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How to get original filename when renaming with Go fsnotify

Complete guide to getting the original filename when renaming with fsnotify in Go. Learn how to use Windows API to track the full path of file renaming in Windows applications.

#file-change-tracking#file-monitoring-go#fsnotify-rename#golang-sys-windows#go-windows-api#rename-events-go#windows-file-system

How to get the original filename before renaming using the fsnotify library in Go?

I’m developing a Go application for Windows that monitors changes in a folder. When processing rename events, fsnotify.Event only provides the new filename but not the old one. The renamedFrom field is private and not accessible for direct access due to Go language restrictions.

The current solution uses heuristics (buffering rename and tracking the next create), but this seems excessive since the logs show an indication of the original file in the CREATE event:

2025/10/05 22:48:31 [CREATE "C:\CODE\temp\afa.docx" ← "C:\CODE\temp\~WRD0000.tmp"]

In the Windows API, there’s FILE_ACTION_RENAMED_NEW_NAME (0x00000005) which allows getting full information about the rename.

How to properly get the original filename at a rename event using fsnotify in Go?

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Getting Original Filename When Renaming in Go’s fsnotify for Windows

In Go’s fsnotify library for Windows, you can obtain the original filename when renaming by making direct Windows API calls through the golang.org/x/sys/windows package, since fsnotify on Windows relies on the Windows API ReadDirectoryChangesW, which provides complete information about rename operations.

Contents

Direct Windows API Usage

The fsnotify library on Windows uses the ReadDirectoryChangesW function from the Windows API, which can provide complete information about rename operations, including the original filename. However, fsnotify hides this information.

For direct access to the Windows API:

go
package main

import (
	"fmt"
	"log"
	"syscall"
	"unsafe"
	
	"golang.org/x/sys/windows"
)

type FILE_NOTIFY_INFORMATION struct {
	NextEntryOffset     uint32
	Action              uint32
	FileNameLength      uint32
	FileName            uint16
}

const (
	FILE_ACTION_RENAMED_OLD_NAME = 0x00000004
	FILE_ACTION_RENAMED_NEW_NAME = 0x00000005
)

func monitorDirectory(path string) error {
	handle, err := windows.CreateFile(
		windows.StringToUTF16Ptr(path),
		windows.FILE_LIST_DIRECTORY,
		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
		nil,
		windows.OPEN_EXISTING,
		windows.FILE_FLAG_BACKUP_SEMANTICS|windows.FILE_FLAG_OVERLAPPED,
		0,
	)
	if err != nil {
		return err
	}
	defer windows.CloseHandle(handle)

	buffer := make([]byte, 4096)
	overlapped := &windows.Overlapped{}
	
	for {
		var bytesReturned uint32
		err := windows.ReadFile(handle, buffer, &bytesReturned, overlapped)
		if err != nil && err != windows.ERROR_IO_PENDING {
			return err
		}

		var offset uint32 = 0
		for offset < bytesReturned {
			notifyInfo := (*FILE_NOTIFY_INFORMATION)(unsafe.Pointer(&buffer[offset]))
			
			switch notifyInfo.Action {
			case FILE_ACTION_RENAMED_OLD_NAME:
				oldName := windows.UTF16ToString((*[1024]uint16)(unsafe.Pointer(&notifyInfo.FileName))[:notifyInfo.FileNameLength/2])
				fmt.Printf("Old name: %s\n", oldName)
			case FILE_ACTION_RENAMED_NEW_NAME:
				newName := windows.UTF16ToString((*[1024]uint16)(unsafe.Pointer(&notifyInfo.FileName))[:notifyInfo.FileNameLength/2])
				fmt.Printf("New name: %s\n", newName)
			}
			
			offset += notifyInfo.NextEntryOffset
			if notifyInfo.NextEntryOffset == 0 {
				break
			}
		}
	}
}

Extending fsnotify for renamedFrom Access

Since renamedFrom is a private field, you can create a wrapper for fsnotify:

go
package fsnotifyext

import (
	"github.com/fsnotify/fsnotify"
)

type EventWithOldName struct {
	fsnotify.Event
	OldName string
}

type ExtendedWatcher struct {
	*fsnotify.Watcher
	eventChan chan EventWithOldName
}

func NewExtendedWatcher() (*ExtendedWatcher, error) {
	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		return nil, err
	}
	
	ew := &ExtendedWatcher{
		Watcher:   watcher,
		eventChan: make(chan EventWithOldName, 100),
	}
	
	go ew.processEvents()
	return ew, nil
}

func (ew *ExtendedWatcher) Events() <-chan EventWithOldName {
	return ew.eventChan
}

func (ew *ExtendedWatcher) processEvents() {
	for {
		select {
		case event, ok := <-ew.Watcher.Events:
			if !ok {
				close(ew.eventChan)
				return
			}
			
			if event.Op&fsnotify.Rename == fsnotify.Rename {
				oldName := ew.getOldName(event.Name)
				ew.eventChan <- EventWithOldName{
					Event:   event,
					OldName: oldName,
				}
			} else {
				ew.eventChan <- EventWithOldName{Event: event}
			}
			
		case err, ok := <-ew.Watcher.Errors():
			if !ok {
				return
			}
			// Handle errors
		}
	}
}

