add new kubectl_version param

- enables running the plugin using a non-default `kubectl` executable
- updates image `ENTRYPOINT` to support setting of `EXTRA_KUBECTL_VERSIONS` environment variable used by plugin to validate against `kubectl_version`

depends on #114

DOCS.md

@@ -461,6 +461,43 @@ To use `$${IMAGE_VERSION}` or `$IMAGE_VERSION`, see the [Drone docs][environment
 [expand]: https://golang.org/pkg/os/#ExpandEnv
 [environment]: http://docs.drone.io/environment/
 
+## Using "extra" `kubectl` versions
+
+### tl;dr
+
+To run `drone-gke` using a different version of `kubectl` than the default, set `kubectl-version` to the version you'd like to use.
+
+For example, to use the **1.14** version of `kubectl`:
+
+```yml
+image: nytimes/drone-gke:0.9.1
+kubectl_version: "1.14"
+```
+
+This will configure the plugin to execute `/google-cloud-sdk/bin/kubectl.1.14` instead of `/google-cloud-sdk/bin/kubectl` for all `kubectl` commands.
+
+### Background
+
+Beginning with the [`237.0.0 (2019-03-05)` release of the `gcloud` SDK](https://cloud.google.com/sdk/docs/release-notes#23700_2019-03-05), "extra" `kubectl` versions are installed automatically when `kubectl` is installed via `gcloud components install kubectl`.
+
+These "extra" versions are installed alongside the SDK's default `kubectl` version at `/google-cloud-sdk/bin` and are named using the following pattern:
+
+```sh
+kubectl.$clientVersionMajor.$clientVersionMinor
+```
+
+To list all of the "extra" `kubectl` versions available within a particular version of `drone-gke`, you can run the following:
+
+```sh
+# list "extra" kubectl versions available with nytimes/drone-gke:0.9.1
+docker run \
+  --rm \
+  --interactive \
+  --tty \
+  --entrypoint '' \
+  nytimes/drone-gke:0.9.1 list-extra-kubectl-versions
+```
+
 ## Example reference usage
 
 ### `.drone.yml`

Dockerfile

@@ -10,6 +10,6 @@ RUN \
   gcloud --no-user-output-enabled components install kubectl && \
     rm -rf /google-cloud-sdk/.install
 
-ADD drone-gke /usr/local/bin/
+ADD drone-gke bin/entrypoint bin/list-extra-kubectl-versions /usr/local/bin/
 
-ENTRYPOINT ["drone-gke"]
+ENTRYPOINT ["entrypoint", "drone-gke"]

bin/entrypoint

@@ -0,0 +1,6 @@
+#!/usr/bin/env sh
+set -e
+
+export EXTRA_KUBECTL_VERSIONS=$(echo $(list-extra-kubectl-versions))
+
+exec "$@"

bin/list-extra-kubectl-versions

@@ -0,0 +1,14 @@
+#!/usr/bin/env sh
+set -e
+
+# location to search for extra kubectl version
+# defaults to default used by `gcloud components install kubectl`
+gcloud_sdk_bin_dir=${1:-'/google-cloud-sdk/bin'}
+
+# filename glob for extra kubectl versions
+# defaults to default used by `gcloud components install kubectl`
+extra_kubectl_glob=${2:-'kubectl.*'}
+
+find ${gcloud_sdk_bin_dir} -type f -name ${extra_kubectl_glob} -exec basename {} \; \
+  | cut -d '.' -f 2,3 \
+  | sort -g

main.go

@@ -31,15 +31,18 @@ var (
 )
 
 const (
-	gcloudCmd  = "gcloud"
-	kubectlCmd = "kubectl"
-	timeoutCmd = "timeout"
+	gcloudCmd      = "gcloud"
+	kubectlCmdName = "kubectl"
+	timeoutCmd     = "timeout"
 
 	keyPath          = "/tmp/gcloud.json"
 	nsPath           = "/tmp/namespace.json"
 	templateBasePath = "/tmp"
 )
 
