fix(tiller): stop printing <no value> when var is missing

Instead of printing "no value", this prints an empty string by
default, but adds a Strict flag on the engine, which (if true)
will cause a template render to error out if a value is not supplied.

Strict is set to false so that developers can instead use `default` to
set default values.

pkg/engine/engine.go

@@ -19,6 +19,7 @@ package engine
 import (
 	"bytes"
 	"fmt"
+	"strings"
 	"text/template"
 
 	"github.com/Masterminds/sprig"
@@ -31,6 +32,9 @@ type Engine struct {
 	// FuncMap contains the template functions that will be passed to each
 	// render call. This may only be modified before the first call to Render.
 	FuncMap template.FuncMap
+	// If strict is enabled, template rendering will fail if a template references
+	// a value that was not passed in.
+	Strict bool
 }
 
 // New creates a new Go template Engine instance.
@@ -93,6 +97,13 @@ func (e *Engine) render(tpls map[string]renderable) (map[string]string, error) {
 	// to share common blocks, but to make the entire thing feel like a file-based
 	// template engine.
 	t := template.New("gotpl")
+	if e.Strict {
+		t.Option("missingkey=error")
+	} else {
+		// Not that zero will attempt to add default values for types it knows,
+		// but will still emit <no value> for others. We mitigate that later.
+		t.Option("missingkey=zero")
+	}
 	files := []string{}
 	for fname, r := range tpls {
 		t = t.New(fname).Funcs(e.FuncMap)
@@ -108,7 +119,10 @@ func (e *Engine) render(tpls map[string]renderable) (map[string]string, error) {
 		if err := t.ExecuteTemplate(&buf, file, tpls[file].vals); err != nil {
 			return map[string]string{}, fmt.Errorf("render error in %q: %s", file, err)
 		}
-		rendered[file] = buf.String()
+		// Work around the issue where Go will emit "<no value>" even if Options(missing=zero)
+		// is set. Since missing=error will never get here, we do not need to handle
+		// the Strict case.
+		rendered[file] = strings.Replace(buf.String(), "<no value>", "", -1)
 		buf.Reset()
 	}
 

pkg/engine/engine_test.go

@@ -46,6 +46,7 @@ func TestRender(t *testing.T) {
 		Templates: []*chart.Template{
 			{Name: "test1", Data: []byte("{{.outer | title }} {{.inner | title}}")},
 			{Name: "test2", Data: []byte("{{.global.callme | lower }}")},
+			{Name: "test3", Data: []byte("{{.noValue}}")},
 		},
 		Values: &chart.Config{
 			Raw: "outer: DEFAULT\ninner: DEFAULT",
@@ -82,6 +83,10 @@ func TestRender(t *testing.T) {
 	if out["test2"] != expect {
 		t.Errorf("Expected %q, got %q", expect, out["test2"])
 	}
+	expect = ""
+	if out["test3"] != expect {
+		t.Errorf("Expected %q, got %q", expect, out["test3"])
+	}
 
 	if _, err := e.Render(c, v); err != nil {
 		t.Errorf("Unexpected error: %s", err)