Switch runc and containerd startup to be entirely Go

At present they use a small shared function called "prepare"
that does the read-write remounts, that I will switch to doing overlay
mounts soon.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

pkg/containerd/cmd/service/main.go

@@ -66,9 +66,8 @@ func main() {
 
 	args := flag.Args()
 	if len(args) < 1 {
-		fmt.Printf("Please specify a command.\n\n")
-		flag.Usage()
-		os.Exit(1)
+		systemInitCmd(args)
+		os.Exit(0)
 	}
 
 	switch args[0] {

pkg/containerd/cmd/service/prepare.go

@@ -0,0 +1,21 @@
+package main
+
+// Please note this file is shared between pkg/runc and pkg/containerd
+// Update it in both places if you make changes
+
+import (
+	"path/filepath"
+	"syscall"
+)
+
+func prepare(path string) error {
+	rootfs := filepath.Join(path, "rootfs")
+	if err := syscall.Mount(rootfs, rootfs, "", syscall.MS_BIND, ""); err != nil {
+		return err
+	}
+	// remount rw
+	if err := syscall.Mount("", rootfs, "", syscall.MS_REMOUNT, ""); err != nil {
+		return err
+	}
+	return nil
+}

pkg/containerd/cmd/service/start.go

@@ -58,6 +58,10 @@ func startCmd(args []string) {
 func start(service, sock, path, dumpSpec string) (string, uint32, string, error) {
 	rootfs := filepath.Join(path, service, "rootfs")
 
+	if err := prepare(filepath.Join(path, service)); err != nil {
+		return "", 0, "preparing rootfs", err
+	}
+
 	client, err := containerd.New(sock)
 	if err != nil {
 		return "", 0, "creating containerd client", err