containerd: add access to default config

when wanting to craft a custom config, but based on the default config,
add a route to output the containerd config to a tempfile.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

cmd/containerd/config.go

@@ -1,6 +1,11 @@
 package main
 
-import "github.com/BurntSushi/toml"
+import (
+	"bytes"
+	"io"
+
+	"github.com/BurntSushi/toml"
+)
 
 func defaultConfig() *config {
 	return &config{
@@ -58,6 +63,15 @@ func (c *config) decodePlugin(name string, v interface{}) error {
 	return c.md.PrimitiveDecode(p, v)
 }
 
+func (c *config) WriteTo(w io.Writer) (int64, error) {
+	buf := bytes.NewBuffer(nil)
+	e := toml.NewEncoder(buf)
+	if err := e.Encode(c); err != nil {
+		return 0, err
+	}
+	return io.Copy(w, buf)
+}
+
 type grpcConfig struct {
 	Socket string `toml:"socket"`
 	Uid    int    `toml:"uid"`

cmd/containerd/main.go

@@ -3,6 +3,7 @@ package main
 import (
 	_ "expvar"
 	"fmt"
+	"io/ioutil"
 	"net/http"
 	_ "net/http/pprof"
 	"os"
@@ -60,7 +61,7 @@ func main() {
 	app.Flags = []cli.Flag{
 		cli.StringFlag{
 			Name:  "config,c",
-			Usage: "path to the configuration file",
+			Usage: "path to the configuration file (Use 'default' to output the default toml)",
 			Value: "/etc/containerd/config.toml",
 		},
 		cli.StringFlag{
@@ -148,6 +149,19 @@ func main() {
 }
 
 func before(context *cli.Context) error {
+	if context.GlobalString("config") == "default" {
+		fh, err := ioutil.TempFile("", "containerd-config.toml.")
+		if err != nil {
+			return err
+		}
+		if _, err := conf.WriteTo(fh); err != nil {
+			fh.Close()
+			return err
+		}
+		fh.Close()
+		log.G(global).Infof("containerd default config written to %q", fh.Name())
+		os.Exit(0)
+	}
 	if err := loadConfig(context.GlobalString("config")); err != nil &&
 		!os.IsNotExist(err) {
 		return err