improves error handling for versions without v prefix

cmd/minikube/cmd/start.go

@@ -390,6 +390,11 @@ func validateKubernetesVersions(old *cfg.Config) string {
 	if nv == "" {
 		nv = constants.DefaultKubernetesVersion
 	}
+
+	if nv[0] != version.VersionPrefix {
+		exit.WithCode(exit.Data, "the --kubernetes-version flag requires the version to start with a 'v'")
+	}
+
 	nvs, err := semver.Make(strings.TrimPrefix(nv, version.VersionPrefix))
 	if err != nil {
 		exit.WithCode(exit.Data, "Unable to parse %q: %v", nv, err)

pkg/minikube/bootstrapper/kubeadm/versions.go

@@ -25,6 +25,7 @@ import (
 
 	"github.com/blang/semver"
 	"github.com/golang/glog"
+	"github.com/kubernetes/minikube/pkg/version"
 	"github.com/pkg/errors"
 	"k8s.io/kubernetes/cmd/kubeadm/app/features"
 	"k8s.io/minikube/pkg/minikube/constants"
@@ -152,9 +153,13 @@ func Supports(featureName string) bool {
 	return false
 }
 
-func ParseKubernetesVersion(version string) (semver.Version, error) {
-	// Strip leading 'v' prefix from version for semver parsing
-	v, err := semver.Make(version[1:])
+// ParseKubernetesVersion validates the kubernetes version as legitimate
+func ParseKubernetesVersion(input string) (semver.Version, error) {
+	if input[0] != version.VersionPrefix {
+		return semver.Version{}, fmt.Errorf("version numbers must begin with %v", version.VersionPrefix)
+	}
+
+	v, err := semver.Make(strings.TrimPrefix(input, version.VersionPrefix))
 	if err != nil {
 		return semver.Version{}, errors.Wrap(err, "parsing kubernetes version")
 	}

pkg/minikube/bootstrapper/kubeadm/versions_test.go

@@ -94,12 +94,45 @@ func TestVersionIsBetween(t *testing.T) {
 }
 
 func TestParseKubernetesVersion(t *testing.T) {
-	version, err := ParseKubernetesVersion("v1.8.0-alpha.5")
-	if err != nil {
-		t.Fatalf("Error parsing version: %v", err)
+	tests := []struct {
+		description string
+		version     string
+		comparison  string
+		expected    boolean
+	}{
+		{
+			description: "regular 1.x.x version",
+			version:     "1.8.0",
+			comparison:  "1.8.0",
+			expected:    true,
+		},
+		{
+			description: "alpha version",
+			version:     "v1.8.0-alpha.5",
+			comparison:  "1.8.0-alpha.5",
+			expected:    true,
+		},
+		{
+			description: "missing `v` prefix: considered invalid",
+			version:     "1.8.0",
+			comparison:  "1.8.0",
+			expected:    false,
+		},
 	}
-	if version.NE(semver.MustParse("1.8.0-alpha.5")) {
-		t.Errorf("Expected: %s, Actual:%s", "1.8.0-alpha.5", version)
+
+	for _, test := range tests {
+		t.Run(test.description, func(t *testing.T) {
+			version, err := ParseKubernetesVersion(test.input)
+			if err != nil {
+				t.Fatalf("Error parsing version: %v", err)
+			}
+
+			comparison := semver.MustParse(test.comparison)
+
+			if version.NE(comparison) == test.expected {
+				t.Errorf("Expected: %s, Actual: %s", test.expected, version)
+			}
+		})
 	}
 }