improve config file watcher to support k8s configmap reloads

The config file watcher assumed a simple configuration where the
existing configuration file is overwritten by a text editor.

In practice this does not detect some more complex configuration change
scenarios such as :
* The configuration file is behind a symlink and the symlink is updated
* The configuration file is behind a double symlink (k8s configmap
  update)
* The configuration file is not updated but removed the re-created

In order to fix these cases :
* Watch the configuration file parent directory instead of the file
itself, this lets us grab events event if the file was removed and a new
file is used.
* In addition to read/write event matching, watch for any change in
the configuration file path. This handles the symlink case where the
file itself hasn't changed but its location did

v2:
- apply gofmt

internal/confwatcher/confwatcher.go

@@ -15,6 +15,7 @@ const (
 // ConfWatcher is a configuration file watcher.
 type ConfWatcher struct {
 	inner       *fsnotify.Watcher
+	watchedPath string
 
 	// out
 	signal chan struct{}
@@ -30,8 +31,9 @@ func New(confPath string) (*ConfWatcher, error) {
 
 	// use absolute path to support Darwin
 	absolutePath, _ := filepath.Abs(confPath)
+	parentPath := filepath.Dir(absolutePath)
 
-	err = inner.Add(absolutePath)
+	err = inner.Add(parentPath)
 	if err != nil {
 		inner.Close()
 		return nil, err
@@ -39,6 +41,7 @@ func New(confPath string) (*ConfWatcher, error) {
 
 	w := &ConfWatcher{
 		inner:       inner,
+		watchedPath: absolutePath,
 		signal:      make(chan struct{}),
 		done:        make(chan struct{}),
 	}
@@ -61,6 +64,7 @@ func (w *ConfWatcher) run() {
 	defer close(w.done)
 
 	var lastCalled time.Time
+	previousWatchedPath, _ := filepath.EvalSymlinks(w.watchedPath)
 
 outer:
 	for {
@@ -70,10 +74,19 @@ outer:
 				continue
 			}
 
-			if (event.Op&fsnotify.Write) == fsnotify.Write ||
-				(event.Op&fsnotify.Create) == fsnotify.Create {
+			currentWatchedPath, _ := filepath.EvalSymlinks(w.watchedPath)
+			eventPath, _ := filepath.Abs(event.Name)
+
+			if currentWatchedPath == "" {
+				// Watched file was removed wait for write event to trigger reload
+				previousWatchedPath = ""
+			} else if currentWatchedPath != previousWatchedPath ||
+				(eventPath == currentWatchedPath &&
+					((event.Op&fsnotify.Write) == fsnotify.Write ||
+						(event.Op&fsnotify.Create) == fsnotify.Create)) {
 				// wait some additional time to allow the writer to complete its job
 				time.Sleep(additionalWait)
+				previousWatchedPath = currentWatchedPath
 
 				lastCalled = time.Now()
 				w.signal <- struct{}{}