Kevin/eng 2022 investigate tpl values templates (#33)

* map include function for .tpl rendering

* testing added include tpl function

* update RenderTpl to take the filePath as parameter
get the template dir from the file path
ISSUES: all templates in the file path are included in the rendered template,
        need to update to only include the explicietly mentioned file

* update RenderTpl test to pass a filepath

* update RenderTpl test remove cruft

* update RenderTpl to only include specified .tpl files

* update the include directive to do a lookup of the named template as opposed to including the template file

template/tpl.go

@@ -2,13 +2,42 @@ package template
 
 import (
 	"bytes"
+	"fmt"
 	"text/template"
 
 	"github.com/Masterminds/sprig/v3"
 )
 
+// include function to include other templates
+func include(tpl *template.Template, name string, data interface{}) (string, error) {
+	// include the named template from the `define` directive in the current template
+	inc := tpl.Lookup(name)
+	if inc == nil {
+		return "", fmt.Errorf("template %s not found", name)
+	}
+
+	var buffer bytes.Buffer
+	err := inc.Execute(&buffer, data)
+	if err != nil {
+		return "", err
+	}
+
+	return buffer.String(), nil
+}
+
+// RenderTpl renders the given Helm .tpl template with the provided bindings and automatically includes additional templates from a directory.
 func RenderTpl(input []byte, bindings map[string]interface{}) ([]byte, error) {
-	tpl, err := template.New("gotpl").Funcs(sprig.TxtFuncMap()).Parse(string(input))
+
+	tpl := template.New("gotpl")
+
+	// Create a new template and add the sprig functions and the include function.
+	tpl.Funcs(sprig.TxtFuncMap()).Funcs(template.FuncMap{
+		"include": func(name string, data interface{}) (string, error) {
+			return include(tpl, name, data)
+		},
+	})
+
+	tpl, err := tpl.Parse(string(input))
 	if err != nil {
 		return nil, err
 	}

test/_simple.tpl

@@ -0,0 +1 @@
+a basic {{ .Template }}