refactor: make PropagateSingleMetrics a renderer

instead of a Map, since it's not really a map as it just updates the
given node.

This is more efficient and also matches what we do in similar
situations elsewhere, e.g. in ContainerWithImageNameRenderer and
ProcessWithContainerNameRenderer.

render/metrics.go

@@ -4,13 +4,26 @@ import (
 	"github.com/weaveworks/scope/report"
 )
 
-// PropagateSingleMetrics puts metrics from one of the children onto the parent
-// iff there is only one child of that type.
-func PropagateSingleMetrics(topology string) MapFunc {
-	return func(n report.Node) report.Nodes {
+// PropagateSingleMetrics creates a renderer which propagates metrics
+// from a node's child to the node. The child is selected based on the
+// specified topology. No metrics are propagated when there is more
+// than one such child.
+func PropagateSingleMetrics(topology string, r Renderer) Renderer {
+	return propagateSingleMetrics{topology: topology, r: r}
+}
+
+type propagateSingleMetrics struct {
+	topology string
+	r        Renderer
+}
+
+func (p propagateSingleMetrics) Render(rpt report.Report) Nodes {
+	nodes := p.r.Render(rpt)
+	outputs := make(report.Nodes, len(nodes.Nodes))
+	for id, n := range nodes.Nodes {
 		var found []report.Node
 		n.Children.ForEach(func(child report.Node) {
-			if child.Topology == topology {
+			if child.Topology == p.topology {
 				if _, ok := child.Latest.Lookup(report.DoesNotMakeConnections); !ok {
 					found = append(found, child)
 				}
@@ -19,6 +32,7 @@ func PropagateSingleMetrics(topology string) MapFunc {
 		if len(found) == 1 {
 			n = n.WithMetrics(found[0].Metrics)
 		}
-		return report.Nodes{n.ID: n}
+		outputs[id] = n
 	}
+	return Nodes{Nodes: outputs, Filtered: nodes.Filtered}
 }

render/metrics_test.go

@@ -159,7 +159,7 @@ func TestPropagateSingleMetrics(t *testing.T) {
 			},
 		},
 	} {
-		got := render.PropagateSingleMetrics(c.topology)(c.input)
+		got := render.PropagateSingleMetrics(c.topology, mockRenderer{report.Nodes{c.input.ID: c.input}}).Render(report.Report{}).Nodes
 		if !reflect.DeepEqual(got, c.output) {
 			t.Errorf("[%s] Diff: %s", c.name, test.Diff(c.output, got))
 		}

render/pod.go

@@ -49,8 +49,7 @@ var PodRenderer = Memoise(ConditionalRenderer(renderKubernetesTopologies,
 			return (!ok || state != kubernetes.StateDeleted)
 		},
 		MakeReduce(
-			MakeMap(
-				PropagateSingleMetrics(report.Container),
+			PropagateSingleMetrics(report.Container,
 				MakeMap(
 					Map2Parent([]string{report.Pod}, UnmanagedID),
 					MakeFilter(
@@ -101,8 +100,7 @@ func renderParents(childTopology string, parentTopologies []string, noParentsPse
 	}
 	return MakeReduce(append(
 		selectors,
-		MakeMap(
-			PropagateSingleMetrics(childTopology),
+		PropagateSingleMetrics(childTopology,
 			MakeMap(
 				Map2Parent(parentTopologies, noParentsPseudoID),
 				childRenderer,