Add RenderConfig option to ContainerPilot.

This is useful in AutoPilot applications like MySQL that need to
rewrite the containerpilot.conf file and force a SIGHUP.

config/render.go

@@ -0,0 +1,73 @@
+package config
+
+import (
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strings"
+)
+
+// A Render Application
+type RenderApp struct {
+	ConfigFlag     string
+	RenderFlag     string
+	RenderedConfig []byte
+}
+
+// EmptyApp creates an empty application
+func EmptyApp() *RenderApp {
+	app := &RenderApp{}
+	return app
+}
+
+// NewRenderApp creates a new Config Rendering application
+func NewRenderApp(configFlag string, renderFlag string) (*RenderApp, error) {
+
+	a := EmptyApp()
+
+	if configFlag == "" {
+		return nil, errors.New("-config flag is required")
+	}
+	if renderFlag != "-" && !strings.HasPrefix(renderFlag, "file://") {
+		return nil, errors.New("-render flag is invalid")
+	}
+
+	a.ConfigFlag = configFlag
+	a.RenderFlag = renderFlag
+
+	var data []byte
+	if strings.HasPrefix(configFlag, "file://") {
+		var err error
+		fName := strings.SplitAfter(configFlag, "file://")[1]
+		if data, err = ioutil.ReadFile(fName); err != nil {
+			return nil, fmt.Errorf("Could not read config file: %s", err)
+		}
+	} else {
+		data = []byte(configFlag)
+	}
+
+	template, err := ApplyTemplate(data)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"Could not apply template to config: %v", err)
+	}
+
+	a.RenderedConfig = template
+
+	return a, nil
+}
+
+func (a *RenderApp) Run() {
+	if a.RenderFlag == "-" {
+		fmt.Printf("%s", a.RenderedConfig)
+		os.Exit(0)
+	}
+	if strings.HasPrefix(a.RenderFlag, "file://") {
+		var err error
+		fName := strings.SplitAfter(a.RenderFlag, "file://")[1]
+		if err = ioutil.WriteFile(fName, a.RenderedConfig, 0644); err != nil {
+			panic(fmt.Errorf("Could not write config file: %s", err))
+		}
+	}
+}

core/app.go

@@ -29,6 +29,10 @@ var (
 	GitHash string
 )
 
+type Runnable interface {
+	Run()
+}
+
 // App encapsulates the state of ContainerPilot after the initial setup.
 // after it is run, it can be reloaded and paused with signals.
 type App struct {
@@ -59,14 +63,17 @@ func EmptyApp() *App {
 }
 
 // LoadApp parses the commandline arguments and loads the config
-func LoadApp() (*App, error) {
+func LoadApp() (Runnable, error) {
 
 	var configFlag string
+	var renderFlag string
 	var versionFlag bool
 
 	if !flag.Parsed() {
 		flag.StringVar(&configFlag, "config", "",
 			"JSON config or file:// path to JSON config file.")
+		flag.StringVar(&renderFlag, "render", "",
+		  "- for stdout or file:// path where to save redenred JSON config file.")
 		flag.BoolVar(&versionFlag, "version", false, "Show version identifier and quit.")
 		flag.Parse()
 	}
@@ -79,7 +86,14 @@ func LoadApp() (*App, error) {
 	}
 
 	os.Setenv("CONTAINERPILOT_PID", fmt.Sprintf("%v", os.Getpid()))
-	app, err := NewApp(configFlag)
+	var app Runnable
+	var err error
+	if renderFlag == "" {
+		app, err = NewApp(configFlag)
+	} else {
+		app, err = config.NewRenderApp(configFlag, renderFlag)
+	}
+
 	if err != nil {
 		return nil, err
 	}