Add option to provide custom template command

Author: Thor77

README.md

@@ -62,6 +62,23 @@ storage:
 {{ call .ReadFile "LICENSE" | call .Indent 12 }}
 ```
 
+### Custom template command
+Instead of using the native go template, you can also use any other command (for example [Helm](https://helm.sh)).
+To do that provide your custom command in the configuration option `flatcar.template_command`.
+It will get passed the hostname as the first argument and `Server` and `SSHKey` in YAML format on stdin.
+```
+hetzner:
+  server:
+    name: ...
+  sshkey:
+    publickey: ...
+```
+Example script to render a helm template with a values file based on the hostname:
+```sh
+#!/bin/sh
+cat - common.yaml "${1}.yaml" | yq -y . | helm template ignition -f -
+```
+
 ## Deployment procedure
 1. check whether vm with the name given as first parameter already exists
 2. create VM (if not already exists)

config.go

@@ -17,10 +17,11 @@ type hcloudConfig struct {
 }
 
 type flatcarConfig struct {
-	InstallScript  string `toml:"install_script"`
-	Version        string
-	ConfigTemplate string            `toml:"config_template"`
-	TemplateStatic map[string]string `toml:"template_static"`
+	InstallScript   string `toml:"install_script"`
+	Version         string
+	ConfigTemplate  string            `toml:"config_template"`
+	TemplateStatic  map[string]string `toml:"template_static"`
+	TemplateCommand string            `toml:"template_command"`
 }
 
 type config struct {

main.go

@@ -11,6 +11,7 @@ import (
 	"log"
 	"net"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"strings"
 	"text/template"
@@ -19,6 +20,7 @@ import (
 	clconfig "github.com/flatcar-linux/container-linux-config-transpiler/config"
 	"github.com/hetznercloud/hcloud-go/hcloud"
 	"github.com/melbahja/goph"
+	"gopkg.in/yaml.v3"
 )
 
 var installScriptSource = "https://raw.githubusercontent.com/flatcar-linux/init/flatcar-master/bin/flatcar-install"
@@ -74,6 +76,15 @@ type templateData struct {
 	Indent   func(int, string) string
 }
 
+type customTemplateDataHetzner struct {
+	Server hcloud.Server
+	SSHKey hcloud.SSHKey
+}
+
+type customTemplateData struct {
+	Hetzner customTemplateDataHetzner
+}
+
 func main() {
 	// TODO: cli parser...
 	if len(os.Args) == 1 {
@@ -196,40 +207,61 @@ func main() {
 		}
 	}
 
-	// render container linux config template
-	// TODO: support name replacement in template path
-	ignitionTemplate := cfg.Flatcar.ConfigTemplate
-	buffer := &bytes.Buffer{}
-	tmpl, err := template.New(filepath.Base(ignitionTemplate)).ParseFiles(ignitionTemplate)
-	if err != nil {
-		log.Fatalf("error loading template: %v\n", err)
-	}
-	err = tmpl.Execute(buffer, templateData{
-		Server: *server,
-		SSHKey: *sshKey,
-		Static: cfg.Flatcar.TemplateStatic,
-		ReadFile: func(filename string) (string, error) {
-			content, err := ioutil.ReadFile(filename)
-			return string(content), err
-		},
-		Indent: func(indent int, input string) string {
-			lines := strings.Split(input, "\n")
-			output := make([]string, len(lines))
-			indentString := strings.Repeat(" ", indent)
-			for i := 0; i < len(output); i++ {
-				output[i] = indentString + lines[i]
-			}
-			return strings.Join(output, "\n")
-		},
-	})
-	if err != nil {
-		log.Fatalf("error rendering template: %v\n", err)
-	}
+	var templateContent []byte
+	if cfg.Flatcar.TemplateCommand == "" {
+		ignitionTemplate := cfg.Flatcar.ConfigTemplate
+		buffer := &bytes.Buffer{}
+		tmpl, err := template.New(filepath.Base(ignitionTemplate)).ParseFiles(ignitionTemplate)
+		if err != nil {
+			log.Fatalf("error loading template: %v\n", err)
+		}
+		err = tmpl.Execute(buffer, templateData{
+			Server: *server,
+			SSHKey: *sshKey,
+			Static: cfg.Flatcar.TemplateStatic,
+			ReadFile: func(filename string) (string, error) {
+				content, err := ioutil.ReadFile(filename)
+				return string(content), err
+			},
+			Indent: func(indent int, input string) string {
+				lines := strings.Split(input, "\n")
+				output := make([]string, len(lines))
+				indentString := strings.Repeat(" ", indent)
+				for i := 0; i < len(output); i++ {
+					output[i] = indentString + lines[i]
+				}
+				return strings.Join(output, "\n")
+			},
+		})
+		if err != nil {
+			log.Fatalf("error rendering template: %v\n", err)
+		}
 
-	// transpile rendered container linux config template into ignition
-	bufferContent, _ := ioutil.ReadAll(buffer)
+		templateContent, _ = ioutil.ReadAll(buffer)
+	} else {
+		// marshal template data for passing it to the custom command
+		templateData := customTemplateData{
+			Hetzner: customTemplateDataHetzner{
+				Server: *server,
+				SSHKey: *sshKey,
+			},
+		}
+		templateDataYAML, err := yaml.Marshal(templateData)
+		if err != nil {
+			log.Fatalf("error marshaling hcloud data to yaml: %v\n", err)
+		}
+
+		// execute custom template command
+		tmplCmd := exec.Command(cfg.Flatcar.TemplateCommand, server.Name)
+		tmplCmd.Stdin = bytes.NewReader(templateDataYAML)
+		templateContent, err = tmplCmd.Output()
+		if err != nil {
+			log.Println(string(err.(*exec.ExitError).Stderr))
+			log.Fatalf("error running template command: %v\n", err)
+		}
+	}
 
-	renderedPath, err := transpileConfig(bufferContent)
+	renderedPath, err := transpileConfig(templateContent)
 	if err != nil {
 		log.Fatalf("error transpiling config: %v\n", err)
 	}