export fswatcher to a separate library

vendor/gitlab.com/etke.cc/go/fswatcher/LICENSE

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+                   GNU LESSER GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
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vendor/gitlab.com/etke.cc/go/fswatcher/README.md

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+# fswatcher
+
+A handy wrapper around [fsnotify](https://github.com/fsnotify/fsnotify) with deduplication
+
+```go
+watcher := fswatcher.New([]string{"/tmp/your-file.txt"}, 0)
+defer watcher.Stop()
+go watcher.Start(func(e fsnotify.Event){
+	// do something with event
+})
+```

vendor/gitlab.com/etke.cc/go/fswatcher/fswatcher.go

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+package fswatcher
+
+import (
+	"math"
+	"path/filepath"
+	"sync"
+	"time"
+
+	"github.com/fsnotify/fsnotify"
+)
+
+// DefaultDelay to avoid unnecessary handler calls
+const DefaultDelay = 100 * time.Millisecond
+
+// Watcher of file system changes
+type Watcher struct {
+	watcher *fsnotify.Watcher
+	delay   time.Duration
+	files   []string
+	mu      sync.Mutex
+	t       map[string]*time.Timer
+}
+
+// Creates FS Watcher
+func New(files []string, delay time.Duration) (*Watcher, error) {
+	if delay <= 0 {
+		delay = DefaultDelay
+	}
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		return nil, err
+	}
+
+	for _, file := range files {
+		err = watcher.Add(filepath.Dir(file))
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	fswatcher := &Watcher{
+		watcher: watcher,
+		delay:   delay,
+		files:   files,
+		t:       make(map[string]*time.Timer),
+	}
+
+	return fswatcher, nil
+}
+
+func (w *Watcher) watch(handler func(e fsnotify.Event)) {
+	for e := range w.watcher.Events {
+		handler(e)
+	}
+}
+
+// Start watcher
+func (w *Watcher) Start(handler func(e fsnotify.Event)) {
+	w.watch(func(e fsnotify.Event) {
+		var found bool
+		for _, f := range w.files {
+			if f == e.Name {
+				found = true
+			}
+		}
+		if !found {
+			return
+		}
+		w.mu.Lock()
+		t, ok := w.t[e.Name]
+		w.mu.Unlock()
+		if !ok {
+			t = time.AfterFunc(math.MaxInt64, func() {
+				handler(e)
+			})
+			t.Stop()
+
+			w.mu.Lock()
+			w.t[e.Name] = t
+			w.mu.Unlock()
+		}
+		t.Reset(w.delay)
+	})
+}
+
+// Stop watcher
+func (w *Watcher) Stop() error {
+	return w.watcher.Close()
+}