From 48f7c70cb1cae85d611efacf3eda9404f57905dc Mon Sep 17 00:00:00 2001
From: Moses Narrow <36607567+0pcom@users.noreply.github.com>
Date: Mon, 30 Mar 2026 18:02:11 -0500
Subject: [PATCH] Fix race between Server.Serve() and Server.Close()
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

If Close() runs before Serve() calls wg.Add(1), the WaitGroup counter
is 0, Wait() returns immediately, and then Serve() calls Add(1) on a
completed WaitGroup — a data race. Check the done channel before
wg.Add(1) so Serve() returns ErrClosed if the server is already shut
down.
---
 pkg/dmsg/server.go | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/pkg/dmsg/server.go b/pkg/dmsg/server.go
index 642b0713..9bd4a622 100644
--- a/pkg/dmsg/server.go
+++ b/pkg/dmsg/server.go
@@ -3,6 +3,7 @@ package dmsg
 
 import (
 	"context"
+	"errors"
 	"net"
 	"sync"
 	"time"
@@ -17,6 +18,9 @@ import (
 	"github.com/skycoin/dmsg/pkg/dmsg/metrics"
 )
 
+// ErrClosed is returned when an operation is attempted on a closed server.
+var ErrClosed = errors.New("server closed")
+
 // PeerEntry represents a peer dmsg server to connect to.
 type PeerEntry struct {
 	PK   cipher.PubKey
@@ -156,7 +160,12 @@ func (s *Server) Serve(lis net.Listener, addr string) error {
 		WithField("local_pk", s.pk)
 
 	log.Info("Serving server.")
-	s.wg.Add(1)
+	select {
+	case <-s.done:
+		return ErrClosed
+	default:
+		s.wg.Add(1)
+	}
 	defer func() {
 		log.Info("Stopped server.")
 		s.wg.Done()