[FIXED] Max streams reached with parallel stream creation (#6502)

Maximum streams reached errors would be returned when creating (the
same) streams in parallel:
- if max streams = 1, the second create request would fail as the
previous create request being inflight made it exceed the limit, even if
it was about the same stream
- if max streams > 1, if the streams already existed would count the
streams twice, once for the streams existing already, and another time
due to being inflight

Signed-off-by: Maurice van Veen <github@mauricevanveen.com>

Author: derekcollison

server/jetstream_cluster.go

@@ -6078,7 +6078,14 @@ func (js *jetStream) jsClusteredStreamLimitsCheck(acc *Account, cfg *StreamConfi
 	numStreams, reservations := tieredStreamAndReservationCount(asa, tier, cfg)
 	// Check for inflight proposals...
 	if cc := js.cluster; cc != nil && cc.inflight != nil {
-		numStreams += len(cc.inflight[acc.Name])
+		streams := cc.inflight[acc.Name]
+		numStreams += len(streams)
+		// If inflight contains the same stream, don't count toward exceeding maximum.
+		if cfg != nil {
+			if _, ok := streams[cfg.Name]; ok {
+				numStreams--
+			}
+		}
 	}
 	if selectedLimits.MaxStreams > 0 && numStreams >= selectedLimits.MaxStreams {
 		return NewJSMaximumStreamsLimitError()
@@ -6186,7 +6193,7 @@ func (s *Server) jsClusteredStreamRequest(ci *ClientInfo, acc *Account, subject,
 		// On success, add this as an inflight proposal so we can apply limits
 		// on concurrent create requests while this stream assignment has
 		// possibly not been processed yet.
-		if streams, ok := cc.inflight[acc.Name]; ok {
+		if streams, ok := cc.inflight[acc.Name]; ok && self == nil {
 			streams[cfg.Name] = &inflightInfo{rg, syncSubject}
 		}
 	}

server/jetstream_cluster_1_test.go

@@ -3198,6 +3198,77 @@ func TestJetStreamClusterAccountInfoAndLimits(t *testing.T) {
 	}
 }
 
+func TestJetStreamClusterMaxStreamsReached(t *testing.T) {
+	c := createJetStreamClusterExplicit(t, "R3S", 3)
+	defer c.shutdown()
+
+	nc, js := jsClientConnect(t, c.randomNonLeader())
+	defer nc.Close()
+
+	// Adjust our limits.
+	c.updateLimits("$G", map[string]JetStreamAccountLimits{
+		_EMPTY_: {
+			MaxMemory:  1024,
+			MaxStore:   1024,
+			MaxStreams: 1,
+		},
+	})
+
+	// Many stream creations in parallel for the same stream should not result in
+	// maximum number of streams reached error. All should have a successful response.
+	var wg sync.WaitGroup
+	for i := 0; i < 15; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			_, err := js.AddStream(&nats.StreamConfig{
+				Name:     "TEST",
+				Subjects: []string{"foo"},
+				Replicas: 1,
+			})
+			require_NoError(t, err)
+		}()
+	}
+	wg.Wait()
+	require_NoError(t, js.DeleteStream("TEST"))
+
+	// Adjust our limits.
+	c.updateLimits("$G", map[string]JetStreamAccountLimits{
+		_EMPTY_: {
+			MaxMemory:  1024,
+			MaxStore:   1024,
+			MaxStreams: 2,
+		},
+	})
+
+	// Setup streams beforehand.
+	for d := 0; d < 2; d++ {
+		_, err := js.AddStream(&nats.StreamConfig{
+			Name:     fmt.Sprintf("TEST-%d", d),
+			Subjects: []string{fmt.Sprintf("foo.%d", d)},
+			Replicas: 1,
+		})
+		require_NoError(t, err)
+	}
+
+	// Many stream creations in parallel for streams that already exist should not result in
+	// maximum number of streams reached error. All should have a successful response.
+	for i := 0; i < 15; i++ {
+		wg.Add(1)
+		d := i % 2
+		go func() {
+			defer wg.Done()
+			_, err := js.AddStream(&nats.StreamConfig{
+				Name:     fmt.Sprintf("TEST-%d", d),
+				Subjects: []string{fmt.Sprintf("foo.%d", d)},
+				Replicas: 1,
+			})
+			require_NoError(t, err)
+		}()
+	}
+	wg.Wait()
+}
+
 func TestJetStreamClusterStreamLimits(t *testing.T) {
 	c := createJetStreamClusterExplicit(t, "R3S", 3)
 	defer c.shutdown()