Add template function returning the latest image

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Co-authored-by: Peter Rifel <rifelpet@users.noreply.github.com>

docs/operations/cluster_template.md

@@ -16,8 +16,8 @@ metadata:
   kops.k8s.io/cluster: {{ '{{.clusterName}}.{{.dnsZone}}' }}
   name: nodes
 spec:
-  image: coreos.com/CoreOS-stable-1409.6.0-hvm
-  kubernetesVersion: {{ '{{.kubernetesVersion}}' }}
+  image: {{ ChannelRecommendedImage .cloud .kubernetesVersion }}
+  kubernetesVersion: {{ '{{ ChannelRecommendedKubernetesUpgradeVersion .kubernetesVersion }}' }}
   machineType: m4.large
   maxPrice: "0.5"
   maxSize: 20
@@ -127,9 +127,13 @@ spec:
 
 This function returns the kubernetes version recommended for the running kops version.
 
-##### ChannelRecommendedKopsUpgradeVersion <kuberneteVersion>
+##### ChannelRecommendedKubernetesUpgradeVersion <kuberneteVersion>
 
-This function returns the kubernetes upgrade recommendation given that you run `<kubernetesVersion>`. Typically this is the latest minor version supported by the given channel.
+This function returns the recommended kubernetes version given that you currently run `<kubernetesVersion>`. Typically this is the latest patch version supported by the given channel.
+
+##### ChannelRecommendedImage <cloudProvider> <kuberneteVersion>
+
+This function returns the recommended image for the given cloud provider and kubernetes version.
 
 #### Sprig functions
 

pkg/util/templater/BUILD.bazel

@@ -13,6 +13,7 @@ go_library(
         "//pkg/apis/kops:go_default_library",
         "//pkg/apis/kops/util:go_default_library",
         "//vendor/github.com/Masterminds/sprig/v3:go_default_library",
+        "//vendor/github.com/blang/semver/v4:go_default_library",
     ],
 )
 

pkg/util/templater/template_functions.go

@@ -20,6 +20,7 @@ import (
 	"text/template"
 
 	"github.com/Masterminds/sprig/v3"
+	"github.com/blang/semver/v4"
 	"k8s.io/kops"
 	kopsapi "k8s.io/kops/pkg/apis/kops"
 	"k8s.io/kops/pkg/apis/kops/util"
@@ -62,5 +63,11 @@ func (r *Templater) templateFuncsMap(tm *template.Template) template.FuncMap {
 
 	}
 
+	funcs["ChannelRecommendedImage"] = func(cloud, k8sVersion string) string {
+		ver, _ := semver.ParseTolerant(k8sVersion)
+		imageSpec := r.channel.FindImage(kopsapi.CloudProviderID(cloud), ver)
+		return imageSpec.Name
+	}
+
 	return funcs
 }

pkg/util/templater/templater_test.go

@@ -95,6 +95,11 @@ func TestRenderChannelFunctions(t *testing.T) {
 			Template: `{{ ChannelRecommendedKubernetesUpgradeVersion "1.4.2" }}`,
 			Expected: "1.4.8",
 		},
+		{
+			Context:  map[string]interface{}{},
+			Template: `{{ ChannelRecommendedImage "aws" "1.4.2" }}`,
+			Expected: "kope.io/k8s-1.4-debian-jessie-amd64-hvm-ebs-2016-10-21",
+		},
 	}
 	makeRenderTests(t, cases)
 }