cmd/config: Add examples and tutorials for config functions

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

functions/examples/template-go-nginx/image/main.go

@@ -0,0 +1,169 @@
+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strconv"
+	"text/template"
+
+	"sigs.k8s.io/kustomize/kyaml/kio"
+	"sigs.k8s.io/kustomize/kyaml/kio/filters"
+	"sigs.k8s.io/kustomize/kyaml/yaml"
+)
+
+func main() {
+	rw := &kio.ByteReadWriter{
+		Reader:                os.Stdin,
+		Writer:                os.Stdout,
+		KeepReaderAnnotations: true,
+	}
+
+	err := kio.Pipeline{
+		Inputs: []kio.Reader{rw},
+		Filters: []kio.Filter{
+			&filter{rw: rw},       // generate the Resources from the template
+			filters.MergeFilter{}, // merge the generated template
+			// set Resource filenames
+			&filters.FileSetter{FilenamePattern: filepath.Join("config", "%n.yaml")},
+			filters.FormatFilter{}, // format the output
+		},
+		Outputs: []kio.Writer{rw},
+	}.Execute()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "%v\n", err)
+		os.Exit(1)
+	}
+}
+
+// define the input API schema as a struct
+type API struct {
+	Metadata struct {
+		// Name is the Deployment Resource and Container name
+		Name string `yaml:"name"`
+	} `yaml:"metadata"`
+
+	Spec struct {
+		// Replicas is the number of Deployment replicas
+		// Defaults to the REPLICAS env var, or 1
+		Replicas *int `yaml:"replicas"`
+	} `yaml:"spec"`
+}
+
+// filter implements kio.Filter
+type filter struct {
+	rw *kio.ByteReadWriter
+}
+
+// Filter checks each input and ensures that all containers have cpu and memory
+// reservations set, otherwise it returns an error.
+func (f *filter) Filter(in []*yaml.RNode) ([]*yaml.RNode, error) {
+	api := f.parseAPI()
+
+	// execute the service template
+	buff := &bytes.Buffer{}
+	t := template.Must(template.New("nginx-service").Parse(serviceTemplate))
+	if err := t.Execute(buff, api); err != nil {
+		return nil, err
+	}
+	s, err := yaml.Parse(buff.String())
+	if err != nil {
+		return nil, err
+	}
+
+	// execute the deployment template
+	buff = &bytes.Buffer{}
+	t = template.Must(template.New("nginx-deployment").Parse(deploymentTemplate))
+	if err := t.Execute(buff, api); err != nil {
+		return nil, err
+	}
+	d, err := yaml.Parse(buff.String())
+	if err != nil {
+		return nil, err
+	}
+
+	// add the template generated Resources to the output -- these will get merged by the next
+	// filter
+	in = append(in, s, d)
+	return in, nil
+}
+
+// parseAPI parses the functionConfig into an API struct, and validates the input
+func (f *filter) parseAPI() API {
+	// parse the input function config -- TODO: simplify this
+	var api API
+	if err := yaml.Unmarshal([]byte(f.rw.FunctionConfig.MustString()), &api); err != nil {
+		fmt.Fprintf(os.Stderr, "%v\n", err)
+		os.Exit(1)
+	}
+
+	// Default functionConfig values from environment variables if they are not set
+	// in the functionConfig
+	r := os.Getenv("REPLICAS")
+	if r != "" && api.Spec.Replicas == nil {
+		replicas, err := strconv.Atoi(r)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "%v\n", err)
+			os.Exit(1)
+		}
+		api.Spec.Replicas = &replicas
+	}
+	if api.Spec.Replicas == nil {
+		r := 1
+		api.Spec.Replicas = &r
+	}
+	if api.Metadata.Name == "" {
+		fmt.Fprintf(os.Stderr, "must specify metadata.name\n")
+		os.Exit(1)
+	}
+
+	return api
+}
+
+var serviceTemplate = `
+apiVersion: v1
+kind: Service
+metadata:
+  name: {{ .Metadata.Name }}
+  labels:
+    app: nginx
+    instance: {{ .Metadata.Name }}
+spec:
+  ports:
+  - port: 80
+    targetPort: 80
+    name: http
+  selector:
+    app: nginx
+    instance: {{ .Metadata.Name }}
+`
+
+var deploymentTemplate = `apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: {{ .Metadata.Name }}
+  labels:
+    app: nginx
+    instance: {{ .Metadata.Name }}
+spec:
+  replicas: {{ .Spec.Replicas }}
+  selector:
+    matchLabels:
+      app: nginx
+      instance: {{ .Metadata.Name }}
+  template:
+    metadata:
+      labels:
+        app: nginx
+        instance: {{ .Metadata.Name }}
+    spec:
+      containers:
+      - name: nginx
+        image: nginx:1.7.9
+        ports:
+        - containerPort: 80
+`