Fix race with store when fetching messages by a consumer (#6003)

We were releasing and acquiring again lock while fetching messages to
reduce contention. This could cause a consumer getting stuck in certain
scenarios, like when `max messages per subject` was set to `1` and
message was republished while consumer was trying to `NAK` the first
one.

- [ ] add a proper test

Signed-off-by: Tomasz Pietrek <[email protected]>

---------

Signed-off-by: Tomasz Pietrek <[email protected]>

server/consumer.go

@@ -3736,9 +3736,7 @@ func (o *consumer) getNextMsg() (*jsPubMsg, uint64, error) {
 	var pmsg = getJSPubMsgFromPool()
 
 	// Grab next message applicable to us.
-	// We will unlock here in case lots of contention, e.g. WQ.
 	filters, subjf, fseq := o.filters, o.subjf, o.sseq
-	o.mu.Unlock()
 	// Check if we are multi-filtered or not.
 	if filters != nil {
 		sm, sseq, err = store.LoadNextMsgMulti(filters, fseq, &pmsg.StoreMsg)
@@ -3753,8 +3751,6 @@ func (o *consumer) getNextMsg() (*jsPubMsg, uint64, error) {
 		pmsg.returnToPool()
 		pmsg = nil
 	}
-	// Re-acquire lock.
-	o.mu.Lock()
 	// Check if we should move our o.sseq.
 	if sseq >= o.sseq {
 		// If we are moving step by step then sseq == o.sseq.

server/jetstream_consumer_test.go

@@ -18,6 +18,7 @@ package server
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"math/rand"
 	"slices"
@@ -1402,6 +1403,120 @@ func TestJetStreamConsumerPedanticMode(t *testing.T) {
 		}
 	}
 }
+
+func TestJetStreamConsumerStuckAckPending(t *testing.T) {
+	s := RunBasicJetStreamServer(t)
+	defer s.Shutdown()
+
+	nc, js := jsClientConnect(t, s)
+	defer nc.Close()
+
+	type ActiveWorkItem struct {
+		ID     int
+		Expiry time.Time
+	}
+
+	_, err := js.AddStream(&nats.StreamConfig{
+		Name:              "TEST_ACTIVE_WORK_ITEMS",
+		Discard:           nats.DiscardOld,
+		MaxMsgsPerSubject: 1,
+		Subjects:          []string{"TEST_ACTIVE_WORK_ITEMS.>"},
+	})
+	require_NoError(t, err)
+
+	_, err = js.AddConsumer("TEST_ACTIVE_WORK_ITEMS", &nats.ConsumerConfig{
+		Durable:       "testactiveworkitemsconsumer",
+		AckPolicy:     nats.AckExplicitPolicy,
+		MaxAckPending: -1,
+		MaxWaiting:    20000,
+		AckWait:       15 * time.Second,
+	})
+	require_NoError(t, err)
+
+	sub, err := js.PullSubscribe("TEST_ACTIVE_WORK_ITEMS.>", "testactiveworkitemsconsumer", nats.BindStream("TEST_ACTIVE_WORK_ITEMS"))
+	require_NoError(t, err)
+
+	errs := make(chan error)
+	go func() {
+		for {
+			msgs, err := sub.Fetch(200)
+			if err != nil {
+				// test is done. stop the loop.
+				if errors.Is(err, nats.ErrSubscriptionClosed) || errors.Is(err, nats.ErrConnectionClosed) {
+					return
+				}
+				if !errors.Is(err, nats.ErrTimeout) {
+					errs <- err
+					return
+				}
+				continue
+			}
+			for _, msg := range msgs {
+				msg := msg
+				var workItem ActiveWorkItem
+				if err := json.Unmarshal(msg.Data, &workItem); err != nil {
+					errs <- err
+					return
+				}
+
+				now := time.Now()
+				// If the work item has not expired, nak it with the respective delay.
+				if workItem.Expiry.After(now) {
+					msg.NakWithDelay(workItem.Expiry.Sub(now))
+				} else {
+					msg.Ack()
+				}
+			}
+		}
+	}()
+
+	for i := 0; i < 25_000; i++ {
+		// Publish item to TEST_ACTIVE_WORK_ITEMS stream with an expiry time.
+		workItem := ActiveWorkItem{ID: i, Expiry: time.Now().Add(30 * time.Second)}
+		data, err := json.Marshal(workItem)
+		require_NoError(t, err)
+
+		_, err = js.Publish(fmt.Sprintf("TEST_ACTIVE_WORK_ITEMS.%d", i), data)
+		require_NoError(t, err)
+
+		// Update expiry time and republish item to TEST_ACTIVE_WORK_ITEMS stream.
+		workItem.Expiry = time.Now().Add(3 * time.Second)
+		data, err = json.Marshal(workItem)
+		require_NoError(t, err)
+		_, err = js.Publish(fmt.Sprintf("TEST_ACTIVE_WORK_ITEMS.%d", i), data)
+		require_NoError(t, err)
+	}
+	noChange := false
+	lastNumAckPending := 0
+	checkFor(t, 60*time.Second, 3*time.Second, func() error {
+		select {
+		case err := <-errs:
+			t.Fatalf("consumer goroutine failed: %v", err)
+		default:
+		}
+		ci, err := js.ConsumerInfo("TEST_ACTIVE_WORK_ITEMS", "testactiveworkitemsconsumer")
+		require_NoError(t, err)
+
+		if lastNumAckPending != 0 && lastNumAckPending == ci.NumAckPending {
+			noChange = true
+		}
+		lastNumAckPending = ci.NumAckPending
+
+		// If we have no change since last check, we can fail the test before `totalWait` timeout.
+		if ci.NumAckPending > 0 && ci.NumPending == 0 {
+			if noChange {
+				_, err := sub.Fetch(1)
+				if err != nil && errors.Is(err, nats.ErrTimeout) {
+
+					t.Fatalf("num ack pending: %d\t num pending: %v\n", ci.NumAckPending, ci.NumPending)
+				}
+			}
+			return fmt.Errorf("num ack pending: %d\t num pending: %v\n", ci.NumAckPending, ci.NumPending)
+		}
+		return nil
+	})
+}
+
 func Benchmark____JetStreamConsumerIsFilteredMatch(b *testing.B) {
 	subject := "foo.bar.do.not.match.any.filter.subject"
 	for n := 1; n <= 1024; n *= 2 {