Fix empty new file edge case (elastic#36076)

It's possible that file scanning happens when the file was created but
was not yet written to. In this case the size is 0.

We should not spawn any resources (e.g. harvesters) for such files until they actually
have some content. We create events only when read something from a
file, so having a harvester on an empty file is not useful.

Not handling this edge case also causes our tests to be flaky,
sometimes an expected size does not match an actual size (0).

filebeat/input/filestream/fswatch.go

@@ -199,6 +199,11 @@ func (w *fileWatcher) watch(ctx unison.Canceler) {
 
 	// remaining files in newFiles are newly created files
 	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)
+			continue
+		}
 		select {
 		case <-ctx.Done():
 			return

filebeat/input/filestream/fswatch_test.go

@@ -30,6 +30,7 @@ import (
 
 	loginp "github.com/elastic/beats/v7/filebeat/input/filestream/internal/input-logfile"
 	conf "github.com/elastic/elastic-agent-libs/config"
+	"github.com/elastic/elastic-agent-libs/logp"
 )
 
 func TestFileWatcher(t *testing.T) {
@@ -219,10 +220,10 @@ scanner:
 		paths := []string{filepath.Join(dir, "*.log")}
 		cfgStr := `
 scanner:
-  check_interval: 100ms
+  check_interval: 10ms
 `
 
-		ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+		ctx, cancel := context.WithTimeout(context.Background(), 1000*time.Millisecond)
 		defer cancel()
 
 		fw := createWatcherWithConfig(t, paths, cfgStr)
@@ -252,16 +253,69 @@ scanner:
 		require.Equal(t, loginp.OpDone, e.Op)
 	})
 
+	t.Run("does not emit events for empty files", func(t *testing.T) {
+		dir := t.TempDir()
+		paths := []string{filepath.Join(dir, "*.log")}
+		cfgStr := `
+scanner:
+  check_interval: 10ms
+`
+
+		ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
+		defer cancel()
+
+		err := logp.DevelopmentSetup(logp.ToObserverOutput())
+		require.NoError(t, err)
+
+		fw := createWatcherWithConfig(t, paths, cfgStr)
+		go fw.Run(ctx)
+
+		basename := "created.log"
+		filename := filepath.Join(dir, basename)
+		err = os.WriteFile(filename, nil, 0777)
+		require.NoError(t, err)
+
+		t.Run("issues a warning in logs", func(t *testing.T) {
+			var lastWarning string
+			expLogMsg := fmt.Sprintf("file %q has no content yet, skipping", filename)
+			require.Eventually(t, func() bool {
+				logs := logp.ObserverLogs().FilterLevelExact(logp.WarnLevel.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)
+		})
+
+		t.Run("emits a create event once something is written to the empty file", func(t *testing.T) {
+			err = os.WriteFile(filename, []byte("hello"), 0777)
+			require.NoError(t, err)
+
+			e := fw.Event()
+			expEvent := loginp.FSEvent{
+				NewPath: filename,
+				OldPath: filename,
+				Op:      loginp.OpWrite,
+				Descriptor: loginp.FileDescriptor{
+					Filename: filename,
+					Info:     testFileInfo{name: basename, size: 5}, // +5 bytes appended
+				},
+			}
+			requireEqualEvents(t, expEvent, e)
+		})
+	})
+
 	t.Run("does not emit an event for a fingerprint collision", func(t *testing.T) {
 		dir := t.TempDir()
 		paths := []string{filepath.Join(dir, "*.log")}
 		cfgStr := `
 scanner:
-  check_interval: 100ms
+  check_interval: 10ms
   fingerprint.enabled: true
 `
 
-		ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+		ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
 		defer cancel()
 
 		fw := createWatcherWithConfig(t, paths, cfgStr)