feat(pkg/engine): add support for custom template funcs

Enhances the template engine and action config to allow users to inject custom template functions via an action config when using Helm as a library.

Closes #30733

Signed-off-by: Stepan Paksashvili <stepan.paksashvili@flant.com>

pkg/action/action.go

@@ -25,6 +25,7 @@ import (
 	"path"
 	"path/filepath"
 	"strings"
+	"text/template"
 
 	"github.com/pkg/errors"
 	"k8s.io/apimachinery/pkg/api/meta"
@@ -80,6 +81,9 @@ type Configuration struct {
 	// Capabilities describes the capabilities of the Kubernetes cluster.
 	Capabilities *chartutil.Capabilities
 
+	// CustomTemplateFuncs is defined by users to provide custom template funcs
+	CustomTemplateFuncs template.FuncMap
+
 	// HookOutputFunc called with container name and returns and expects writer that will receive the log output.
 	HookOutputFunc func(namespace, pod, container string) io.Writer
 }
@@ -118,6 +122,8 @@ func (cfg *Configuration) renderResources(ch *chart.Chart, values chartutil.Valu
 		}
 		e := engine.New(restConfig)
 		e.EnableDNS = enableDNS
+		e.CustomTemplateFuncs = cfg.CustomTemplateFuncs
+
 		files, err2 = e.Render(ch, values)
 	} else {
 		var e engine.Engine

pkg/engine/engine.go

@@ -44,6 +44,8 @@ type Engine struct {
 	clientProvider *ClientProvider
 	// EnableDNS tells the engine to allow DNS lookups when rendering templates
 	EnableDNS bool
+	// CustomTemplateFuncs is defined by users to provide custom template funcs
+	CustomTemplateFuncs template.FuncMap
 }
 
 // New creates a new instance of Engine using the passed in rest config.
@@ -244,6 +246,11 @@ func (e Engine) initFunMap(t *template.Template) {
 		}
 	}
 
+	// Set custom template func, do it here to give opportunity to override standard functions
+	for k, v := range e.CustomTemplateFuncs {
+		funcMap[k] = v
+	}
+
 	t.Funcs(funcMap)
 }
 

pkg/engine/engine_test.go

@@ -1300,3 +1300,49 @@ func TestRenderTplMissingKeyString(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestRenderCustomTemplateFuncs(t *testing.T) {
+	// Create a chart with a single template that uses a custom function "exclaim"
+	c := &chart.Chart{
+		Metadata: &chart.Metadata{Name: "CustomFunc"},
+		Templates: []*chart.File{
+			{
+				Name: "templates/manifest",
+				Data: []byte(`{{exclaim .Values.message}}`),
+			},
+		},
+	}
+	v := chartutil.Values{
+		"Values": chartutil.Values{
+			"message": "hello",
+		},
+		"Chart": c.Metadata,
+		"Release": chartutil.Values{
+			"Name": "TestRelease",
+		},
+	}
+
+	// Define a custom template function "exclaim" that appends "!!!" to a string.
+	customFuncs := template.FuncMap{
+		"exclaim": func(input string) string {
+			return input + "!!!"
+		},
+	}
+
+	// Create an engine instance and set the CustomTemplateFuncs.
+	e := new(Engine)
+	e.CustomTemplateFuncs = customFuncs
+
+	// Render the chart.
+	out, err := e.Render(c, v)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Expected output should be "hello!!!".
+	expected := "hello!!!"
+	key := "CustomFunc/templates/manifest"
+	if rendered, ok := out[key]; !ok || rendered != expected {
+		t.Errorf("Expected %q, got %q", expected, rendered)
+	}
+}