pkg/printers: Support base64 decode in kubectl go-template

Adds a `base64decode` function to templates in `kubectl` so that
it's possible to extract secret data in plaintext instead of
base64 without requiring a separate executable.

Sample usage:

```sh
kubectl get secret SECRET -o go-template='{{ .data.KEY | base64decode }}'
```

pkg/printers/BUILD

@@ -69,7 +69,10 @@ filegroup(
 
 go_test(
     name = "go_default_test",
-    srcs = ["humanreadable_test.go"],
+    srcs = [
+        "humanreadable_test.go",
+        "template_test.go",
+    ],
     embed = [":go_default_library"],
     deps = [
         "//pkg/apis/core:go_default_library",

pkg/printers/template.go

@@ -17,6 +17,7 @@ limitations under the License.
 package printers
 
 import (
+	"encoding/base64"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -33,7 +34,10 @@ type TemplatePrinter struct {
 
 func NewTemplatePrinter(tmpl []byte) (*TemplatePrinter, error) {
 	t, err := template.New("output").
-		Funcs(template.FuncMap{"exists": exists}).
+		Funcs(template.FuncMap{
+			"exists":       exists,
+			"base64decode": base64decode,
+		}).
 		Parse(string(tmpl))
 	if err != nil {
 		return nil, err
@@ -112,3 +116,11 @@ func (p *TemplatePrinter) safeExecute(w io.Writer, obj interface{}) error {
 	}
 	return retErr
 }
+
+func base64decode(v string) (string, error) {
+	data, err := base64.StdEncoding.DecodeString(v)
+	if err != nil {
+		return "", fmt.Errorf("base64 decode failed: %v", err)
+	}
+	return string(data), nil
+}

pkg/printers/template_test.go

@@ -0,0 +1,97 @@
+/*
+Copyright 2018 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package printers
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"k8s.io/apimachinery/pkg/runtime"
+	api "k8s.io/kubernetes/pkg/apis/core"
+)
+
+func TestTemplate(t *testing.T) {
+	testCase := map[string]struct {
+		template  string
+		obj       runtime.Object
+		expectOut string
+		expectErr func(error) (string, bool)
+	}{
+		"support base64 decoding of secret data": {
+			template: "{{ .Data.username | base64decode }}",
+			obj: &api.Secret{
+				Data: map[string][]byte{
+					"username": []byte("hunter"),
+				},
+			},
+			expectOut: "hunter",
+		},
+		"test error path for base64 decoding": {
+			template: "{{ .Data.username | base64decode }}",
+			obj:      &badlyMarshaledSecret{},
+			expectErr: func(err error) (string, bool) {
+				matched := strings.Contains(err.Error(), "base64 decode")
+				return "a base64 decode error", matched
+			},
+		},
+	}
+	for name, test := range testCase {
+		buffer := &bytes.Buffer{}
+
+		p, err := NewTemplatePrinter([]byte(test.template))
+		if err != nil {
+			if test.expectErr == nil {
+				t.Errorf("[%s]expected success but got:\n %v\n", name, err)
+				continue
+			}
+			if expected, ok := test.expectErr(err); !ok {
+				t.Errorf("[%s]expect:\n %v\n but got:\n %v\n", name, expected, err)
+			}
+			continue
+		}
+
+		err = p.PrintObj(test.obj, buffer)
+		if err != nil {
+			if test.expectErr == nil {
+				t.Errorf("[%s]expected success but got:\n %v\n", name, err)
+				continue
+			}
+			if expected, ok := test.expectErr(err); !ok {
+				t.Errorf("[%s]expect:\n %v\n but got:\n %v\n", name, expected, err)
+			}
+			continue
+		}
+
+		if test.expectErr != nil {
+			t.Errorf("[%s]expect:\n error\n but got:\n no error\n", name)
+			continue
+		}
+
+		if test.expectOut != buffer.String() {
+			t.Errorf("[%s]expect:\n %v\n but got:\n %v\n", name, test.expectOut, buffer.String())
+		}
+	}
+}
+
+type badlyMarshaledSecret struct {
+	api.Secret
+}
+
+func (a badlyMarshaledSecret) MarshalJSON() ([]byte, error) {
+	return []byte(`{"apiVersion":"v1","Data":{"username":"--THIS IS NOT BASE64--"},"kind":"Secret"}`), nil
+}