Fix FileWatcher robustness issues (bd-71)

- Fix Debouncer race condition with sequence numbers to prevent double-fire
- Add parent directory watch to catch file creates/renames
- Add .git/HEAD watch for branch change detection
- Implement retry/backoff (50-400ms) for re-establishing JSONL watch
- Handle Create/Chmod events in addition to Write
- Add .git/HEAD polling in polling mode
- All 18 debouncer and watcher tests pass

Amp-Thread-ID: https://ampcode.com/threads/T-4029d643-b4b4-4d3b-bd85-74461f78cd7f
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/daemon_debouncer.go

@@ -12,6 +12,7 @@ type Debouncer struct {
 	timer    *time.Timer
 	duration time.Duration
 	action   func()
+	seq      uint64 // Sequence number to prevent stale timer fires
 }
 
 // NewDebouncer creates a new debouncer with the given duration and action.
@@ -34,11 +35,21 @@ func (d *Debouncer) Trigger() {
 		d.timer.Stop()
 	}
 
+	// Increment sequence number to invalidate any pending timers
+	d.seq++
+	currentSeq := d.seq
+
 	d.timer = time.AfterFunc(d.duration, func() {
-		d.action()
 		d.mu.Lock()
+		defer d.mu.Unlock()
+
+		// Only fire if this is still the latest trigger
+		if d.seq == currentSeq {
 			d.timer = nil
-		d.mu.Unlock()
+			d.mu.Unlock() // Unlock before calling action to avoid holding lock during callback
+			d.action()
+			d.mu.Lock() // Re-lock for defer
+		}
 	})
 }
 

cmd/bd/daemon_watcher.go

@@ -16,12 +16,16 @@ type FileWatcher struct {
 	watcher        *fsnotify.Watcher
 	debouncer      *Debouncer
 	jsonlPath      string
+	parentDir      string
 	pollingMode    bool
 	lastModTime    time.Time
 	lastExists     bool
 	lastSize       int64
 	pollInterval   time.Duration
 	gitRefsPath    string
+	gitHeadPath    string
+	lastHeadModTime time.Time
+	lastHeadExists bool
 	cancel         context.CancelFunc
 }
 
@@ -31,6 +35,7 @@ type FileWatcher struct {
 func NewFileWatcher(jsonlPath string, onChanged func()) (*FileWatcher, error) {
 	fw := &FileWatcher{
 		jsonlPath:    jsonlPath,
+		parentDir:    filepath.Dir(jsonlPath),
 		debouncer:    NewDebouncer(500*time.Millisecond, onChanged),
 		pollInterval: 5 * time.Second,
 	}
@@ -46,8 +51,16 @@ func NewFileWatcher(jsonlPath string, onChanged func()) (*FileWatcher, error) {
 	fallbackEnv := os.Getenv("BEADS_WATCHER_FALLBACK")
 	fallbackDisabled := fallbackEnv == "false" || fallbackEnv == "0"
 
-	// Store git refs path for filtering
+	// Store git paths for filtering
-	fw.gitRefsPath = filepath.Join(filepath.Dir(jsonlPath), "..", ".git", "refs", "heads")
+	gitDir := filepath.Join(fw.parentDir, "..", ".git")
+	fw.gitRefsPath = filepath.Join(gitDir, "refs", "heads")
+	fw.gitHeadPath = filepath.Join(gitDir, "HEAD")
+
+	// Get initial git HEAD state for polling
+	if stat, err := os.Stat(fw.gitHeadPath); err == nil {
+		fw.lastHeadModTime = stat.ModTime()
+		fw.lastHeadExists = true
+	}
 
 	watcher, err := fsnotify.NewWatcher()
 	if err != nil {
@@ -63,8 +76,17 @@ func NewFileWatcher(jsonlPath string, onChanged func()) (*FileWatcher, error) {
 
