Improvements to kyaml fn framework

This commit creates a new version of the alpha configuration functions framework. Goals include:
- Make it easy to build multi-version APIs with the framework (not previously facilitated at all).
- Simplify the framework's APIs where redundant configuration options exist (leaving the most powerful, replacing others with helpers to maintain usability they provided).
- Make the Framework's APIs more consistent (e.g. between the various template types, usage of kio.Filter, field names)
- Decouple responsibilities (e.g. command creation, resource list processing, generation of templating functions).
- Make the framework even more powerfully pluggable (e.g. any kio.Filter can be a selector, and the selector the framework provides is itself a filter built from reusable abstractions).
- Improve documentation.
- Make container patches merge fields (notably list fields like `env`) correctly.

kyaml/fn/framework/template.go

@@ -0,0 +1,80 @@
+// Copyright 2021 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package framework
+
+import (
+	"bytes"
+	"strings"
+	"text/template"
+
+	"sigs.k8s.io/kustomize/kyaml/errors"
+	"sigs.k8s.io/kustomize/kyaml/kio"
+	"sigs.k8s.io/kustomize/kyaml/yaml"
+)
+
+// ResourceTemplate generates resources from templates.
+type ResourceTemplate struct {
+	// Templates is a function that returns a list of templates to render into one or more resources.
+	Templates TemplatesFunc
+
+	// TemplateData is the data to use when rendering the templates provided by the Templates field.
+	TemplateData interface{}
+}
+
+// DefaultTemplateData sets TemplateData to the provided default values if it has not already
+// been set.
+func (rt *ResourceTemplate) DefaultTemplateData(data interface{}) {
+	if rt.TemplateData == nil {
+		rt.TemplateData = data
+	}
+}
+
+// Render renders the Templates into resource nodes using TemplateData.
+func (rt *ResourceTemplate) Render() ([]*yaml.RNode, error) {
+	var items []*yaml.RNode
+
+	if rt.Templates == nil {
+		return items, nil
+	}
+
+	templates, err := rt.Templates()
+	if err != nil {
+		return nil, errors.WrapPrefixf(err, "failed to retrieve ResourceTemplates")
+	}
+
+	for i := range templates {
+		newItems, err := rt.doTemplate(templates[i])
+		if err != nil {
+			return nil, err
+		}
+		items = append(items, newItems...)
+	}
+	return items, nil
+}
+
+func (rt *ResourceTemplate) doTemplate(t *template.Template) ([]*yaml.RNode, error) {
+	// invoke the template
+	var b bytes.Buffer
+	err := t.Execute(&b, rt.TemplateData)
+	if err != nil {
+		return nil, errors.WrapPrefixf(err, "failed to render template %v", t.DefinedTemplates())
+	}
+	var items []*yaml.RNode
+
+	// split the resources so the error messaging is better
+	for _, s := range strings.Split(b.String(), "\n---\n") {
+		s = strings.TrimSpace(s)
+		if s == "" {
+			continue
+		}
+		newItems, err := (&kio.ByteReader{Reader: bytes.NewBufferString(s)}).Read()
+		if err != nil {
+			return nil, errors.WrapPrefixf(err,
+				"failed to parse rendered template into a resource:\n%s\n", addLineNumbers(s))
+		}
+
+		items = append(items, newItems...)
+	}
+	return items, nil
+}