Add structcheck, unused, and varcheck linters.

Warn on unused and dead code

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

fs/diff_unix.go

@@ -5,36 +5,12 @@ package fs
 import (
 	"bytes"
 	"os"
-	"path/filepath"
-	"strings"
 	"syscall"
 
 	"github.com/containerd/continuity/sysx"
 	"github.com/pkg/errors"
 )
 
-// whiteouts are files with a special meaning for the layered filesystem.
-// Docker uses AUFS whiteout files inside exported archives. In other
-// filesystems these files are generated/handled on tar creation/extraction.
-
-// whiteoutPrefix prefix means file is a whiteout. If this is followed by a
-// filename this means that file has been removed from the base layer.
-const whiteoutPrefix = ".wh."
-
-// whiteoutMetaPrefix prefix means whiteout has a special meaning and is not
-// for removing an actual file. Normally these files are excluded from exported
-// archives.
-const whiteoutMetaPrefix = whiteoutPrefix + whiteoutPrefix
-
-// whiteoutLinkDir is a directory AUFS uses for storing hardlink links to other
-// layers. Normally these should not go into exported archives and all changed
-// hardlinks should be copied to the top layer.
-const whiteoutLinkDir = whiteoutMetaPrefix + "plnk"
-
-// whiteoutOpaqueDir file means directory has been made opaque - meaning
-// readdir calls to this directory do not follow to lower layers.
-const whiteoutOpaqueDir = whiteoutMetaPrefix + ".opq"
-
 // detectDirDiff returns diff dir options if a directory could
 // be found in the mount info for upper which is the direct
 // diff with the provided lower directory
@@ -45,26 +21,6 @@ func detectDirDiff(upper, lower string) *diffDirOptions {
 	return nil
 }
 
-func aufsMetadataSkip(path string) (skip bool, err error) {
-	skip, err = filepath.Match(string(os.PathSeparator)+whiteoutMetaPrefix+"*", path)
-	if err != nil {
-		skip = true
-	}
-	return
-}
-
-func aufsDeletedFile(root, path string, fi os.FileInfo) (string, error) {
-	f := filepath.Base(path)
-
-	// If there is a whiteout, then the file was removed
-	if strings.HasPrefix(f, whiteoutPrefix) {
-		originalFile := f[len(whiteoutPrefix):]
-		return filepath.Join(filepath.Dir(path), originalFile), nil
-	}
-
-	return "", nil
-}
-
 // compareSysStat returns whether the stats are equivalent,
 // whether the files are considered the same file, and
 // an error