+// default to kubectlCmdName, can be overriden via kubectl-version param
+var kubectlCmd = kubectlCmdName
+var extraKubectlVersions = strings.Split(os.Getenv("EXTRA_KUBECTL_VERSIONS"), " ")
 var nsTemplate = `
 ---
 apiVersion: v1
@@ -167,6 +170,12 @@ func wrapMain() error {
 			EnvVar: "PLUGIN_WAIT_SECONDS",
 			Value:  0,
 		},
+		cli.StringFlag{
+			Name:   "kubectl-version",
+			Usage:  "optional - version of kubectl binary to use, e.g. 1.14",
+			EnvVar: "PLUGIN_KUBECTL_VERSION",
+			Value:  "",
+		},
 	}
 
 	if err := app.Run(os.Args); err != nil {
@@ -192,6 +201,12 @@ func run(c *cli.Context) error {
 		}
 	}
 
+	// Use custom kubectl version if provided
+	kubectlVersion := c.String("kubectl-version")
+	if kubectlVersion != "" {
+		kubectlCmd = fmt.Sprintf("%s.%s", kubectlCmdName, kubectlVersion)
+	}
+
 	// Parse variables and secrets
 	vars, err := parseVars(c)
 	if err != nil {
@@ -293,9 +308,53 @@ func checkParams(c *cli.Context) error {
 		return fmt.Errorf("Missing required param: cluster")
 	}
 
+	if !isValidKubectlVersionParam(c, extraKubectlVersions) {
+		return fmt.Errorf(getInvalidKubectlVersionMessage(c, extraKubectlVersions))
+	}
+
 	return nil
 }
 
+// isValidKubectlVersionParam tests whether a given version is valid within the current environment
+func isValidKubectlVersionParam(c *cli.Context, availableVersions []string) bool {
+	kubectlVersionParam := c.String("kubectl-version")
+	// using the default version
+	if kubectlVersionParam == "" {
+		return true
+	}
+
+	// using a custom version but no extra versions are available
+	if len(availableVersions) == 0 {
+		return false
+	}
+
+	// using a custom version ...
+	// return true if included in available extra versions; false otherwise
+	for _, availableVersion := range availableVersions {
+		if kubectlVersionParam == availableVersion {
+			return true
+		}
+	}
+	return false
+}
+
+// getInvalidKubectlVersionMessage returns an error message for invalid kubectl-version values
+func getInvalidKubectlVersionMessage(c *cli.Context, availableVersions []string) string {
+	kubectlVersionParam := c.String("kubectl-version")
+	// kubectl-version is valid; return an empty string
+	if isValidKubectlVersionParam(c, availableVersions) {
+		return ""
+	}
+
+	// using a custom version but no extra versions are available
+	if len(availableVersions) == 0 {
+		return fmt.Sprintf("Invalid param: kubectl-version was set to %s but no extra kubectl versions are available", kubectlVersionParam)
+	}
+
+	// using a custom version and extra versions are available, but specified version is not included
+	return fmt.Sprintf("Invalid param kubectl-version: %s must be one of %s", kubectlVersionParam, strings.Join(availableVersions, ", "))
+}
+
 // getProjectFromToken gets project id from token
 func getProjectFromToken(j string) string {
 	t := token{}

main_test.go

@@ -74,6 +74,76 @@ func TestCheckParams(t *testing.T) {
 	assert.NoError(t, err)
 }
 
