coordinator: add watch function to store interface

Author: davidweisse

coordinator/history/aferostore.go

@@ -59,3 +59,21 @@ func (s *AferoStore) CompareAndSwap(key string, oldVal, newVal []byte) error {
 	}
 	return s.fs.WriteFile(key, newVal, 0o644)
 }
+
+// Watch watches for changes to the value of key.
+func (s *AferoStore) Watch(_ string) (Watcher, error) {
+	return &AferoStoreWatcher{}, nil
+}
+
+// AferoStoreWatcher implements the Watcher interface for AferoStore.
+type AferoStoreWatcher struct{}
+
+// Stop is a no-op for AferoStoreWatcher.
+func (w *AferoStoreWatcher) Stop() {
+	// No-op.
+}
+
+// ResultChan returns a nil channel.
+func (w *AferoStoreWatcher) ResultChan() <-chan []byte {
+	return nil
+}

coordinator/history/history.go

@@ -251,4 +251,18 @@ type Store interface {
 	// If the current value is not equal to oldVal, an error must be returned. The comparison must
 	// treat a nil slice the same as an empty slice.
 	CompareAndSwap(key string, oldVal, newVal []byte) error
+
+	// Watch watches for changes to the value of key.
+	//
+	// If the value of key changes, the new value is sent on the channel.
+	Watch(key string) (Watcher, error)
+}
+
+// Watcher watches and reports changes to a key in a Store.
+type Watcher interface {
+	// Stop watching the key and close the result channel.
+	Stop()
+
+	// ResultChan returns a channel which will receive the new value of the key when it changes.
+	ResultChan() <-chan []byte
 }