From af43dd3c745875f566eb80d32232df06bd8b981a Mon Sep 17 00:00:00 2001
From: Derek Collison <derek@nats.io>
Date: Sat, 11 Jun 2022 14:12:05 -0700
Subject: [PATCH] When internal system messages were destined for a queue
 subscriber across a route the reply subject would be empty but not nil which
 caused mangling of the RMSG proto.

Signed-off-by: Derek Collison <derek@nats.io>
---
 server/client.go      |  6 +++---
 server/events_test.go | 23 +++++++++++++++++++++++
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/server/client.go b/server/client.go
index d7a8da09..37add98e 100644
--- a/server/client.go
+++ b/server/client.go
@@ -2954,7 +2954,7 @@ func (c *client) msgHeaderForRouteOrLeaf(subj, reply []byte, rt *routeTarget, ac
 	mh = append(mh, ' ')
 
 	if len(rt.qs) > 0 {
-		if reply != nil {
+		if len(reply) > 0 {
 			mh = append(mh, "+ "...) // Signal that there is a reply.
 			mh = append(mh, reply...)
 			mh = append(mh, ' ')
@@ -2962,7 +2962,7 @@ func (c *client) msgHeaderForRouteOrLeaf(subj, reply []byte, rt *routeTarget, ac
 			mh = append(mh, "| "...) // Only queues
 		}
 		mh = append(mh, rt.qs...)
-	} else if reply != nil {
+	} else if len(reply) > 0 {
 		mh = append(mh, reply...)
 		mh = append(mh, ' ')
 	}
@@ -4180,7 +4180,7 @@ func (c *client) processMsgResults(acc *Account, r *SublistResult, msg, deliver,
 	// leaf nodes or routes even if there are no queue filters since we collect
 	// them above and do not process inline like normal clients.
 	// However, do select queue subs if asked to ignore empty queue filter.
-	if (c.kind == LEAF || c.kind == ROUTER || c.kind == GATEWAY) && qf == nil && flags&pmrIgnoreEmptyQueueFilter == 0 {
+	if (c.kind == LEAF || c.kind == ROUTER || c.kind == GATEWAY) && len(qf) == 0 && flags&pmrIgnoreEmptyQueueFilter == 0 {
 		goto sendToRoutesOrLeafs
 	}
 
diff --git a/server/events_test.go b/server/events_test.go
index e2f12b51..987510f6 100644
--- a/server/events_test.go
+++ b/server/events_test.go
@@ -2371,3 +2371,26 @@ func TestServerEventsFilteredByTag(t *testing.T) {
 	require_Contains(t, string(m1.Data)+string(m2.Data), "srv-A", "srv-B", "foo", "bar", "baz")
 	require_Len(t, len(msgs), 0)
 }
+
+// https://github.com/nats-io/nats-server/issues/3177
+func TestServerEventsAndDQSubscribers(t *testing.T) {
+	c := createJetStreamClusterWithTemplate(t, jsClusterAccountsTempl, "DDQ", 3)
+	defer c.shutdown()
+
+	nc, err := nats.Connect(c.randomServer().ClientURL(), nats.UserInfo("admin", "s3cr3t!"))
+	require_NoError(t, err)
+	defer nc.Close()
+
+	sub, err := nc.QueueSubscribeSync("$SYS.ACCOUNT.*.DISCONNECT", "qq")
+	require_NoError(t, err)
+	nc.Flush()
+
+	// Create and disconnect 10 random connections.
+	for i := 0; i < 10; i++ {
+		nc, err := nats.Connect(c.randomServer().ClientURL())
+		require_NoError(t, err)
+		nc.Close()
+	}
+
+	checkSubsPending(t, sub, 10)
+}