Move RootPath to fs package

This moves the RootPath function out of the archive package and into the
fs package for external use.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

archive/path.go

@@ -1,107 +1 @@
 package archive
-
-import (
-	"os"
-	"path/filepath"
-	"strings"
-
-	"github.com/pkg/errors"
-)
-
-var (
-	errTooManyLinks = errors.New("too many links")
-)
-
-// rootPath joins a path with a root, evaluating and bounding any
-// symlink to the root directory.
-// TODO(dmcgowan): Expose and move to fs package or continuity path driver
-func rootPath(root, path string) (string, error) {
-	if path == "" {
-		return root, nil
-	}
-	var linksWalked int // to protect against cycles
-	for {
-		i := linksWalked
-		newpath, err := walkLinks(root, path, &linksWalked)
-		if err != nil {
-			return "", err
-		}
-		path = newpath
-		if i == linksWalked {
-			newpath = filepath.Join("/", newpath)
-			if path == newpath {
-				return filepath.Join(root, newpath), nil
-			}
-			path = newpath
-		}
-	}
-}
-
-func walkLink(root, path string, linksWalked *int) (newpath string, islink bool, err error) {
-	if *linksWalked > 255 {
-		return "", false, errTooManyLinks
-	}
-
-	path = filepath.Join("/", path)
-	if path == "/" {
-		return path, false, nil
-	}
-	realPath := filepath.Join(root, path)
-
-	fi, err := os.Lstat(realPath)
-	if err != nil {
-		// If path does not yet exist, treat as non-symlink
-		if os.IsNotExist(err) {
-			return path, false, nil
-		}
-		return "", false, err
-	}
-	if fi.Mode()&os.ModeSymlink == 0 {
-		return path, false, nil
-	}
-	newpath, err = os.Readlink(realPath)
-	if err != nil {
-		return "", false, err
-	}
-	if filepath.IsAbs(newpath) && strings.HasPrefix(newpath, root) {
-		newpath = newpath[:len(root)]
-		if !strings.HasPrefix(newpath, "/") {
-			newpath = "/" + newpath
-		}
-	}
-	*linksWalked++
-	return newpath, true, nil
-}
-
-func walkLinks(root, path string, linksWalked *int) (string, error) {
-	switch dir, file := filepath.Split(path); {
-	case dir == "":
-		newpath, _, err := walkLink(root, file, linksWalked)
-		return newpath, err
-	case file == "":
-		if os.IsPathSeparator(dir[len(dir)-1]) {
-			if dir == "/" {
-				return dir, nil
-			}
-			return walkLinks(root, dir[:len(dir)-1], linksWalked)
-		}
-		newpath, _, err := walkLink(root, dir, linksWalked)
-		return newpath, err
-	default:
-		newdir, err := walkLinks(root, dir, linksWalked)
-		if err != nil {
-			return "", err
-		}
-		newpath, islink, err := walkLink(root, filepath.Join(newdir, file), linksWalked)
-		if err != nil {
-			return "", err
-		}
-		if !islink {
-			return newpath, nil
-		}
-		if filepath.IsAbs(newpath) {
-			return newpath, nil
-		}
-		return filepath.Join(newdir, newpath), nil
-	}
-}