Fix empty file edge case

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

@@ -365,6 +365,11 @@ func (s *fileScanner) GetFiles() map[string]loginp.FileDescriptor {
 				continue
 			}
 
+			if it.info.Size() == 0 {
+				s.log.Warnf("file %q has no content yet, skipping", filename)
+				continue
+			}
+
 			fd, err := s.toFileDescriptor(&it)
 			if err != nil {
 				s.log.Warnf("cannot create a file descriptor for an ingest target %q: %s", filename, err)

filebeat/input/filestream/fswatch_test.go

@@ -218,10 +218,10 @@ 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(), 100*time.Millisecond)
 		defer cancel()
 
 		fw := createWatcherWithConfig(t, paths, cfgStr)
@@ -251,6 +251,30 @@ 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()
+
+		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)
+
+		require.NoError(t, err)
+		e := fw.Event()
+		require.Equal(t, loginp.OpDone, e.Op)
+	})
+
 	t.Run("does not emit an event for a fingerprint collision", func(t *testing.T) {
 		dir := t.TempDir()
 		paths := []string{filepath.Join(dir, "*.log")}
@@ -260,7 +284,7 @@ scanner:
   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)