From 41eef2a5be3edb3e2fd1db897a08522c00525460 Mon Sep 17 00:00:00 2001
From: "R.I.Pienaar" <rip@devco.net>
Date: Mon, 16 Nov 2020 15:52:39 +0100
Subject: [PATCH] ensure response message goes to the reply subject in
 RespondMsg

Signed-off-by: R.I.Pienaar <rip@devco.net>
---
 nats.go      |  1 +
 nats_test.go | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/nats.go b/nats.go
index aa4db8347..2728a92d5 100644
--- a/nats.go
+++ b/nats.go
@@ -3888,6 +3888,7 @@ func (m *Msg) RespondMsg(msg *Msg) error {
 	if m.Reply == "" {
 		return ErrMsgNoReply
 	}
+	msg.Subject = m.Reply
 	m.Sub.mu.Lock()
 	nc := m.Sub.conn
 	m.Sub.mu.Unlock()
diff --git a/nats_test.go b/nats_test.go
index 75cfd1b45..951c42de0 100644
--- a/nats_test.go
+++ b/nats_test.go
@@ -2645,3 +2645,40 @@ func TestJSSubscribe(t *testing.T) {
 		t.Fatalf("subscriber not subscribed to the delivery subject")
 	}
 }
+
+func TestMsg_RespondMsg(t *testing.T) {
+	s := RunServerOnPort(-1)
+	defer s.Shutdown()
+
+	nc, err := Connect(s.ClientURL())
+	if err != nil {
+		t.Fatalf("Expected to connect to server, got %v", err)
+	}
+	defer nc.Close()
+
+	sub, err := nc.SubscribeSync(NewInbox())
+	if err != nil {
+		t.Fatalf("subscribe failed: %s", err)
+	}
+
+	nc.PublishMsg(&Msg{Reply: sub.Subject, Subject: sub.Subject, Data: []byte("request")})
+	req, err := sub.NextMsg(time.Second)
+	if err != nil {
+		t.Fatalf("NextMsg failed: %s", err)
+	}
+
+	// verifies that RespondMsg sets the reply subject on msg based on req
+	err = req.RespondMsg(&Msg{Data: []byte("response")})
+	if err != nil {
+		t.Fatalf("RespondMsg failed: %s", err)
+	}
+
+	resp, err := sub.NextMsg(time.Second)
+	if err != nil {
+		t.Fatalf("NextMsg failed: %s", err)
+	}
+
+	if !bytes.Equal(resp.Data, []byte("response")) {
+		t.Fatalf("did not get correct response: %q", resp.Data)
+	}
+}