Fix panic if event is dropped by queue

CHANGELOG.asciidoc

@@ -43,6 +43,7 @@ https://github.com/elastic/beats/compare/v6.0.0-rc2...master[Check the HEAD diff
 *Packetbeat*
 
 - Fix missing length check in the PostgreSQL module. {pull}5457[5457]
+- Fix panic in ACK handler if event is dropped on blocked queue {issue}5524[5524]
 
 *Winlogbeat*
 

libbeat/publisher/queue/memqueue/ackloop.go

@@ -53,13 +53,20 @@ func (l *ackLoop) run() {
 			count, events := lst.count()
 			l.lst.concat(&lst)
 
-			// log.Debugf("ackloop: scheduledACKs count=%v events=%v\n", count, events)
+			// log.Debug("ACK List:")
+			// for current := l.lst.head; current != nil; current = current.next {
+			// 	log.Debugf("  ack entry(seq=%v, start=%v, count=%v",
+			// 		current.seq, current.start, current.count)
+			// }
+
 			l.batchesSched += uint64(count)
 			l.totalSched += uint64(events)
 
 		case <-l.sig:
 			acked += l.handleBatchSig()
-			acks = l.broker.acks
+			if acked > 0 {
+				acks = l.broker.acks
+			}
 		}
 
 		// log.Debug("ackloop INFO")
@@ -87,12 +94,14 @@ func (l *ackLoop) handleBatchSig() int {
 		count += current.count
 	}
 
-	if e := l.broker.eventer; e != nil {
-		e.OnACK(count)
-	}
+	if count > 0 {
+		if e := l.broker.eventer; e != nil {
+			e.OnACK(count)
+		}
 
-	// report acks to waiting clients
-	l.processACK(lst, count)
+		// report acks to waiting clients
+		l.processACK(lst, count)
+	}
 
 	for !lst.empty() {
 		releaseACKChan(lst.pop())
@@ -110,15 +119,15 @@ func (l *ackLoop) collectAcked() chanList {
 	lst := chanList{}
 
 	acks := l.lst.pop()
+	l.onACK(acks)
 	lst.append(acks)
 
 	done := false
 	for !l.lst.empty() && !done {
 		acks := l.lst.front()
 		select {
 		case <-acks.ch:
-			l.broker.logger.Debugf("ackloop: receive ack [%v: %v, %v]", acks.seq, acks.start, acks.count)
-			l.batchesACKed++
+			l.onACK(acks)
 			lst.append(l.lst.pop())
 
 		default:
@@ -128,3 +137,8 @@ func (l *ackLoop) collectAcked() chanList {
 
 	return lst
 }
+
+func (l *ackLoop) onACK(acks *ackChan) {
+	l.batchesACKed++
+	l.broker.logger.Debugf("ackloop: receive ack [%v: %v, %v]", acks.seq, acks.start, acks.count)
+}

libbeat/publisher/queue/memqueue/broker.go

@@ -200,6 +200,10 @@ func (l *chanList) prepend(ch *ackChan) {
 }
 
 func (l *chanList) concat(other *chanList) {
+	if other.head == nil {
+		return
+	}
+
 	if l.head == nil {
 		*l = *other
 		return

libbeat/publisher/queue/memqueue/produce.go

@@ -21,6 +21,7 @@ type ackProducer struct {
 }
 
 type openState struct {
+	log    logger
 	isOpen atomic.Bool
 	done   chan struct{}
 	events chan pushRequest
@@ -37,6 +38,7 @@ type ackHandler func(count int)
 
 func newProducer(b *Broker, cb ackHandler, dropCB func(beat.Event), dropOnCancel bool) queue.Producer {
 	openState := openState{
+		log:    b.logger,
 		isOpen: atomic.MakeBool(true),
 		done:   make(chan struct{}),
 		events: b.events,
@@ -69,11 +71,18 @@ func (p *forgetfullProducer) Cancel() int {
 }
 
 func (p *ackProducer) Publish(event publisher.Event) bool {
-	return p.openState.publish(p.makeRequest(event))
+	return p.updSeq(p.openState.publish(p.makeRequest(event)))
 }
 
 func (p *ackProducer) TryPublish(event publisher.Event) bool {
-	return p.openState.tryPublish(p.makeRequest(event))
+	return p.updSeq(p.openState.tryPublish(p.makeRequest(event)))
+}
+
+func (p *ackProducer) updSeq(ok bool) bool {
+	if ok {
+		p.seq++
+	}
+	return ok
 }
 
 func (p *ackProducer) makeRequest(event publisher.Event) pushRequest {
@@ -82,7 +91,6 @@ func (p *ackProducer) makeRequest(event publisher.Event) pushRequest {
 		seq:   p.seq,
 		state: &p.state,
 	}
-	p.seq++
 	return req
 }
 
@@ -126,6 +134,7 @@ func (st *openState) tryPublish(req pushRequest) bool {
 		st.events = nil
 		return false
 	default:
+		st.log.Debugf("Dropping event, queue is blocked (seq=%v) ", req.seq)
 		return false
 	}
 }