feat(helm): add support for required properties

Adds the `required` function in enginge.go to support required
properties in values.yml. When a chart developer wishes to specify
intent in requiring a value, they can use this function to declare
an error message that gets returned when chart rendering fails
when a required value is not present in values.yml.

Closes #1580

docs/charts.md

@@ -436,6 +436,9 @@ anything.
 **NOTE:** If the `--set` flag is used on `helm install` or `helm upgrade`, those
 values are simply converted to YAML on the client side.
 
+**NOTE:** If any required entries in the values file exist, they can be declared
+as required in the chart template by using the ['required' function](charts_tips_and_tricks.md)
+
 Any of these values are then accessible inside of templates using the
 `.Values` object:
 

docs/charts_tips_and_tricks.md

@@ -14,8 +14,8 @@ First, we added almost all of the functions in the
 for security reasons: `env` and `expandenv` (which would have given chart authors
 access to Tiller's environment).
 
-We also added one special template function: `include`. The `include` function
-allows you to bring in another template, and then pass the results to other
+We also added two special template functions: `include` and `required`. The `include`
+function allows you to bring in another template, and then pass the results to other
 template functions.
 
 For example, this template snippet includes a template called `mytpl.tpl`, then
@@ -25,6 +25,17 @@ lowercases the result, then wraps that in double quotes.
 value: {{include "mytpl.tpl" . | lower | quote}}
 ```
 
+The `required` function allows you to declare a particular
+values entry as required for template rendering.  If the value is empty, the template
+rendering will fail with a user submitted error message.
+
+The following example of the `required` function declares an entry for .Values.who
+is required, and will print an error message when that entry is missing:
+
+```yaml
+value: {{required "A valid .Values.who entry required!" .Values.who }}
+```
+
 ## Quote Strings, Don't Quote Integers
 
 When you are working with string data, you are always safer quoting the
@@ -61,6 +72,30 @@ Because YAML ascribes significance to indentation levels and whitespace,
 this is one great way to include snippets of code, but handle
 indentation in a relevant context.
 
+## Using the 'required' function
+
+Go provides a way for setting template options to control behavior
+when a map is indexed with a key that's not present in the map. This
+is typically set with template.Options("missingkey=option"), where option
+can be default, zero, or error. While setting this option to error will
+stop execution with an arror, this would apply to every missing key in the
+map. There may be situations where a chart developer wants to enforce this
+behavior for select values in the values.yml file.
+
+The `required` function gives developers the ability to declare a value entry
+as required for template rendering. If the entry is empty in values.yml, the
+template will not render and will return an error message supplied by the
+developer.
+
+For example:
+
+```
+{{ required "A valid foo is required!" .Values.foo }}
+```
+
+The above will render the template when .Values.foo is defined, but will fail
+to render and exit when .Values.foo is undefined.
+
 ## Automatically Roll Deployments When ConfigMaps or Secrets change
 
 Often times configmaps or secrets are injected as configuration

pkg/engine/engine.go

@@ -63,6 +63,8 @@ func New() *Engine {
 //
 //	- "include": This is late-bound in Engine.Render(). The version
 //	   included in the FuncMap is a placeholder.
+//      - "required": This is late-bound in Engine.Render(). The version
+//	   included in thhe FuncMap is a placeholder.
 func FuncMap() template.FuncMap {
 	f := sprig.TxtFuncMap()
 	delete(f, "env")
@@ -80,6 +82,7 @@ func FuncMap() template.FuncMap {
 		// late-bound to a template. By declaring it here, we preserve the
 		// integrity of the linter.
 		"include":  func(string, interface{}) string { return "not implemented" },
+		"required": func(string, interface{}) interface{} { return "not implemented" },
 	}
 
 	for k, v := range extra {
@@ -143,6 +146,18 @@ func (e *Engine) alterFuncMap(t *template.Template) template.FuncMap {
 		return buf.String()
 	}
 
+	// Add the 'required' function here
+	funcMap["required"] = func(warn string, val interface{}) (interface{}, error) {
+		if val == nil {
+			return val, fmt.Errorf(warn)
+		} else if _, ok := val.(string); ok {
+			if val == "" {
+				return val, fmt.Errorf(warn)
+			}
+		}
+		return val, nil
+	}
+
 	return funcMap
 }
 

pkg/engine/engine_test.go

@@ -49,7 +49,7 @@ func TestFuncMap(t *testing.T) {
 	}
 
 	// Test for Engine-specific template functions.
-	expect := []string{"include", "toYaml", "fromYaml", "toToml", "toJson", "fromJson"}
+	expect := []string{"include", "required", "toYaml", "fromYaml", "toToml", "toJson", "fromJson"}
 	for _, f := range expect {
 		if _, ok := fns[f]; !ok {
 			t.Errorf("Expected add-on function %q", f)
@@ -409,4 +409,40 @@ func TestAlterFuncMap(t *testing.T) {
 	if got := out["conrad/templates/quote"]; got != expect {
 		t.Errorf("Expected %q, got %q (%v)", expect, got, out)
 	}
+
+	reqChart := &chart.Chart{
+		Metadata: &chart.Metadata{Name: "conan"},
+		Templates: []*chart.Template{
+			{Name: "templates/quote", Data: []byte(`All your base are belong to {{ required "A valid 'who' is required" .Values.who }}`)},
+			{Name: "templates/bases", Data: []byte(`All {{ required "A valid 'bases' is required" .Values.bases }} of them!`)},
+		},
+		Values:       &chart.Config{Raw: ``},
+		Dependencies: []*chart.Chart{},
+	}
+
+	reqValues := chartutil.Values{
+		"Values": chartutil.Values{
+			"who":   "us",
+			"bases": 2,
+		},
+		"Chart": reqChart.Metadata,
+		"Release": chartutil.Values{
+			"Name": "That 90s meme",
+		},
+	}
+
+	outReq, err := New().Render(reqChart, reqValues)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	expectStr := "All your base are belong to us"
+	if gotStr := outReq["conan/templates/quote"]; gotStr != expectStr {
+		t.Errorf("Expected %q, got %q (%v)", expectStr, gotStr, outReq)
+	}
+	expectNum := "All 2 of them!"
+	if gotNum := outReq["conan/templates/bases"]; gotNum != expectNum {
+		t.Errorf("Expected %q, got %q (%v)", expectNum, gotNum, outReq)
+	}
+
 }