Add RootFs field to configuration options in libcontainer's Config

Since currently the container.json file does not include the pathname
to a container's root filesystem, we need to parse /proc/mounts which
is slow and error-prone.  This patch addresses this issue by adding a
new RootFs field.

Signed-off-by: Saied Kazemi <saied@google.com>

config.go

@@ -15,6 +15,9 @@ type Config struct {
 	// Mount specific options.
 	MountConfig *MountConfig `json:"mount_config,omitempty"`
 
+	// Pathname to container's root filesystem
+	RootFs string `json:"root_fs,omitempty"`
+
 	// Hostname optionally sets the container's hostname if provided
 	Hostname string `json:"hostname,omitempty"`
 

namespaces/exec.go

@@ -21,7 +21,7 @@ import (
 // Move this to libcontainer package.
 // Exec performs setup outside of a namespace so that a container can be
 // executed.  Exec is a high level function for working with container namespaces.
-func Exec(container *libcontainer.Config, stdin io.Reader, stdout, stderr io.Writer, console string, rootfs, dataPath string, args []string, createCommand CreateCommand, startCallback func()) (int, error) {
+func Exec(container *libcontainer.Config, stdin io.Reader, stdout, stderr io.Writer, console, dataPath string, args []string, createCommand CreateCommand, startCallback func()) (int, error) {
 	var (
 		err error
 	)
@@ -34,7 +34,7 @@ func Exec(container *libcontainer.Config, stdin io.Reader, stdout, stderr io.Wri
 	}
 	defer syncPipe.Close()
 
-	command := createCommand(container, console, rootfs, dataPath, os.Args[0], syncPipe.Child(), args)
+	command := createCommand(container, console, container.RootFs, dataPath, os.Args[0], syncPipe.Child(), args)
 	// Note: these are only used in non-tty mode
 	// if there is a tty for the container it will be opened within the namespace and the
 	// fds will be duped to stdin, stdiout, and stderr