Add method to add a single string into Sets

This avoids creating and discarding a StringSet just to pass the parameter

probe/awsecs/reporter.go

@@ -182,10 +182,10 @@ func (r Reporter) Tag(rpt report.Report) (report.Report, error) {
 
 			// parents sets to merge into all matching container nodes
 			parentsSets := report.MakeSets()
-			parentsSets = parentsSets.Add(report.ECSTask, report.MakeStringSet(taskID))
+			parentsSets = parentsSets.AddString(report.ECSTask, taskID)
 			if serviceName, ok := ecsInfo.TaskServiceMap[taskArn]; ok {
 				serviceID := report.MakeECSServiceNodeID(cluster, serviceName)
-				parentsSets = parentsSets.Add(report.ECSService, report.MakeStringSet(serviceID))
+				parentsSets = parentsSets.AddString(report.ECSService, serviceID)
 				// in addition, make service parent of task
 				rpt.ECSTask.Nodes[taskID] = rpt.ECSTask.Nodes[taskID].WithParents(report.MakeSets().Add(report.ECSService, report.MakeStringSet(serviceID)))
 			}

probe/kubernetes/pod.go

@@ -56,7 +56,7 @@ func (p *pod) UID() string {
 }
 
 func (p *pod) AddParent(topology, id string) {
-	p.parents = p.parents.Add(topology, report.MakeStringSet(id))
+	p.parents = p.parents.AddString(topology, id)
 }
 
 func (p *pod) State() string {

probe/kubernetes/reporter.go

@@ -357,7 +357,7 @@ func (r *Reporter) hostTopology(services []Service) report.Topology {
 	t := report.MakeTopology()
 	t.AddNode(
 		report.MakeNode(report.MakeHostNodeID(r.hostID)).
-			WithSets(report.MakeSets().Add(host.LocalNetworks, report.MakeStringSet(serviceNetwork.String()))))
+			WithSets(report.MakeSets().AddString(host.LocalNetworks, serviceNetwork.String())))
 	return t
 }
 

render/container.go

@@ -150,7 +150,7 @@ func (r containerWithImageNameRenderer) Render(rpt report.Report) Nodes {
 		c = propagateLatest(docker.ImageLabelPrefix+"works.weave.role", image, c)
 		c.Parents = c.Parents.
 			Delete(report.ContainerImage).
-			Add(report.ContainerImage, report.MakeStringSet(imageNodeID))
+			AddString(report.ContainerImage, imageNodeID)
 		outputs[id] = c
 	}
 	return Nodes{Nodes: outputs, Filtered: containers.Filtered}

report/sets.go

@@ -46,6 +46,21 @@ func (s Sets) Add(key string, value StringSet) Sets {
 	}
 }
 
+// AddString adds a single string under a key, creating a new StringSet if necessary.
+func (s Sets) AddString(key string, str string) Sets {
+	if s.psMap == nil {
+		s = emptySets
+	}
+	value, found := s.Lookup(key)
+	if found && value.Contains(str) {
+		return s
+	}
+	value = value.Add(str)
+	return Sets{
+		psMap: s.psMap.Set(key, value),
+	}
+}
+
 // Delete the given set from the Sets.
 func (s Sets) Delete(key string) Sets {
 	if s.psMap == nil {