 	fw.watcher = watcher
 
-	// Watch the JSONL file
+	// Watch the parent directory (catches creates/renames)
+	if err := watcher.Add(fw.parentDir); err != nil {
+		fmt.Fprintf(os.Stderr, "Warning: failed to watch parent directory %s: %v\n", fw.parentDir, err)
+	}
+
+	// Watch the JSONL file (may not exist yet)
 	if err := watcher.Add(jsonlPath); err != nil {
+		if os.IsNotExist(err) {
+			// File doesn't exist yet - rely on parent dir watch
+			fmt.Fprintf(os.Stderr, "Info: JSONL file %s doesn't exist yet, watching parent directory\n", jsonlPath)
+		} else {
 			watcher.Close()
 			if fallbackDisabled {
 				return nil, fmt.Errorf("failed to watch JSONL and BEADS_WATCHER_FALLBACK is disabled: %w", err)
@@ -76,9 +98,11 @@ func NewFileWatcher(jsonlPath string, onChanged func()) (*FileWatcher, error) {
 			fw.watcher = nil
 			return fw, nil
 		}
+	}
 
-	// Also watch .git/refs/heads for branch changes (best effort)
+	// Also watch .git/refs/heads and .git/HEAD for branch changes (best effort)
 	_ = watcher.Add(fw.gitRefsPath) // Ignore error - not all setups have this
+	_ = watcher.Add(fw.gitHeadPath)  // Ignore error - not all setups have this
 
 	return fw, nil
 }
@@ -97,6 +121,8 @@ func (fw *FileWatcher) Start(ctx context.Context, log daemonLogger) {
 	}
 
 	go func() {
+		jsonlBase := filepath.Base(fw.jsonlPath)
+
 		for {
 			select {
 			case event, ok := <-fw.watcher.Events:
@@ -104,30 +130,43 @@ func (fw *FileWatcher) Start(ctx context.Context, log daemonLogger) {
 					return
 				}
 
-				// Handle JSONL write events
+				// Handle parent directory events (file create/replace)
-				if event.Name == fw.jsonlPath && event.Op&fsnotify.Write != 0 {
+				if event.Name == filepath.Join(fw.parentDir, jsonlBase) && event.Op&fsnotify.Create != 0 {
-					log.log("File change detected: %s", event.Name)
+					log.log("JSONL file created: %s", event.Name)
+					// Ensure we're watching the file directly
+					_ = fw.watcher.Add(fw.jsonlPath)
 					fw.debouncer.Trigger()
+					continue
+				}
+
+				// Handle JSONL write/chmod events
+				if event.Name == fw.jsonlPath && event.Op&(fsnotify.Write|fsnotify.Create|fsnotify.Chmod) != 0 {
+					log.log("File change detected: %s (op: %v)", event.Name, event.Op)
+					fw.debouncer.Trigger()
+					continue
 				}
 
 				// Handle JSONL removal/rename (e.g., git checkout)
 				if event.Name == fw.jsonlPath && (event.Op&fsnotify.Remove != 0 || event.Op&fsnotify.Rename != 0) {
 					log.log("JSONL removed/renamed, re-establishing watch")
 					fw.watcher.Remove(fw.jsonlPath)
-					// Brief wait for file to be recreated
+					// Retry with exponential backoff
-					time.Sleep(100 * time.Millisecond)
+					fw.reEstablishWatch(ctx, log)
-					if err := fw.watcher.Add(fw.jsonlPath); err != nil {
+					continue
-						log.log("Failed to re-watch JSONL: %v", err)
-					} else {
-						// File was recreated, trigger to reload
-						fw.debouncer.Trigger()
 				}
+
+				// Handle .git/HEAD changes (branch switches)
+				if event.Name == fw.gitHeadPath && event.Op&(fsnotify.Write|fsnotify.Create) != 0 {
+					log.log("Git HEAD change detected: %s", event.Name)
+					fw.debouncer.Trigger()
+					continue
 				}
 
 				// Handle git ref changes (only events under gitRefsPath)
 				if event.Op&fsnotify.Write != 0 && strings.HasPrefix(event.Name, fw.gitRefsPath) {
 					log.log("Git ref change detected: %s", event.Name)
 					fw.debouncer.Trigger()
+					continue
 				}
 
 			case err, ok := <-fw.watcher.Errors:
@@ -143,6 +182,32 @@ func (fw *FileWatcher) Start(ctx context.Context, log daemonLogger) {
 	}()
 }
 
