Fix panic in multiListener

If Close() is called concurrently, both goroutines could pass the select’s default case before either of them executes close(), resulting in the channel being closed twice

Author: denpeshkov

net/multi_listen.go

@@ -21,6 +21,7 @@ import (
 	"fmt"
 	"net"
 	"sync"
+	"sync/atomic"
 )
 
 // connErrPair pairs conn and error which is returned by accept on sub-listeners.
@@ -38,6 +39,7 @@ type multiListener struct {
 	connCh chan connErrPair
 	// stopCh communicates from parent to child listeners.
 	stopCh chan struct{}
+	closed atomic.Bool
 }
 
 // compile time check to ensure *multiListener implements net.Listener
@@ -150,10 +152,8 @@ func (ml *multiListener) Accept() (net.Conn, error) {
 // the go-routines to exit.
 func (ml *multiListener) Close() error {
 	// Make sure this can be called repeatedly without explosions.
-	select {
-	case <-ml.stopCh:
+	if !ml.closed.CompareAndSwap(false, true) {
 		return fmt.Errorf("use of closed network connection")
-	default:
 	}
 
 	// Tell all sub-listeners to stop.