[FIXED] JetStream: possible panic on stream info when leader not elected

It is possible that a stream info request would be handled at a
time where the raft group would not yet be set/created, causing
a panic.

Resolves #3626 (at least the panic reports there)

Signed-off-by: Ivan Kozlovic <ivan@synadia.com>

server/jetstream_api.go

@@ -1780,7 +1780,10 @@ func (s *Server) jsStreamInfoRequest(sub *subscription, c *client, a *Account, s
 			if cc.meta != nil {
 				ourID = cc.meta.ID()
 			}
-			bail := !rg.isMember(ourID)
+			// We have seen cases where rg or rg.node is nil at this point,
+			// so check explicitly on those conditions and bail if that is
+			// the case.
+			bail := rg == nil || rg.node == nil || !rg.isMember(ourID)
 			if !bail {
 				// We know we are a member here, if this group is new and we are preferred allow us to answer.
 				bail = rg.Preferred != ourID || time.Since(rg.node.Created()) > lostQuorumIntervalDefault

server/jetstream_cluster_3_test.go

@@ -1373,3 +1373,42 @@ func TestJetStreamClusterInterestStreamConsumer(t *testing.T) {
 		t.Fatalf("Should not have any messages left: %d of %d", si.State.Msgs, n)
 	}
 }
+
+func TestJetStreamClusterNoPanicOnStreamInfoWhenNoLeaderYet(t *testing.T) {
+	c := createJetStreamClusterExplicit(t, "R3S", 3)
+	defer c.shutdown()
+
+	nc := natsConnect(t, c.randomServer().ClientURL())
+	defer nc.Close()
+
+	js, _ := nc.JetStream(nats.MaxWait(500 * time.Millisecond))
+
+	wg := sync.WaitGroup{}
+	wg.Add(1)
+	ch := make(chan struct{})
+	go func() {
+		defer wg.Done()
+
+		for {
+			js.StreamInfo("TEST")
+			select {
+			case <-ch:
+				return
+			case <-time.After(15 * time.Millisecond):
+			}
+		}
+	}()
+
+	time.Sleep(250 * time.Millisecond)
+
+	// Don't care if this succeeds or not (could get a context deadline
+	// due to the low MaxWait() when creating the context).
+	js.AddStream(&nats.StreamConfig{
+		Name:     "TEST",
+		Subjects: []string{"foo"},
+		Replicas: 3,
+	})
+
+	close(ch)
+	wg.Wait()
+}