Restore memory queue's internal event cleanup after a batch is vended (…

…#41356)

Fix #41355, where event data in the memory queue was not being freed when event batches were acknowledged, but only gradually as the queue buffer was overwritten by later events. This gave the same effect as if all beat instances, even low-volume ones, were running with a full / saturated event queue.

The root cause, found by @swiatekm, is [this PR](#39584), an unrelated cleanup of old code that accidentally included one live call along with the deprecated ones. (There was an old `FreeEntries` hook in pipeline batches that was only used for deprecated shipper configs, but the cleanup also removed the `FreeEntries` call _inside_ the queue which was essential for releasing event memory.)

(cherry picked from commit fdb912a)

CHANGELOG.next.asciidoc

@@ -69,6 +69,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - Support Elastic Agent control protocol chunking support {pull}37343[37343]
 - Lower logging level to debug when attempting to configure beats with unknown fields from autodiscovered events/environments {pull}[37816][37816]
 - Set timeout of 1 minute for FQDN requests {pull}37756[37756]
+- Fix issue where old data could be saved in the memory queue after acknowledgment, increasing memory use {pull}41356[41356]
 
 *Auditbeat*
 

libbeat/publisher/pipeline/ttl_batch.go

@@ -77,6 +77,7 @@ func newBatch(retryer retryer, original queue.Batch, ttl int) *ttlBatch {
 			events = append(events, event)
 		}
 	}
+	original.FreeEntries()
 
 	b := &ttlBatch{
 		done:    original.Done,

libbeat/publisher/pipeline/ttl_batch_test.go

@@ -112,6 +112,12 @@ func TestBatchCallsDoneAndFreesEvents(t *testing.T) {
 	require.True(t, doneCalled, "Calling batch.Drop should invoke the done callback")
 }
 
+func TestNewBatchFreesEvents(t *testing.T) {
+	queueBatch := &mockQueueBatch{}
+	_ = newBatch(nil, queueBatch, 0)
+	assert.Equal(t, 1, queueBatch.freeEntriesCalled, "Creating a new ttlBatch should call FreeEntries on the underlying queue.Batch")
+}
+
 type mockQueueBatch struct {
 	freeEntriesCalled int
 }
@@ -127,6 +133,10 @@ func (b *mockQueueBatch) Entry(i int) queue.Entry {
 	return fmt.Sprintf("event %v", i)
 }
 
+func (b *mockQueueBatch) FreeEntries() {
+	b.freeEntriesCalled++
+}
+
 type mockRetryer struct {
 	batches []*ttlBatch
 }

libbeat/publisher/queue/diskqueue/consumer.go

@@ -97,6 +97,9 @@ func (batch *diskQueueBatch) Entry(i int) queue.Entry {
 	return batch.frames[i].event
 }
 
+func (batch *diskQueueBatch) FreeEntries() {
+}
+
 func (batch *diskQueueBatch) Done() {
 	batch.queue.acks.addFrames(batch.frames)
 }

libbeat/publisher/queue/memqueue/broker.go

@@ -398,6 +398,15 @@ func (b *batch) Entry(i int) queue.Entry {
 	return b.rawEntry(i).event
 }
 
+func (b *batch) FreeEntries() {
+	// This signals that the event data has been copied out of the batch, and is
+	// safe to free from the queue buffer, so set all the event pointers to nil.
+	for i := 0; i < b.count; i++ {
+		index := (b.start + i) % len(b.queue.buf)
+		b.queue.buf[index].event = nil
+	}
+}
+
 func (b *batch) Done() {
 	b.doneChan <- batchDoneMsg{}
 }

libbeat/publisher/queue/memqueue/queue_test.go

@@ -262,3 +262,41 @@ func TestAdjustInputQueueSize(t *testing.T) {
 		assert.Equal(t, int(float64(mainQueue)*maxInputQueueSizeRatio), AdjustInputQueueSize(mainQueue, mainQueue))
 	})
 }
+
+func TestBatchFreeEntries(t *testing.T) {
+	const queueSize = 10
+	const batchSize = 5
+	// 1. Add 10 events to the queue, request two batches with 5 events each
+	// 2. Make sure the queue buffer has 10 non-nil events
+	// 3. Call FreeEntries on the second batch
+	// 4. Make sure only events 6-10 are nil
+	// 5. Call FreeEntries on the first batch
+	// 6. Make sure all events are nil
+	testQueue := NewQueue(nil, nil, Settings{Events: queueSize, MaxGetRequest: batchSize, FlushTimeout: time.Second}, 0, nil)
+	producer := testQueue.Producer(queue.ProducerConfig{})
+	for i := 0; i < queueSize; i++ {
+		_, ok := producer.Publish(i)
+		require.True(t, ok, "Queue publish must succeed")
+	}
+	batch1, err := testQueue.Get(batchSize)
+	require.NoError(t, err, "Queue read must succeed")
+	require.Equal(t, batchSize, batch1.Count(), "Returned batch size must match request")
+	batch2, err := testQueue.Get(batchSize)
+	require.NoError(t, err, "Queue read must succeed")
+	require.Equal(t, batchSize, batch2.Count(), "Returned batch size must match request")
+	// Slight concurrency subtlety: we check events are non-nil after the queue
+	// reads, since if we do it before we have no way to be sure the insert
+	// has been completed.
+	for i := 0; i < queueSize; i++ {
+		require.NotNil(t, testQueue.buf[i].event, "All queue events must be non-nil")
+	}
+	batch2.FreeEntries()
+	for i := 0; i < batchSize; i++ {
+		require.NotNilf(t, testQueue.buf[i].event, "Queue index %v: batch 1's events should be unaffected by calling FreeEntries on Batch 2", i)
+		require.Nilf(t, testQueue.buf[batchSize+i].event, "Queue index %v: batch 2's events should be nil after FreeEntries", batchSize+i)
+	}
+	batch1.FreeEntries()
+	for i := 0; i < queueSize; i++ {
+		require.Nilf(t, testQueue.buf[i].event, "Queue index %v: all events should be nil after calling FreeEntries on both batches")
+	}
+}

libbeat/publisher/queue/queue.go

@@ -112,6 +112,10 @@ type Batch interface {
 	Count() int
 	Entry(i int) Entry
 	Done()
+	// Release internal references to the contained events if supported
+	// (the disk queue does not currently implement this).
+	// Entry() should not be used after this call.
+	FreeEntries()
 }
 
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