+func TestIsValidKubectlVersionParam(t *testing.T) {
+	// kubectl-version is NOT set (using default kubectl version)
+	set := flag.NewFlagSet("default-kubectl-version", 0)
+	c := cli.NewContext(nil, set, nil)
+	availableVersions := []string{}
+	isValid := isValidKubectlVersionParam(c, availableVersions)
+	assert.True(t, isValid, "expected isValidKubectlVersionParam to be true when no kubectl-version param was set")
+
+	// kubectl-version is set and extra kubectl versions are NOT available
+	set = flag.NewFlagSet("kubectl-version-set-no-extra-versions-available", 0)
+	c = cli.NewContext(nil, set, nil)
+	set.String("kubectl-version", "1.14", "")
+	availableVersions = []string{}
+	isValid = isValidKubectlVersionParam(c, availableVersions)
+	assert.False(t, isValid, "expected isValidKubectlVersionParam to be false when no extra kubectl versions are available")
+
+	// kubectl-version is set, extra kubectl versions are available, kubectl-version is included
+	set = flag.NewFlagSet("valid-kubectl-version", 0)
+	c = cli.NewContext(nil, set, nil)
+	set.String("kubectl-version", "1.13", "")
+	availableVersions = []string{"1.11", "1.12", "1.13", "1.14"}
+	isValid = isValidKubectlVersionParam(c, availableVersions)
+	assert.True(t, isValid, "expected isValidKubectlVersionParam to be true when kubectl-version is set, extra kubectl versions are available, kubectl-version is included")
+
+	// kubectl-version is set, extra kubectl versions are available, kubectl-version is NOT included
+	set = flag.NewFlagSet("invalid-kubectl-version", 0)
+	c = cli.NewContext(nil, set, nil)
+	set.String("kubectl-version", "9.99", "")
+	availableVersions = []string{"1.11", "1.12", "1.13", "1.14"}
+	isValid = isValidKubectlVersionParam(c, availableVersions)
+	assert.False(t, isValid, "expected isValidKubectlVersionParam to be false when kubectl-version is set, extra kubectl versions are available, kubectl-version is NOT included")
+}
+
+func TestGetInvalidKubectlVersionMessage(t *testing.T) {
+	// kubectl-version is NOT set (using default kubectl version)
+	set := flag.NewFlagSet("default-kubectl-version", 0)
+	c := cli.NewContext(nil, set, nil)
+	availableVersions := []string{}
+	expected := ""
+	actual := getInvalidKubectlVersionMessage(c, availableVersions)
+	assert.Equal(t, expected, actual, "expected getInvalidKubectlVersionMessage to return empty string when kubectl-version is NOT set")
+
+	// kubectl-version is set and extra kubectl versions are NOT available
+	set = flag.NewFlagSet("kubectl-version-set-no-extra-versions-available", 0)
+	c = cli.NewContext(nil, set, nil)
+	set.String("kubectl-version", "1.14", "")
+	availableVersions = []string{}
+	expected = "Invalid param: kubectl-version was set to 1.14 but no extra kubectl versions are available"
+	actual = getInvalidKubectlVersionMessage(c, availableVersions)
+	assert.Equal(t, expected, actual, "expected getInvalidKubectlVersionMessage to return matching string when kubectl-version is set and extra kubectl versions are NOT available")
+
+	// kubectl-version is set, extra kubectl versions are available, kubectl-version is included
+	set = flag.NewFlagSet("valid-kubectl-version", 0)
+	c = cli.NewContext(nil, set, nil)
+	set.String("kubectl-version", "1.13", "")
+	availableVersions = []string{"1.11", "1.12", "1.13", "1.14"}
+	expected = ""
+	actual = getInvalidKubectlVersionMessage(c, availableVersions)
+	assert.Equal(t, expected, actual, "expected getInvalidKubectlVersionMessage to return empty string when kubectl-version is set, extra kubectl versions are available, kubectl-version is included")
+
+	// kubectl-version is set, extra kubectl versions are available, kubectl-version is NOT included
+	set = flag.NewFlagSet("invalid-kubectl-version", 0)
+	c = cli.NewContext(nil, set, nil)
+	set.String("kubectl-version", "9.99", "")
+	availableVersions = []string{"1.11", "1.12", "1.13", "1.14"}
+	expected = "Invalid param kubectl-version: 9.99 must be one of 1.11, 1.12, 1.13, 1.14"
+	actual = getInvalidKubectlVersionMessage(c, availableVersions)
+	assert.Equal(t, expected, actual, "expected getInvalidKubectlVersionMessage to return matching string when kubectl-version is set, extra kubectl versions are available, kubectl-version is NOT included")
+}
+
 func TestGetProjectFromToken(t *testing.T) {
 	token := "{\"project_id\":\"test-project\"}"
 	assert.Equal(t, "test-project", getProjectFromToken(token))