libct: StartInitialization: decouple from factory

StartInitialization does not have to be a method of Factory (while
it is clear why it was done that way initially, now we only have
Linux containers so it does not make sense).

Fix callers and docs accordingly.

No change in functionality.

Also, since this was the only user of libcontainer.New with the empty
string as an argument, the corresponding check can now be removed
from it.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

init.go

@@ -32,8 +32,7 @@ func init() {
 		logrus.SetFormatter(new(logrus.JSONFormatter))
 		logrus.Debug("child process in init()")
 
-		factory, _ := libcontainer.New("")
-		if err := factory.StartInitialization(); err != nil {
+		if err := libcontainer.StartInitialization(); err != nil {
 			// as the error is sent back to the parent there is no need to log
 			// or write it to stderr because the parent process will handle this
 			os.Exit(1)

libcontainer/README.md

@@ -32,8 +32,7 @@ func init() {
 	if len(os.Args) > 1 && os.Args[1] == "init" {
 		runtime.GOMAXPROCS(1)
 		runtime.LockOSThread()
-		factory, _ := libcontainer.New("")
-		if err := factory.StartInitialization(); err != nil {
+		if err := libcontainer.StartInitialization(); err != nil {
 			logrus.Fatal(err)
 		}
 		panic("--this line should have never been executed, congratulations--")

libcontainer/factory_linux.go

@@ -29,10 +29,8 @@ var idRegex = regexp.MustCompile(`^[\w+-\.]+$`)
 
 // New returns a linux based container factory based in the root directory.
 func New(root string) (*LinuxFactory, error) {
-	if root != "" {
-		if err := os.MkdirAll(root, 0o700); err != nil {
-			return nil, err
-		}
+	if err := os.MkdirAll(root, 0o700); err != nil {
+		return nil, err
 	}
 	return &LinuxFactory{
 		Root: root,
@@ -169,9 +167,10 @@ func (l *LinuxFactory) Load(id string) (Container, error) {
 	return c, nil
 }
 
-// StartInitialization loads a container by opening the pipe fd from the parent to read the configuration and state
-// This is a low level implementation detail of the reexec and should not be consumed externally
-func (l *LinuxFactory) StartInitialization() (err error) {
+// StartInitialization loads a container by opening the pipe fd from the parent
+// to read the configuration and state. This is a low level implementation
+// detail of the reexec and should not be consumed externally.
+func StartInitialization() (err error) {
 	// Get the INITPIPE.
 	envInitPipe := os.Getenv("_LIBCONTAINER_INITPIPE")
 	pipefd, err := strconv.Atoi(envInitPipe)

libcontainer/integration/init_test.go

@@ -19,11 +19,7 @@ func init() {
 	}
 	runtime.GOMAXPROCS(1)
 	runtime.LockOSThread()
-	factory, err := libcontainer.New("")
-	if err != nil {
-		logrus.Fatalf("unable to initialize for container: %s", err)
-	}
-	if err := factory.StartInitialization(); err != nil {
+	if err := libcontainer.StartInitialization(); err != nil {
 		logrus.Fatal(err)
 	}
 }