		return err1
 	}

tun/tun_windows.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"sync"
 	"sync/atomic"
 	"time"
 	_ "unsafe"
@@ -42,6 +43,7 @@ type NativeTun struct {
 	rate      rateJuggler
 	session   wintun.Session
 	readWait  windows.Handle
+	closeOnce sync.Once
 }
 
 var WintunPool, _ = wintun.MakePool("WireGuard")
@@ -122,13 +124,15 @@ func (tun *NativeTun) Events() chan Event {
 }
 
 func (tun *NativeTun) Close() error {
+	var err error
+	tun.closeOnce.Do(func() {
 		tun.close = true
 		tun.session.End()
-	var err error
 		if tun.wt != nil {
 			_, err = tun.wt.Delete(false)
 		}
 		close(tun.events)
+	})
 	return err
 }