From 57618be9325e81e75b9ba12881d3ab991a944a64 Mon Sep 17 00:00:00 2001
From: Tom Wilkie <tom.wilkie@gmail.com>
Date: Tue, 3 May 2016 17:05:44 +0100
Subject: [PATCH] Add a condition to rendering any kubernetes nodes at all

---
 render/pod.go    | 16 ++++++++++------
 render/render.go | 27 +++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/render/pod.go b/render/pod.go
index 7f7c540ab..f93741b17 100644
--- a/render/pod.go
+++ b/render/pod.go
@@ -14,10 +14,14 @@ const (
 	UnmanagedMajor = "Unmanaged"
 )
 
+func renderKubernetesTopologies(rpt report.Report) bool {
+	return len(rpt.Pod.Nodes)+len(rpt.Service.Nodes) > 1
+}
+
 // PodRenderer is a Renderer which produces a renderable kubernetes
 // graph by merging the container graph and the pods topology.
-var PodRenderer = ApplyDecorators(
-	MakeFilter(
+var PodRenderer = ConditionalRenderer(renderKubernetesTopologies,
+	ApplyDecorators(MakeFilter(
 		func(n report.Node) bool {
 			state, ok := n.Latest.Lookup(kubernetes.PodState)
 			return (!ok || state != kubernetes.StateDeleted)
@@ -36,13 +40,13 @@ var PodRenderer = ApplyDecorators(
 			),
 			SelectPod,
 		),
-	),
+	)),
 )
 
 // PodServiceRenderer is a Renderer which produces a renderable kubernetes services
 // graph by merging the pods graph and the services topology.
-var PodServiceRenderer = ApplyDecorators(
-	FilterEmpty(report.Pod,
+var PodServiceRenderer = ConditionalRenderer(renderKubernetesTopologies,
+	ApplyDecorators(FilterEmpty(report.Pod,
 		MakeReduce(
 			MakeMap(
 				MapPod2Service,
@@ -50,7 +54,7 @@ var PodServiceRenderer = ApplyDecorators(
 			),
 			SelectService,
 		),
-	),
+	)),
 )
 
 // MapContainer2Pod maps container Nodes to pod
diff --git a/render/render.go b/render/render.go
index 9ef5ffadb..9a9078a3e 100644
--- a/render/render.go
+++ b/render/render.go
@@ -158,3 +158,30 @@ func propagateLatest(key string, from, to report.Node) report.Node {
 	}
 	return to
 }
+
+// Condition is a predecate over the entire report that can evaluate to true or false.
+type Condition func(report.Report) bool
+
+type conditionalRenderer struct {
+	Condition
+	Renderer
+}
+
+// ConditionalRenderer renders nothing if the condition is false, otherwise it defers
+// to the wrapped Renderer.
+func ConditionalRenderer(c Condition, r Renderer) Renderer {
+	return Memoise(conditionalRenderer{c, r})
+}
+
+func (cr conditionalRenderer) Render(rpt report.Report, dct Decorator) report.Nodes {
+	if cr.Condition(rpt) {
+		return cr.Renderer.Render(rpt, dct)
+	}
+	return report.Nodes{}
+}
+func (cr conditionalRenderer) Stats(rpt report.Report, dct Decorator) Stats {
+	if cr.Condition(rpt) {
+		return cr.Renderer.Stats(rpt, dct)
+	}
+	return Stats{}
+}