kubeadm: make sure we only track >= 3 etcd versions at at time

There is no point to track more than 3 etcd versions at a time
where each etcd versions maps to a k8s CP version.

It's 3 instead of 2 (k8s CP / kubeadm version skew size) because
there is a period of time where the 3rd version (newest) will
be WIP at k/k master - e.g. at the time of this commit it's 1.31.

Add a unit test to block on this.

Also fixate the min etcd version to 3.5.11.

cmd/kubeadm/app/constants/constants.go

@@ -317,7 +317,7 @@ const (
 	KubeletHealthzPort = 10248
 
 	// MinExternalEtcdVersion indicates minimum external etcd version which kubeadm supports
-	MinExternalEtcdVersion = "3.4.13-4"
+	MinExternalEtcdVersion = "3.5.11-0"
 
 	// DefaultEtcdVersion indicates the default etcd version that kubeadm uses
 	DefaultEtcdVersion = "3.5.13-0"
@@ -479,13 +479,6 @@ var (
 
 	// SupportedEtcdVersion lists officially supported etcd versions with corresponding Kubernetes releases
 	SupportedEtcdVersion = map[uint8]string{
-		22: "3.5.13-0",
-		23: "3.5.13-0",
-		24: "3.5.13-0",
-		25: "3.5.13-0",
-		26: "3.5.13-0",
-		27: "3.5.13-0",
-		28: "3.5.13-0",
 		29: "3.5.13-0",
 		30: "3.5.13-0",
 		31: "3.5.13-0",

cmd/kubeadm/app/constants/constants_test.go

@@ -124,11 +124,15 @@ func TestGetStaticPodFilepath(t *testing.T) {
 	}
 }
 
+func TestEtcdSupportedVersionLength(t *testing.T) {
+	const max = 3
+	if len(SupportedEtcdVersion) > max {
+		t.Fatalf("SupportedEtcdVersion must not include more than %d versions", max)
+	}
+}
+
 func TestEtcdSupportedVersion(t *testing.T) {
 	var supportedEtcdVersion = map[uint8]string{
-		13: "3.2.24",
-		14: "3.3.10",
-		15: "3.3.10",
 		16: "3.3.17-0",
 		17: "3.4.3-0",
 		18: "3.4.3-0",
@@ -147,7 +151,7 @@ func TestEtcdSupportedVersion(t *testing.T) {
 		},
 		{
 			kubernetesVersion: "1.10.1",
-			expectedVersion:   version.MustParseSemantic("3.2.24"),
+			expectedVersion:   version.MustParseSemantic("3.3.17-0"),
 			expectedWarning:   true,
 			expectedError:     false,
 		},