Enable to export layers from Additional Layer Store

Currently, layers aquired from additional layer store cannot be exported
(e.g. `podman save`, `podman push`).

This is because the current additional layer store exposes only *extracted view*
of layers. Tar is not reproducible so the runtime cannot reproduce the tar
archive that has the same diff ID as the original.

This commit solves this issue by introducing a new API "`blob`" to the
additional layer store. This file exposes the raw contents of that layer. When
*(c/storage).layerStore.Diff is called, it acquires the diff contents from this
`blob` file which the same digest as the original layer.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

drivers/driver.go

@@ -230,6 +230,9 @@ type AdditionalLayer interface {
 	// Info returns arbitrary information stored along with this layer (i.e. `info` file)
 	Info() (io.ReadCloser, error)
 
+	// Blob returns a reader of the raw contents of this layer.
+	Blob() (io.ReadCloser, error)
+
 	// Release tells the additional layer store that we don't use this handler.
 	Release()
 }
@@ -243,6 +246,10 @@ type AdditionalLayerStoreDriver interface {
 	// LookupAdditionalLayer looks up additional layer store by the specified
 	// digest and ref and returns an object representing that layer.
 	LookupAdditionalLayer(d digest.Digest, ref string) (AdditionalLayer, error)
+
+	// LookupAdditionalLayer looks up additional layer store by the specified
+	// ID and returns an object representing that layer.
+	LookupAdditionalLayerByID(id string) (AdditionalLayer, error)
 }
 
 // DiffGetterDriver is the interface for layered file system drivers that

drivers/overlay/overlay.go

@@ -712,6 +712,7 @@ func (d *Driver) Cleanup() error {
 // LookupAdditionalLayer looks up additional layer store by the specified
 // digest and ref and returns an object representing that layer.
 // This API is experimental and can be changed without bumping the major version number.
+// TODO: to remove the comment once it's no longer experimental.
 func (d *Driver) LookupAdditionalLayer(dgst digest.Digest, ref string) (graphdriver.AdditionalLayer, error) {
 	l, err := d.getAdditionalLayerPath(dgst, ref)
 	if err != nil {
@@ -727,6 +728,25 @@ func (d *Driver) LookupAdditionalLayer(dgst digest.Digest, ref string) (graphdri
 	}, nil
 }
 
+// LookupAdditionalLayerByID looks up additional layer store by the specified
+// ID and returns an object representing that layer.
+// This API is experimental and can be changed without bumping the major version number.
+// TODO: to remove the comment once it's no longer experimental.
+func (d *Driver) LookupAdditionalLayerByID(id string) (graphdriver.AdditionalLayer, error) {
+	l, err := d.getAdditionalLayerPathByID(id)
+	if err != nil {
+		return nil, err
+	}
+	// Tell the additional layer store that we use this layer.
+	// This will increase reference counter on the store's side.
+	// This will be decreased on Release() method.
+	notifyUseAdditionalLayer(l)
+	return &additionalLayer{
+		path: l,
+		d:    d,
+	}, nil
+}
+
 // CreateFromTemplate creates a layer with the same contents and parent as another layer.
 func (d *Driver) CreateFromTemplate(id, template string, templateIDMappings *idtools.IDMappings, parent string, parentIDMappings *idtools.IDMappings, opts *graphdriver.CreateOpts, readWrite bool) error {
 	if readWrite {
@@ -1818,9 +1838,7 @@ func (d *Driver) getAdditionalLayerPath(dgst digest.Digest, ref string) (string,
 		for _, p := range []string{
 			filepath.Join(target, "diff"),
 			filepath.Join(target, "info"),
-			// TODO(ktock): We should have an API to expose the stream data of this layer
-			//              to enable the client to retrieve the entire contents of this
-			//              layer when it exports this layer.
+			filepath.Join(target, "blob"),
 		} {
 			if _, err := os.Stat(p); err != nil {
 				return "", errors.Wrapf(graphdriver.ErrLayerUnknown,
@@ -1835,8 +1853,8 @@ func (d *Driver) getAdditionalLayerPath(dgst digest.Digest, ref string) (string,
 }
 
 func (d *Driver) releaseAdditionalLayerByID(id string) {
-	if al, err := ioutil.ReadFile(path.Join(d.dir(id), "additionallayer")); err == nil {
-		notifyReleaseAdditionalLayer(string(al))
+	if al, err := d.getAdditionalLayerPathByID(id); err == nil {
+		notifyReleaseAdditionalLayer(al)
 	} else if !os.IsNotExist(err) {
 		logrus.Warnf("unexpected error on reading Additional Layer Store pointer %v", err)
 	}
@@ -1851,12 +1869,19 @@ type additionalLayer struct {
 
