untangle plugin runner a bit

add some docs and make it clearer what is actually going on
(parallel rule execution on start and then on timer)

pkg/custompluginmonitor/plugin/plugin.go

@@ -61,13 +61,32 @@ func (p *Plugin) Run() {
 	runTicker := time.NewTicker(*p.config.PluginGlobalConfig.InvokeInterval)
 	defer runTicker.Stop()
 
-	runner := func() {
+	// on boot run once
+	select {
+	case <-p.tomb.Stopping():
+		return
+	default:
+		p.runRules()
+	}
+
+	// run every InvokeInterval
+	for {
+		select {
+		case <-runTicker.C:
+			p.runRules()
+		case <-p.tomb.Stopping():
+			return
+		}
+	}
+}
+
+// run each rule in parallel and wait for them to complete
+func (p *Plugin) runRules() {
 	glog.Info("Start to run custom plugins")
 
 	for _, rule := range p.config.Rules {
 		p.syncChan <- struct{}{}
 		p.Add(1)
-
 		go func(rule *cpmtypes.CustomRule) {
 			defer p.Done()
 			defer func() {
@@ -96,23 +115,6 @@ func (p *Plugin) Run() {
 	glog.Info("Finish running custom plugins")
 }
 
-	select {
-	case <-p.tomb.Stopping():
-		return
-	default:
-		runner()
-	}
-
-	for {
-		select {
-		case <-runTicker.C:
-			runner()
-		case <-p.tomb.Stopping():
-			return
-		}
-	}
-}
-
 func (p *Plugin) run(rule cpmtypes.CustomRule) (exitStatus cpmtypes.Status, output string) {
 	var ctx context.Context
 	var cancel context.CancelFunc