+// reEstablishWatch attempts to re-add the JSONL watch with exponential backoff.
+func (fw *FileWatcher) reEstablishWatch(ctx context.Context, log daemonLogger) {
+	delays := []time.Duration{50 * time.Millisecond, 100 * time.Millisecond, 200 * time.Millisecond, 400 * time.Millisecond}
+	
+	for _, delay := range delays {
+		select {
+		case <-ctx.Done():
+			return
+		case <-time.After(delay):
+			if err := fw.watcher.Add(fw.jsonlPath); err != nil {
+				if os.IsNotExist(err) {
+					log.log("JSONL still missing after %v, retrying...", delay)
+					continue
+				}
+				log.log("Failed to re-watch JSONL after %v: %v", delay, err)
+				return
+			}
+			// Success!
+			log.log("Successfully re-established JSONL watch after %v", delay)
+			fw.debouncer.Trigger()
+			return
+		}
+	}
+	log.log("Failed to re-establish JSONL watch after all retries")
+}
+
 // startPolling begins polling for file changes using a ticker.
 func (fw *FileWatcher) startPolling(ctx context.Context, log daemonLogger) {
 	log.log("Starting polling mode with %v interval", fw.pollInterval)
@@ -152,6 +217,9 @@ func (fw *FileWatcher) startPolling(ctx context.Context, log daemonLogger) {
 		for {
 			select {
 			case <-ticker.C:
+				changed := false
+
+				// Check JSONL file
 				stat, err := os.Stat(fw.jsonlPath)
 				if err != nil {
 					if os.IsNotExist(err) {
@@ -161,14 +229,12 @@ func (fw *FileWatcher) startPolling(ctx context.Context, log daemonLogger) {
 							fw.lastModTime = time.Time{}
 							fw.lastSize = 0
 							log.log("File missing (polling): %s", fw.jsonlPath)
-							fw.debouncer.Trigger()
+							changed = true
-						}
-						continue
 						}
+					} else {
 						log.log("Polling error: %v", err)
-					continue
 					}
-
+				} else {
 					// File exists
 					if !fw.lastExists {
 						// File appeared
@@ -176,17 +242,48 @@ func (fw *FileWatcher) startPolling(ctx context.Context, log daemonLogger) {
 						fw.lastModTime = stat.ModTime()
 						fw.lastSize = stat.Size()
 						log.log("File appeared (polling): %s", fw.jsonlPath)
-					fw.debouncer.Trigger()
+						changed = true
-					continue
+					} else if !stat.ModTime().Equal(fw.lastModTime) || stat.Size() != fw.lastSize {
-				}
-
 						// File exists and existed before - check for changes
-				if !stat.ModTime().Equal(fw.lastModTime) || stat.Size() != fw.lastSize {
 						fw.lastModTime = stat.ModTime()
 						fw.lastSize = stat.Size()
 						log.log("File change detected (polling): %s", fw.jsonlPath)
+						changed = true
+					}
+				}
+
+				// Check .git/HEAD for branch changes
+				headStat, err := os.Stat(fw.gitHeadPath)
+				if err != nil {
+					if os.IsNotExist(err) {
+						if fw.lastHeadExists {
+							fw.lastHeadExists = false
+							fw.lastHeadModTime = time.Time{}
+							log.log("Git HEAD missing (polling): %s", fw.gitHeadPath)
+							changed = true
+						}
+					}
+					// Ignore other errors for HEAD - it's optional
+				} else {
+					// HEAD exists
+					if !fw.lastHeadExists {
+						// HEAD appeared
+						fw.lastHeadExists = true
+						fw.lastHeadModTime = headStat.ModTime()
+						log.log("Git HEAD appeared (polling): %s", fw.gitHeadPath)
+						changed = true
+					} else if !headStat.ModTime().Equal(fw.lastHeadModTime) {
+						// HEAD changed (branch switch)
+						fw.lastHeadModTime = headStat.ModTime()
+						log.log("Git HEAD change detected (polling): %s", fw.gitHeadPath)
+						changed = true
+					}
+				}
+
+				if changed {
 					fw.debouncer.Trigger()
 				}
+
 			case <-ctx.Done():
 				return
 			}