resources: Improve notification system and notify refreshes

Resources can send "refresh" notifications along edges. These messages
are sent whenever the upstream (initiating vertex) changes state. When
the changed state propagates downstream, it will be paired with a
refresh flag which can be queried in the CheckApply method of that
resource.

Future work will include a stateful refresh tracking mechanism so that
if a refresh event is generated and not consumed, it will be saved
across an interrupt (shutdown) or a crash so that it can be re-applied
on the subsequent run. This is important because the unapplied refresh
is a form of hysteresis which needs to be tracked and remembered or we
won't be able to determine that the state is wrong!

Still to do:
* Update the autogrouping code to handle the edge notify properties!
* Actually finish the stateful bool code

resources/timer.go

@@ -33,6 +33,8 @@ func init() {
 type TimerRes struct {
 	BaseRes  `yaml:",inline"`
 	Interval int `yaml:"interval"` // Interval : Interval between runs
+
+	ticker *time.Ticker
 }
 
 // TimerUID is the UID struct for TimerRes.
@@ -65,6 +67,11 @@ func (obj *TimerRes) Validate() error {
 	return nil
 }
 
+// newTicker creates a new ticker
+func (obj *TimerRes) newTicker() *time.Ticker {
+	return time.NewTicker(time.Duration(obj.Interval) * time.Second)
+}
+
 // Watch is the primary listener for this resource and it outputs events.
 func (obj *TimerRes) Watch(processChan chan event.Event) error {
 	if obj.IsWatching() {
@@ -84,16 +91,16 @@ func (obj *TimerRes) Watch(processChan chan event.Event) error {
 		return time.After(time.Duration(500) * time.Millisecond) // 1/2 the resolution of converged timeout
 	}
 
-	// Create a time.Ticker for the given interval
-	ticker := time.NewTicker(time.Duration(obj.Interval) * time.Second)
-	defer ticker.Stop()
+	// create a time.Ticker for the given interval
+	obj.ticker = obj.newTicker()
+	defer obj.ticker.Stop()
 
 	var send = false
 
 	for {
 		obj.SetState(ResStateWatching)
 		select {
-		case <-ticker.C: // received the timer event
+		case <-obj.ticker.C: // received the timer event
 			send = true
 			log.Printf("%s[%s]: received tick", obj.Kind(), obj.GetName())
 
@@ -121,6 +128,22 @@ func (obj *TimerRes) Watch(processChan chan event.Event) error {
 	}
 }
 
+// CheckApply method for Timer resource. Triggers a timer reset on notify.
+func (obj *TimerRes) CheckApply(apply bool) (bool, error) {
+	// because there are no checks to run, this resource has a less
+	// traditional pattern than what is seen in most resources...
+	if !obj.Refresh() { // this works for apply || !apply
+		return true, nil // state is always okay if no refresh to do
+	} else if !apply { // we had a refresh to do
+		return false, nil // therefore state is wrong
+	}
+
+	// reset the timer since apply && refresh
+	obj.ticker.Stop()
+	obj.ticker = obj.newTicker()
+	return false, nil
+}
+
 // GetUIDs includes all params to make a unique identification of this object.
 // Most resources only return one, although some resources can return multiple.
 func (obj *TimerRes) GetUIDs() []ResUID {
@@ -158,8 +181,3 @@ func (obj *TimerRes) Compare(res Res) bool {
 	}
 	return true
 }
-
-// CheckApply method for Timer resource. Does nothing, returns happy!
-func (obj *TimerRes) CheckApply(apply bool) (bool, error) {
-	return true, nil // state is always okay
-}