// Implementation for getting old name should go here
func (ew *ExtendedWatcher) getOldName(newName string) string {
	// Use Windows API or heuristics
	return ""
}

Alternative Approaches

1. Using a Buffered Watcher

go
func bufferedRenameHandler(watcher *fsnotify.Watcher) {
	renameQueue := make(map[string]string)
	
	for {
		select {
		case event, ok := <-watcher.Events:
			if !ok {
				return
			}
			
			if event.Op&fsnotify.Create == fsnotify.Create {
				// Check if this is a new name after a rename
				if oldName, exists := renameQueue[event.Name]; exists {
					fmt.Printf("Renamed: %s -> %s\n", oldName, event.Name)
					delete(renameQueue, event.Name)
				}
			} else if event.Op&fsnotify.Rename == fsnotify.Rename {
				// Save the new name for later lookup
				renameQueue[event.Name] = "" // Old name unknown for now
			}
			
		case err, ok := <-watcher.Errors:
			if !ok {
				return
			}
			log.Println("Error:", err)
		}
	}
}

2. Combining Events

go
type RenameEvent struct {
	OldPath string
	NewPath string
	Timestamp time.Time
}

func trackRenameEvents(watcher *fsnotify.Watcher) chan RenameEvent {
	renameChan := make(chan RenameEvent, 10)
	
	go func() {
		pendingRenames := make(map[string]time.Time)
		
		for {
			select {
			case event, ok := <-watcher.Events:
				if !ok {
					close(renameChan)
					return
				}
				
				if event.Op&fsnotify.Rename == fsnotify.Rename {
					pendingRenames[event.Name] = time.Now()
				} else if event.Op&fsnotify.Create == fsnotify.Create {
					// Look for corresponding rename event
					for newPath, renameTime := range pendingRenames {
						if time.Since(renameTime) < 5*time.Second {
							renameChan <- RenameEvent{
								OldPath:   getOldPathHeuristic(newPath),
								NewPath:   newPath,
								Timestamp: renameTime,
							}
							delete(pendingRenames, newPath)
							break
						}
					}
				}
				
			case err, ok := <-watcher.Errors:
				if !ok {
					return
				}
				log.Println("Error:", err)
			}
		}
	}()
	
	return renameChan
}

Practical Implementation

Here’s a complete implementation using Windows API through syscall:

go
package main

import (
	"fmt"
	"log"
	"strings"
	"syscall"
	"time"
	"unsafe"
	
	"golang.org/x/sys/windows"
)

type WindowsFileWatcher struct {
	handle      windows.Handle
	notifyChan  chan FileEvent
	errorChan   chan error
	done        chan struct{}
	buffer      []byte
}

type FileEvent struct {
	Path  string
	Op    uint32
	OldPath string // For renames
}

const (
	FILE_ACTION_ADDED         = 0x00000001
	FILE_ACTION_REMOVED       = 0x00000002
	FILE_ACTION_MODIFIED      = 0x00000003
	FILE_ACTION_RENAMED_OLD_NAME = 0x00000004
	FILE_ACTION_RENAMED_NEW_NAME = 0x00000005
)

func NewWindowsFileWatcher(path string) (*WindowsFileWatcher, error) {
	watcher := &WindowsFileWatcher{
		notifyChan: make(chan FileEvent, 100),
		errorChan:  make(chan error, 10),
		done:       make(chan struct{}),
		buffer:     make([]byte, 4096),
	}
	
	var err error
	watcher.handle, err = windows.CreateFile(
		windows.StringToUTF16Ptr(path),
		windows.FILE_LIST_DIRECTORY,
		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
		nil,
		windows.OPEN_EXISTING,
		windows.FILE_FLAG_BACKUP_SEMANTICS|windows.FILE_FLAG_OVERLAPPED,
		0,
	)
	
	if err != nil {
		return nil, fmt.Errorf("failed to create handle for monitoring: %v", err)
	}
	
	go watcher.monitor()
	return watcher, nil
}

func (w *WindowsFileWatcher) Events() <-chan FileEvent {
	return w.notifyChan
}

func (w *WindowsFileWatcher) Errors() <-chan error {
	return w.errorChan
}

func (w *WindowsFileWatcher) Close() error {
	close(w.done)
	return windows.CloseHandle(w.handle)
}

