Add notify based formatter

Close: #509

walk/walk.go

@@ -22,6 +22,7 @@ const (
 	Stdin
 	Filesystem
 	Git
+	Watch
 
 	BatchSize = 1024
 )
@@ -215,6 +216,8 @@ func NewReader(
 		reader = NewFilesystemReader(root, path, statz, BatchSize)
 	case Git:
 		reader, err = NewGitReader(root, path, statz)
+	case Watch:
+		reader, err = NewWatchReader(root, path, statz)
 
 	default:
 		return nil, fmt.Errorf("unknown walk type: %v", walkType)

walk/watch.go

@@ -0,0 +1,149 @@
+package walk
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/fsnotify/fsnotify"
+	"github.com/numtide/treefmt/v2/stats"
+	"golang.org/x/sync/errgroup"
+)
+
+type WatchReader struct {
+	root string
+	path string
+
+	log   *log.Logger
+	stats *stats.Stats
+
+	eg      *errgroup.Group
+	watcher *fsnotify.Watcher
+}
+
+func (f *WatchReader) Read(ctx context.Context, files []*File) (n int, err error) {
+	// ensure we record how many files we traversed
+	defer func() {
+		f.stats.Add(stats.Traversed, n)
+	}()
+
+LOOP:
+	// keep filling files up to it's length
+	for n < len(files) {
+		select {
+		// exit early if the context was cancelled
+		case <-ctx.Done():
+			err = ctx.Err()
+			if err == nil {
+				return n, fmt.Errorf("context cancelled: %w", ctx.Err())
+			}
+
+			return n, nil
+
+		// read the next event from the channel
+		case event, ok := <-f.watcher.Events:
+			if !ok {
+				// channel was closed, exit the loop
+				err = io.EOF
+
+				break LOOP
+			}
+
+			// skip if the event is a chmod event since it doesn't change the
+			// file contents
+			if event.Has(fsnotify.Chmod) {
+				continue
+			}
+
+			file, err := os.Open(event.Name)
+			if err != nil {
+				return n, fmt.Errorf("failed to stat file %s: %w", event.Name, err)
+			}
+			defer file.Close()
+			info, err := file.Stat()
+			if err != nil {
+				return n, fmt.Errorf("failed to stat file %s: %w", event.Name, err)
+			}
+
+			// determine the absolute path since fsnotify only provides the
+			// relative path relative to the path we're watching
+			path := filepath.Clean(filepath.Join(f.root, f.path, event.Name))
+
+			// determine a path relative to the root
+			relPath, err := filepath.Rel(f.root, path)
+			if err != nil {
+				return n, fmt.Errorf("failed to determine a relative path for %s: %w", path, err)
+			}
+
+			// add to the file array and increment n
+			files[n] = &File{
+				Path:    path,
+				RelPath: relPath,
+				Info:    info,
+			}
+			n++
+
+		case err, ok := <-f.watcher.Errors:
+			if !ok {
+				return n, fmt.Errorf("failed to read from watcher: %w", err)
+			}
+			f.log.Printf("error: %s", err)
+		}
+	}
+
+	return n, err
+}
+
+// Close waits for all watcher processing to complete.
+func (f *WatchReader) Close() error {
+	if err := f.watcher.Close(); err != nil {
+		return fmt.Errorf("failed to close watcher: %w", err)
+	}
+	if err := f.eg.Wait(); err != nil {
+		return fmt.Errorf("failed to wait for processing to complete: %w", err)
+	}
+	return nil
+}
+
+func NewWatchReader(
+	root string,
+	path string,
+	statz *stats.Stats,
+) (*WatchReader, error) {
+	// create an error group for managing the processing loop
+	eg := errgroup.Group{}
+
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		log.Fatalf("failed to create watcher: %v", err)
+	}
+
+	r := WatchReader{
+		root:    root,
+		path:    path,
+		log:     log.Default(),
+		stats:   statz,
+		eg:      &eg,
+		watcher: watcher,
+	}
+
+	// f.path is relative to the root, so we create a fully qualified version
+	// we also clean the path up in case there are any ../../ components etc.
+	fqPath := filepath.Clean(filepath.Join(root, path))
+
+	// ensure the path is within the root
+	if !strings.HasPrefix(fqPath, root) {
+		return nil, fmt.Errorf("path '%s' is outside of the root '%s'", fqPath, root)
+	}
+
+	// start watching the path
+	if err := watcher.Add(fqPath); err != nil {
+		return nil, fmt.Errorf("failed to watch path %s: %w", fqPath, err)
+	}
+
+	return &r, nil
+}