[chore][pkg/stanza] Adjust length of knownFiles based on number of ma…

…tches (#28646)

Follows
#28493

This adjusts the length of `knownFiles` to be roughly 4x the number of
matches per poll cycle. In other words, we will remember files for up to
4 poll cycles.

Resolves
#28567

pkg/stanza/fileconsumer/config.go

@@ -182,7 +182,7 @@ func (c Config) buildManager(logger *zap.SugaredLogger, emit emit.Callback, spli
 		maxBatchFiles:     c.MaxConcurrentFiles / 2,
 		maxBatches:        c.MaxBatches,
 		previousPollFiles: make([]*reader.Reader, 0, c.MaxConcurrentFiles/2),
-		knownFiles:        make([]*reader.Metadata, 0, 10*c.MaxConcurrentFiles),
+		knownFiles:        []*reader.Metadata{},
 	}, nil
 }
 

pkg/stanza/fileconsumer/file.go

@@ -34,12 +34,26 @@ type Manager struct {
 
 	previousPollFiles []*reader.Reader
 	knownFiles        []*reader.Metadata
+
+	// This value approximates the expected number of files which we will find in a single poll cycle.
+	// It is updated each poll cycle using a simple moving average calculation which assigns 20% weight
+	// to the most recent poll cycle.
+	// It is used to regulate the size of knownFiles. The goal is to allow knownFiles
+	// to contain checkpoints from a few previous poll cycles, but not grow unbounded.
+	movingAverageMatches int
 }
 
 func (m *Manager) Start(persister operator.Persister) error {
 	ctx, cancel := context.WithCancel(context.Background())
 	m.cancel = cancel
 
+	if matches, err := m.fileMatcher.MatchFiles(); err != nil {
+		m.Warnf("finding files: %v", err)
+	} else {
+		m.movingAverageMatches = len(matches)
+		m.knownFiles = make([]*reader.Metadata, 0, 4*len(matches))
+	}
+
 	if persister != nil {
 		m.persister = persister
 		offsets, err := checkpoint.Load(ctx, m.persister)
@@ -53,19 +67,15 @@ func (m *Manager) Start(persister operator.Persister) error {
 		}
 	}
 
-	if _, err := m.fileMatcher.MatchFiles(); err != nil {
-		m.Warnf("finding files: %v", err)
-	}
-
 	// Start polling goroutine
 	m.startPoller(ctx)
 
 	return nil
 }
 
 func (m *Manager) closePreviousFiles() {
-	if forgetNum := len(m.previousPollFiles) + len(m.knownFiles) - cap(m.knownFiles); forgetNum > 0 {
-		m.knownFiles = m.knownFiles[forgetNum:]
+	if len(m.knownFiles) > 4*m.movingAverageMatches {
+		m.knownFiles = m.knownFiles[m.movingAverageMatches:]
 	}
 	for _, r := range m.previousPollFiles {
 		m.knownFiles = append(m.knownFiles, r.Close())
@@ -116,6 +126,8 @@ func (m *Manager) poll(ctx context.Context) {
 	matches, err := m.fileMatcher.MatchFiles()
 	if err != nil {
 		m.Debugf("finding files: %v", err)
+	} else {
+		m.movingAverageMatches = (m.movingAverageMatches*3 + len(matches)) / 4
 	}
 	m.Debugf("matched files", zap.Strings("paths", matches))