From 43b2641591d4726a1d6b776f7bc3b83864132b58 Mon Sep 17 00:00:00 2001
From: Ivan Kozlovic <ivan@synadia.com>
Date: Thu, 14 Apr 2022 12:53:25 -0600
Subject: [PATCH] [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: https://github.com/nats-io/nats.go/pull/958

Signed-off-by: Ivan Kozlovic <ivan@synadia.com>
---
 server/jetstream_test.go | 10 ++++++++++
 server/stream.go         |  8 ++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/server/jetstream_test.go b/server/jetstream_test.go
index 1b84974609..12d79c737f 100644
--- a/server/jetstream_test.go
+++ b/server/jetstream_test.go
@@ -3471,6 +3471,16 @@ 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")
diff --git a/server/stream.go b/server/stream.go
index 51dab7c6f8..75828de2a0 100644
--- a/server/stream.go
+++ b/server/stream.go
@@ -2761,12 +2761,12 @@ 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.
@@ -2954,7 +2954,7 @@ 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()