Make log messages in the file scanner less noisy (#38421) (#38454)

* Demote warning about empty files to the Debug level
* Demote warnings about files too small to the Debug level
* Improve wording, so it does not sound like an error

(cherry picked from commit 4df595f)

Co-authored-by: Denis <denis.rechkunov@elastic.co>

CHANGELOG-developer.next.asciidoc

@@ -181,6 +181,7 @@ The list below covers the major changes between 7.0.0-rc2 and main only.
 - Elide retryable HTTP client construction in Filebeat HTTPJSON and CEL inputs if not needed. {pull}36916[36916]
 - Allow assignment of packetbeat protocols to interfaces. {issue}36574[36564] {pull}36852[36852]
 - Add Active Directory entity collector for Filebeat entity analytics. {pull}37854[37854]
+- Make logs for empty and small files less noisy when using fingerprint file identity in filestream. {pull}38421[38421]
 
 ==== Deprecated
 

filebeat/input/filestream/fswatch.go

@@ -20,6 +20,7 @@ package filestream
 import (
 	"crypto/sha256"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"hash"
 	"io"
@@ -45,6 +46,10 @@ const (
 	watcherDebugKey              = "file_watcher"
 )
 
+var (
+	errFileTooSmall = errors.New("file size is too small for ingestion")
+)
+
 type fileWatcherConfig struct {
 	// Interval is the time between two scans.
 	Interval time.Duration `config:"check_interval"`
@@ -202,7 +207,7 @@ func (w *fileWatcher) watch(ctx unison.Canceler) {
 	for path, fd := range newFilesByName {
 		// no need to react on empty new files
 		if fd.Info.Size() == 0 {
-			w.log.Warnf("file %q has no content yet, skipping", fd.Filename)
+			w.log.Debugf("file %q has no content yet, skipping", fd.Filename)
 			delete(paths, path)
 			continue
 		}
@@ -385,6 +390,10 @@ func (s *fileScanner) GetFiles() map[string]loginp.FileDescriptor {
 			}
 
 			fd, err := s.toFileDescriptor(&it)
+			if errors.Is(err, errFileTooSmall) {
+				s.log.Debugf("cannot start ingesting from file %q: %s", filename, err)
+				continue
+			}
 			if err != nil {
 				s.log.Warnf("cannot create a file descriptor for an ingest target %q: %s", filename, err)
 				continue
@@ -473,7 +482,7 @@ func (s *fileScanner) toFileDescriptor(it *ingestTarget) (fd loginp.FileDescript
 		// we should not open the file if we know it's too small
 		minSize := s.cfg.Fingerprint.Offset + s.cfg.Fingerprint.Length
 		if fileSize < minSize {
-			return fd, fmt.Errorf("filesize of %q is %d bytes, expected at least %d bytes for fingerprinting", fd.Filename, fileSize, minSize)
+			return fd, fmt.Errorf("filesize of %q is %d bytes, expected at least %d bytes for fingerprinting: %w", fd.Filename, fileSize, minSize, errFileTooSmall)
 		}
 
 		file, err := os.Open(it.originalFilename)

filebeat/input/filestream/fswatch_test.go

@@ -276,17 +276,20 @@ scanner:
 		err := os.WriteFile(filename, nil, 0777)
 		require.NoError(t, err)
 
-		t.Run("issues a warning in logs", func(t *testing.T) {
-			var lastWarning string
+		t.Run("issues a debug message in logs", func(t *testing.T) {
 			expLogMsg := fmt.Sprintf("file %q has no content yet, skipping", filename)
 			require.Eventually(t, func() bool {
-				logs := logp.ObserverLogs().FilterLevelExact(logp.WarnLevel.ZapLevel()).TakeAll()
+				logs := logp.ObserverLogs().FilterLevelExact(logp.DebugLevel.ZapLevel()).TakeAll()
 				if len(logs) == 0 {
 					return false
 				}
-				lastWarning = logs[len(logs)-1].Message
-				return strings.Contains(lastWarning, expLogMsg)
-			}, 100*time.Millisecond, 10*time.Millisecond, "required a warning message %q but got %q", expLogMsg, lastWarning)
+				for _, l := range logs {
+					if strings.Contains(l.Message, expLogMsg) {
+						return true
+					}
+				}
+				return false
+			}, 100*time.Millisecond, 10*time.Millisecond, "required a debug message %q but never found", expLogMsg)
 		})
 
 		t.Run("emits a create event once something is written to the empty file", func(t *testing.T) {
@@ -797,6 +800,25 @@ scanner:
 		})
 	}
 
+	t.Run("does not issue warnings when file is too small", func(t *testing.T) {
+		cfgStr := `
+scanner:
+  fingerprint:
+    enabled: true
+    offset: 0
+    length: 1024
+`
+		logp.DevelopmentSetup(logp.ToObserverOutput())
+
+		// this file is 128 bytes long
+		paths := []string{filepath.Join(dir, undersizedBasename)}
+		s := createScannerWithConfig(t, paths, cfgStr)
+		files := s.GetFiles()
+		require.Empty(t, files)
+		logs := logp.ObserverLogs().FilterLevelExact(logp.WarnLevel.ZapLevel()).TakeAll()
+		require.Empty(t, logs, "there must be no warning logs for files too small")
+	})
+
 	t.Run("returns error when creating scanner with a fingerprint too small", func(t *testing.T) {
 		cfgStr := `
 scanner: