Fix consumer start sequence when sequence not yet in stream

Signed-off-by: Neil Twigg <neil@nats.io>

server/consumer.go

@@ -4983,12 +4983,16 @@ func (o *consumer) selectStartingSeqNo() {
 			o.sseq = o.cfg.OptStartSeq
 		}
 
-		if state.FirstSeq == 0 {
-			o.sseq = 1
-		} else if o.sseq < state.FirstSeq {
-			o.sseq = state.FirstSeq
-		} else if o.sseq > state.LastSeq {
-			o.sseq = state.LastSeq + 1
+		// Only clip the sseq if the OptStartSeq is not provided, otherwise
+		// it's possible that the stream just doesn't contain OptStartSeq yet.
+		if o.cfg.OptStartSeq == 0 {
+			if state.FirstSeq == 0 {
+				o.sseq = 1
+			} else if o.sseq < state.FirstSeq {
+				o.sseq = state.FirstSeq
+			} else if o.sseq > state.LastSeq {
+				o.sseq = state.LastSeq + 1
+			}
 		}
 	}
 

server/jetstream_test.go

@@ -23869,3 +23869,59 @@ func TestJetStreamConsumerInfoNumPending(t *testing.T) {
 	require_NoError(t, err)
 	require_Equal(t, ci.NumPending, 100)
 }
+
+func TestJetStreamConsumerStartSequenceNotInStream(t *testing.T) {
+	// This test is checking that we still correctly set the start
+	// sequence of a consumer if that start sequence doesn't appear
+	// in the stream yet. Previously this would have been clipped
+	// back to between the first and last seq from the stream state.
+
+	s := RunBasicJetStreamServer(t)
+	defer s.Shutdown()
+
+	nc, js := jsClientConnect(t, s)
+	defer nc.Close()
+
+	_, err := js.AddStream(&nats.StreamConfig{
+		Name:     "TEST",
+		Subjects: []string{"test"},
+	})
+	require_NoError(t, err)
+
+	sub, err := js.PullSubscribe("test", "test_consumer", nats.StartSequence(10))
+	require_NoError(t, err)
+
+	stream, err := s.gacc.lookupStream("TEST")
+	require_NoError(t, err)
+	consumer := stream.lookupConsumer("test_consumer")
+
+	func() {
+		consumer.mu.RLock()
+		defer consumer.mu.RUnlock()
+
+		require_Equal(t, consumer.dseq, 1)
+		require_Equal(t, consumer.sseq, 10)
+	}()
+
+	for i := 1; i <= 10; i++ {
+		_, err = js.Publish("test", []byte{byte(i)})
+		require_NoError(t, err)
+	}
+
+	msgs, err := sub.Fetch(1)
+	require_NoError(t, err)
+	require_Len(t, len(msgs), 1)
+	require_Equal(t, msgs[0].Data[0], 10)
+
+	require_NoError(t, msgs[0].AckSync())
+
+	func() {
+		consumer.mu.RLock()
+		defer consumer.mu.RUnlock()
+
+		require_Equal(t, consumer.dseq, 2)
+		require_Equal(t, consumer.adflr, 1)
+		require_Equal(t, consumer.sseq, 11)
+		require_Equal(t, consumer.asflr, 10)
+	}()
+}