Implement activation service

Signed-off-by: Daniel Weiße <dw@edgeless.systems>

activation/watcher/watcher.go

@@ -0,0 +1,108 @@
+package watcher
+
+import (
+	"fmt"
+
+	"github.com/fsnotify/fsnotify"
+	"k8s.io/klog/v2"
+)
+
+// FileWatcher watches for changes to the file and calls the waiter's Update method.
+type FileWatcher struct {
+	updater updater
+	watcher eventWatcher
+	done    chan struct{}
+}
+
+// New creates a new FileWatcher for the given validator.
+func New(updater updater) (*FileWatcher, error) {
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		return nil, err
+	}
+
+	return &FileWatcher{
+		watcher: &fsnotifyWatcher{watcher},
+		updater: updater,
+		done:    make(chan struct{}, 1),
+	}, nil
+}
+
+// Close closes the watcher.
+// It should only be called once.
+func (f *FileWatcher) Close() error {
+	err := f.watcher.Close()
+	<-f.done
+	return err
+}
+
+// Watch starts watching the file at the given path.
+// It will call the watcher's Update method when the file is modified.
+func (f *FileWatcher) Watch(file string) error {
+	defer func() { f.done <- struct{}{} }()
+	if err := f.watcher.Add(file); err != nil {
+		return err
+	}
+
+	for {
+		select {
+		case event, ok := <-f.watcher.Events():
+			if !ok {
+				klog.V(4).Infof("watcher closed")
+				return nil
+			}
+
+			// file changes may be indicated by either a WRITE, CHMOD, CREATE or RENAME event
+			if event.Op&(fsnotify.Write|fsnotify.Chmod|fsnotify.Create|fsnotify.Rename) != 0 {
+				if err := f.updater.Update(); err != nil {
+					klog.Errorf("failed to update activation validator: %s", err)
+				}
+			}
+
+			// if a file gets removed, e.g. by a rename event, we need to re-add the file to the watcher
+			if event.Op&fsnotify.Remove == fsnotify.Remove {
+				if err := f.watcher.Add(event.Name); err != nil {
+					klog.Errorf("failed to re-add file %q to watcher: %s", event.Name, err)
+					return fmt.Errorf("failed to re-add file %q to watcher: %w", event.Name, err)
+				}
+			}
+
+		case err := <-f.watcher.Errors():
+			if err != nil {
+				klog.Errorf("watching for measurements updates: %s", err)
+				return fmt.Errorf("watching for measurements updates: %w", err)
+			}
+		}
+	}
+}
+
+type updater interface {
+	Update() error
+}
+
+type eventWatcher interface {
+	Add(string) error
+	Close() error
+	Events() <-chan fsnotify.Event
+	Errors() <-chan error
+}
+
+type fsnotifyWatcher struct {
+	watcher *fsnotify.Watcher
+}
+
+func (w *fsnotifyWatcher) Add(file string) error {
+	return w.watcher.Add(file)
+}
+
+func (w *fsnotifyWatcher) Close() error {
+	return w.watcher.Close()
+}
+
+func (w *fsnotifyWatcher) Events() <-chan fsnotify.Event {
+	return w.watcher.Events
+}
+
+func (w *fsnotifyWatcher) Errors() <-chan error {
+	return w.watcher.Errors
+}