Fix e2e tests writing after the tester tun is closed causing a panic (#1681)

udp/udp_tester.go

@@ -7,7 +7,7 @@ import (
 	"io"
 	"net/netip"
 	"os"
-	"sync/atomic"
+	"sync"
 
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
@@ -37,7 +37,15 @@ type TesterConn struct {
 	RxPackets chan *Packet // Packets to receive into nebula
 	TxPackets chan *Packet // Packets transmitted outside by nebula
 
-	closed atomic.Bool
+	// done is closed exactly once by Close. Senders select on it so they
+	// never race with a channel close; readers exit when it fires. The
+	// packet channels are intentionally never closed - that was the source
+	// of `send on closed channel` panics when a WriteTo/Send from another
+	// goroutine passed the close check and reached the send just after
+	// Close ran.
+	done      chan struct{}
+	closeOnce sync.Once
+
 	l *logrus.Logger
 }
 
@@ -46,6 +54,7 @@ func NewListener(l *logrus.Logger, ip netip.Addr, port int, _ bool, _ int) (Conn
 		Addr:      netip.AddrPortFrom(ip, uint16(port)),
 		RxPackets: make(chan *Packet, 10),
 		TxPackets: make(chan *Packet, 10),
+		done:      make(chan struct{}),
 		l:         l,
 	}, nil
 }
@@ -54,10 +63,6 @@ func NewListener(l *logrus.Logger, ip netip.Addr, port int, _ bool, _ int) (Conn
 // this is an encrypted packet or a handshake message in most cases
 // packets were transmitted from another nebula node, you can send them with Tun.Send
 func (u *TesterConn) Send(packet *Packet) {
-	if u.closed.Load() {
-		return
-	}
-
 	h := &header.H{}
 	if err := h.Parse(packet.Data); err != nil {
 		panic(err)
@@ -68,7 +73,10 @@ func (u *TesterConn) Send(packet *Packet) {
 			WithField("dataLen", len(packet.Data)).
 			Debug("UDP receiving injected packet")
 	}
-	u.RxPackets <- packet
+	select {
+	case <-u.done:
+	case u.RxPackets <- packet:
+	}
 }
 
 // Get will pull a UdpPacket from the transmit queue
@@ -76,7 +84,12 @@ func (u *TesterConn) Send(packet *Packet) {
 // packets were ingested from the tun side (in most cases), you can send them with Tun.Send
 func (u *TesterConn) Get(block bool) *Packet {
 	if block {
-		return <-u.TxPackets
+		select {
+		case <-u.done:
+			return nil
+		case p := <-u.TxPackets:
+			return p
+		}
 	}
 
 	select {
@@ -92,10 +105,6 @@ func (u *TesterConn) Get(block bool) *Packet {
 //********************************************************************************************************************//
 
 func (u *TesterConn) WriteTo(b []byte, addr netip.AddrPort) error {
-	if u.closed.Load() {
-		return io.ErrClosedPipe
-	}
-
 	p := &Packet{
 		Data: make([]byte, len(b), len(b)),
 		From: u.Addr,
@@ -103,18 +112,23 @@ func (u *TesterConn) WriteTo(b []byte, addr netip.AddrPort) error {
 	}
 
 	copy(p.Data, b)
-	u.TxPackets <- p
+	select {
+	case <-u.done:
+		return io.ErrClosedPipe
+	case u.TxPackets <- p:
 		return nil
+	}
 }
 
 func (u *TesterConn) ListenOut(r EncReader) error {
 	for {
-		p, ok := <-u.RxPackets
-		if !ok {
+		select {
+		case <-u.done:
 			return os.ErrClosed
-		}
+		case p := <-u.RxPackets:
 			r(p.From, p.Data)
 		}
+	}
 }
 
 func (u *TesterConn) ReloadConfig(*config.C) {}
@@ -137,9 +151,8 @@ func (u *TesterConn) Rebind() error {
 }
 
 func (u *TesterConn) Close() error {
-	if u.closed.CompareAndSwap(false, true) {
-		close(u.RxPackets)
-		close(u.TxPackets)
-	}
+	u.closeOnce.Do(func() {
+		close(u.done)
+	})
 	return nil
 }