func (w *WindowsFileWatcher) monitor() {
	overlapped := &windows.Overlapped{}
	
	for {
		select {
		case <-w.done:
			return
		default:
			var bytesReturned uint32
			err := windows.ReadFile(w.handle, w.buffer, &bytesReturned, overlapped)
			
			if err != nil && err != windows.ERROR_IO_PENDING {
				select {
				case w.errorChan <- err:
				default:
				}
				continue
			}
			
			if bytesReturned > 0 {
				w.processBuffer(bytesReturned)
			}
			
			time.Sleep(100 * time.Millisecond)
		}
	}
}

func (w *WindowsFileWatcher) processBuffer(bytesReturned uint32) {
	offset := uint32(0)
	
	for offset < bytesReturned {
		info := (*FILE_NOTIFY_INFORMATION)(unsafe.Pointer(&w.buffer[offset]))
		
		path := windows.UTF16ToString((*[1024]uint16)(unsafe.Pointer(&info.FileName))[:info.FileNameLength/2])
		
		event := FileEvent{
			Path: path,
			Op:   info.Action,
		}
		
		// For renames, get the old name from the next record
		if info.Action == FILE_ACTION_RENAMED_NEW_NAME {
			// Assume the previous record was RENAMED_OLD_NAME
			event.OldPath = w.findOldRenamePath(path)
		}
		
		select {
		case w.notifyChan <- event:
		default:
		}
		
		offset += info.NextEntryOffset
		if info.NextEntryOffset == 0 {
			break
		}
	}
}

func (w *WindowsFileWatcher) findOldRenamePath(newPath string) string {
	// In a real implementation, you would need to analyze the event buffer
	// For simplicity, we use a heuristic
	base := filepath.Base(newPath)
	if strings.HasPrefix(base, "~") {
		return strings.ReplaceAll(newPath, "~", "")
	}
	return ""
}

func main() {
	watcher, err := NewWindowsFileWatcher("C:\\CODE\\temp")
	if err != nil {
		log.Fatal(err)
	}
	defer watcher.Close()
	
	for {
		select {
		case event, ok := <-watcher.Events():
			if !ok {
				return
			}
			
			switch event.Op {
			case FILE_ACTION_RENAMED_OLD_NAME:
				fmt.Printf("Old filename: %s\n", event.Path)
			case FILE_ACTION_RENAMED_NEW_NAME:
				fmt.Printf("New filename: %s (from %s)\n", event.Path, event.OldPath)
			case FILE_ACTION_ADDED:
				fmt.Printf("File created: %s\n", event.Path)
			case FILE_ACTION_REMOVED:
				fmt.Printf("File removed: %s\n", event.Path)
			case FILE_ACTION_MODIFIED:
				fmt.Printf("File modified: %s\n", event.Path)
			}
			
		case err, ok := <-watcher.Errors():
			if !ok {
				return
			}
			log.Printf("Error: %v\n", err)
		}
	}
}

Performance Optimization

To improve performance, consider:

  1. Using a buffer pool:
go
var bufferPool = sync.Pool{
	New: func() interface{} {
		return make([]byte, 4096)
	},
}

func (w *WindowsFileWatcher) monitor() {
	for {
		buffer := bufferPool.Get().([]byte)
		defer bufferPool.Put(buffer)
		
		// Use buffer...
	}
}
  1. Event filtering:
go
func (w *WindowsFileWatcher) shouldProcessEvent(path string, op uint32) bool {
	// Skip temporary files
	if strings.Contains(path, "~") || strings.Contains(path, ".tmp") {
		return false
	}
	return true
}
  1. Batch event processing:
go
func (w *WindowsFileWatcher) processBatch() []FileEvent {
	var events []FileEvent
	
	for len(events) < 10 { // Limit batch size
		select {
		case event := <-w.notifyChan:
			events = append(events, event)
		default:
			return events
		}
	}
	return events
}

The key point is to use direct Windows API calls through golang.org/x/sys/windows, since the standard fsnotify library hides all rename information except for the new filename.

Sources

  1. Windows API Documentation - ReadDirectoryChangesW
  2. golang.org/x/sys/windows package documentation
  3. fsnotify library source code
  4. Microsoft Windows File System Change Notifications
How to handle rename events in cross-platform Go applications using fsnotify?What are performant alternatives to direct Windows API calls for tracking file renaming in Go?How to implement robust file rename tracking in Go with error handling and recovery?What are the best practices for file system monitoring in Go applications?How to handle edge cases in file rename detection, such as temporary files and system files?Can fsnotify be extended to support custom event types beyond the standard ones?
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