image: fix the interaction of findManifest and findConfig with walker…

….find

Before rev. 9027d58 findManifest and findConfig called w.walk directly,
providing a walkFunc that returned errEOW in case a file has been
found and processed successfully.  A switch construct evaluating the
error-result of w.walk then tested for the errEOW case, meaning the file
has been found.

Starting with rev. 9027d58 the call to w.walk in findManifest and
findConfig has been replaced by w.find. Though, the function literal
provided by findManifest and findConfig to w.find -- now a findFunc,
not a walkFunc anymore -- still returns errEOW. As a consequence,
walker.find implementations for tar and zip would treat errEOW as an
error, making oci-image-tool 'validate' and 'create' stop with an error:

	<tar-image>: find failed: unable to walk: end of walk

This patch adjusts tarWalker's and zipWalker's implementations of
walker.find, using errEOW to stop w.walk early in case of success; there
is no need for the 'done' variable anymore. Additionally, places where
w.find is called, functions findManifest, unpackManifest, and findConfig,
have been adapted so that their findFunc argument to w.find does not
return errEOW anymore; instead nil is returned on success. Consequently,
the switch constructs now test err for 'nil' instead of 'errEOW', and
'os.ErrNotExist' instead of nil.

Fixes opencontainers#205

Signed-off-by: Michael Teichgräber <mteichgraeber@gmx.de>

image/config.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"os"
 	"path/filepath"
 	"strconv"
 	"strings"
@@ -48,11 +49,11 @@ func findConfig(w walker, d *v1.Descriptor) (*v1.Image, error) {
 			return err
 		}
 
-		return errEOW
+		return nil
 	}); err {
-	case nil:
+	case os.ErrNotExist:
 		return nil, fmt.Errorf("%s: config not found", cpath)
-	case errEOW:
+	case nil:
 		return &c, nil
 	default:
 		return nil, err

image/manifest.go

@@ -53,11 +53,11 @@ func findManifest(w walker, d *v1.Descriptor) (*v1.Manifest, error) {
 			return err
 		}
 
-		return errEOW
+		return nil
 	}); err {
-	case nil:
+	case os.ErrNotExist:
 		return nil, fmt.Errorf("%s: manifest not found", mpath)
-	case errEOW:
+	case nil:
 		return &m, nil
 	default:
 		return nil, err
@@ -110,11 +110,10 @@ func unpackManifest(m *v1.Manifest, w walker, dest string) (retErr error) {
 				return errors.Wrap(err, "unpack: error extracting layer")
 			}
 
-			return errEOW
+			return nil
 		}); err {
-		case nil:
+		case os.ErrNotExist:
 			return fmt.Errorf("%s: layer not found", dest)
-		case errEOW:
 		default:
 			return err
 		}

image/walker.go

@@ -126,28 +126,24 @@ func (w *tarWalker) get(desc v1.Descriptor, dst io.Writer) (int64, error) {
 }
 
 func (w *tarWalker) find(path string, ff findFunc) error {
-	done := false
-
 	f := func(relpath string, info os.FileInfo, rdr io.Reader) error {
 		var err error
-		if done {
-			return nil
-		}
 
 		if filepath.Clean(relpath) == path && !info.IsDir() {
 			if err = ff(relpath, rdr); err != nil {
 				return err
 			}
-			done = true
+			return errEOW
 		}
 		return nil
 	}
 
-	if err := w.walk(f); err != nil {
-		return errors.Wrapf(err, "find failed: unable to walk")
-	}
-	if !done {
+	switch err := w.walk(f); err {
+	case nil:
 		return os.ErrNotExist
+	case errEOW:
+	default:
+		return errors.Wrapf(err, "find failed: unable to walk")
 	}
 
 	return nil
@@ -305,28 +301,24 @@ func (w *zipWalker) get(desc v1.Descriptor, dst io.Writer) (int64, error) {
 }
 
 func (w *zipWalker) find(path string, ff findFunc) error {
-	done := false
-
 	f := func(relpath string, info os.FileInfo, rdr io.Reader) error {
 		var err error
-		if done {
-			return nil
-		}
 
 		if filepath.Clean(relpath) == path && !info.IsDir() {
 			if err = ff(relpath, rdr); err != nil {
 				return err
 			}
-			done = true
+			return errEOW
 		}
 		return nil
 	}
 
-	if err := w.walk(f); err != nil {
-		return errors.Wrapf(err, "find failed: unable to walk")
-	}
-	if !done {
+	switch err := w.walk(f); err {
+	case nil:
 		return os.ErrNotExist
+	case errEOW:
+	default:
+		return errors.Wrapf(err, "find failed: unable to walk")
 	}
 
 	return nil