don't panic if we write to a closed tun device because tun.close() was called

src/yggdrasil/tun.go

@@ -5,6 +5,7 @@ package yggdrasil
 import (
 	"bytes"
 	"errors"
+	"sync"
 	"time"
 
 	"github.com/songgao/packets/ethernet"
@@ -24,6 +25,8 @@ type tunAdapter struct {
 	icmpv6 icmpv6
 	mtu    int
 	iface  *water.Interface
+	mutex  sync.RWMutex // Protects the below
+	isOpen bool
 }
 
 // Gets the maximum supported MTU for the platform based on the defaults in
@@ -50,6 +53,9 @@ func (tun *tunAdapter) start(ifname string, iftapmode bool, addr string, mtu int
 	if err := tun.setup(ifname, iftapmode, addr, mtu); err != nil {
 		return err
 	}
+	tun.mutex.Lock()
+	tun.isOpen = true
+	tun.mutex.Unlock()
 	go func() { panic(tun.read()) }()
 	go func() { panic(tun.write()) }()
 	if iftapmode {
@@ -148,14 +154,28 @@ func (tun *tunAdapter) write() error {
 					len(data))            // Payload length
 				copy(frame[tun_ETHER_HEADER_LENGTH:], data[:])
 				if _, err := tun.iface.Write(frame); err != nil {
+					tun.mutex.RLock()
+					open := tun.isOpen
+					tun.mutex.RUnlock()
+					if !open {
+						return nil
+					} else {
 						panic(err)
 					}
 				}
+			}
 		} else {
 			if _, err := tun.iface.Write(data); err != nil {
+				tun.mutex.RLock()
+				open := tun.isOpen
+				tun.mutex.RUnlock()
+				if !open {
+					return nil
+				} else {
 					panic(err)
 				}
 			}
+		}
 		util.PutBytes(data)
 	}
 }
@@ -173,9 +193,16 @@ func (tun *tunAdapter) read() error {
 	for {
 		n, err := tun.iface.Read(buf)
 		if err != nil {
+			tun.mutex.RLock()
+			open := tun.isOpen
+			tun.mutex.RUnlock()
+			if !open {
+				return nil
+			} else {
 				// panic(err)
 				return err
 			}
+		}
 		o := 0
 		if tun.iface.IsTAP() {
 			o = tun_ETHER_HEADER_LENGTH
@@ -202,6 +229,9 @@ func (tun *tunAdapter) read() error {
 // process stops. Typically this operation will happen quickly, but on macOS
 // it can block until a read operation is completed.
 func (tun *tunAdapter) close() error {
+	tun.mutex.Lock()
+	tun.isOpen = false
+	tun.mutex.Unlock()
 	if tun.iface == nil {
 		return nil
 	}