[CHANGED] JetStream: accept "Nats-Expected-Last-Sequence" with "0"

We use to ignore if the seq was 0, but now would treat it as
a requirement that the stream be empty if header is present but
set to 0.

This relates to client PR: nats-io/nats.go#958

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

server/jetstream_test.go

@@ -3478,6 +3478,16 @@ func TestJetStreamPublishExpect(t *testing.T) {
 		t.Fatalf("Expected an error, got %q", resp.Data)
 	}
 
+	// Or if we expect that there are no messages by setting "0" as the expected last seq
+	m.Header.Set(JSExpectedLastSeq, "0")
+	resp, err = nc.RequestMsg(m, 100*time.Millisecond)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	if pa := getPubAckResponse(resp.Data); pa == nil || pa.Error == nil {
+		t.Fatalf("Expected an error, got %q", resp.Data)
+	}
+
 	// Now send a message with a message ID and make sure we can match that.
 	m = nats.NewMsg("foo.bar")
 	m.Data = []byte("HELLO")

server/stream.go

@@ -3187,12 +3187,12 @@ func getExpectedStream(hdr []byte) string {
 }
 
 // Fast lookup of expected stream.
-func getExpectedLastSeq(hdr []byte) uint64 {
+func getExpectedLastSeq(hdr []byte) (uint64, bool) {
 	bseq := getHeader(JSExpectedLastSeq, hdr)
 	if len(bseq) == 0 {
-		return 0
+		return 0, false
 	}
-	return uint64(parseInt64(bseq))
+	return uint64(parseInt64(bseq)), true
 }
 
 // Fast lookup of rollups.
@@ -3472,7 +3472,7 @@ func (mset *stream) processJetStreamMsg(subject, reply string, hdr, msg []byte,
 			return errors.New("expected stream does not match")
 		}
 		// Expected last sequence.
-		if seq := getExpectedLastSeq(hdr); seq > 0 && seq != mset.lseq {
+		if seq, exists := getExpectedLastSeq(hdr); exists && seq != mset.lseq {
 			mlseq := mset.lseq
 			mset.clfs++
 			mset.mu.Unlock()