cmd/config: Add examples and tutorials for config functions

- Add examples under `functions`
- Add built-in tutorial for functions

functions/examples/validator-resource-requests/image/main.go

@@ -0,0 +1,94 @@
+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package main implements a validator function run by `kustomize config run-fns`
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"sigs.k8s.io/kustomize/kyaml/kio"
+	"sigs.k8s.io/kustomize/kyaml/kio/kioutil"
+	"sigs.k8s.io/kustomize/kyaml/yaml"
+)
+
+func main() {
+	rw := &kio.ByteReadWriter{Reader: os.Stdin, Writer: os.Stdout, KeepReaderAnnotations: true}
+	p := kio.Pipeline{
+		Inputs:  []kio.Reader{rw},       // read the inputs into a slice
+		Filters: []kio.Filter{filter{}}, // run the filter against the inputs
+		Outputs: []kio.Writer{rw}}       // copy the inputs to the output
+	if err := p.Execute(); err != nil {
+		fmt.Fprintf(os.Stderr, "%v\n", err)
+		os.Exit(1)
+	}
+}
+
+// filter implements kio.Filter
+type filter struct{}
+
+// Filter checks each input and ensures that all containers have cpu and memory
+// reservations set, otherwise it returns an error.
+func (filter) Filter(in []*yaml.RNode) ([]*yaml.RNode, error) {
+	// validate each Resource
+	for _, r := range in {
+		if err := validate(r); err != nil {
+			return nil, err
+		}
+	}
+	return in, nil
+}
+
+func validate(r *yaml.RNode) error {
+	meta, err := r.GetMeta()
+	if err != nil {
+		return err
+	}
+
+	// lookup the containers field in the Resource
+	containers, err := r.Pipe(yaml.Lookup("spec", "template", "spec", "containers"))
+	if err != nil {
+		s, _ := r.String()
+		return fmt.Errorf("%v: %s", err, s)
+	}
+	if containers == nil {
+		// doesn't have containers, ignore it
+		return nil
+	}
+
+	// visit each container in the list and validate
+	return containers.VisitElements(func(node *yaml.RNode) error {
+
+		// check cpu is non-nil
+		f, err := node.Pipe(yaml.Lookup("resources", "requests", "cpu"))
+		if err != nil {
+			s, _ := r.String()
+			return fmt.Errorf("%v: %s", err, s)
+		}
+		if f == nil {
+			return fmt.Errorf(
+				"cpu-requests missing for a container in %s %s (%s [%s])",
+				meta.Kind, meta.Name,
+				meta.Annotations[kioutil.PathAnnotation],
+				meta.Annotations[kioutil.IndexAnnotation])
+		}
+
+		// check memory is non-nil
+		f, err = node.Pipe(yaml.Lookup("resources", "requests", "memory"))
+		if err != nil {
+			s, _ := r.String()
+			return fmt.Errorf("%v: %s", err, s)
+		}
+		if f == nil {
+			return fmt.Errorf(
+				"memory-requests missing for a container in %s in %s (%s [%s])",
+				meta.Kind, meta.Name,
+				meta.Annotations[kioutil.PathAnnotation],
+				meta.Annotations[kioutil.IndexAnnotation])
+		}
+
+		// container is valid
+		return nil
+	})
+}