 // Info returns arbitrary information stored along with this layer (i.e. `info` file).
 // This API is experimental and can be changed without bumping the major version number.
+// TODO: to remove the comment once it's no longer experimental.
 func (al *additionalLayer) Info() (io.ReadCloser, error) {
 	return os.Open(filepath.Join(al.path, "info"))
 }
 
+// Blob returns a reader of the raw contents of this leyer.
+func (al *additionalLayer) Blob() (io.ReadCloser, error) {
+	return os.Open(filepath.Join(al.path, "blob"))
+}
+
 // CreateAs creates a new layer from this additional layer.
 // This API is experimental and can be changed without bumping the major version number.
+// TODO: to remove the comment once it's no longer experimental.
 func (al *additionalLayer) CreateAs(id, parent string) error {
 	// TODO: support opts
 	if err := al.d.Create(id, parent, nil); err != nil {
@@ -1876,8 +1901,17 @@ func (al *additionalLayer) CreateAs(id, parent string) error {
 	return os.Symlink(filepath.Join(al.path, "diff"), diffDir)
 }
 
+func (d *Driver) getAdditionalLayerPathByID(id string) (string, error) {
+	al, err := ioutil.ReadFile(path.Join(d.dir(id), "additionallayer"))
+	if err != nil {
+		return "", err
+	}
+	return string(al), nil
+}
+
 // Release tells the additional layer store that we don't use this handler.
 // This API is experimental and can be changed without bumping the major version number.
+// TODO: to remove the comment once it's no longer experimental.
 func (al *additionalLayer) Release() {
 	// Tell the additional layer store that we don't use this layer handler.
 	// This will decrease the reference counter on the store's side, which was

layers.go

@@ -1401,6 +1401,52 @@ func (r *layerStore) Diff(from, to string, options *DiffOptions) (io.ReadCloser,
 		return maybeCompressReadCloser(diff)
 	}
 
+	if ad, ok := r.driver.(drivers.AdditionalLayerStoreDriver); ok {
+		if aLayer, err := ad.LookupAdditionalLayerByID(to); err == nil {
+			// This is an additional layer. We leverage blob API for aquiring the reproduced raw blob.
+			info, err := aLayer.Info()
+			if err != nil {
+				aLayer.Release()
+				return nil, err
+			}
+			defer info.Close()
+			layer := &Layer{}
+			if err := json.NewDecoder(info).Decode(layer); err != nil {
+				aLayer.Release()
+				return nil, err
+			}
+			blob, err := aLayer.Blob()
+			if err != nil {
+				aLayer.Release()
+				return nil, err
+			}
+			// If layer compression type is different from the expected one, decompress and convert it.
+			if compression != layer.CompressionType {
+				diff, err := archive.DecompressStream(blob)
+				if err != nil {
+					if err2 := blob.Close(); err2 != nil {
+						err = errors.Wrapf(err, "failed to close blob file: %v", err2)
+					}
+					aLayer.Release()
+					return nil, err
+				}
+				rc, err := maybeCompressReadCloser(diff)
+				if err != nil {
+					if err2 := closeAll(blob.Close, diff.Close); err2 != nil {
+						err = errors.Wrapf(err, "failed to cleanup: %v", err2)
+					}
+					aLayer.Release()
+					return nil, err
+				}
+				return ioutils.NewReadCloserWrapper(rc, func() error {
+					defer aLayer.Release()
+					return closeAll(blob.Close, rc.Close)
+				}), nil
+			}
+			return ioutils.NewReadCloserWrapper(blob, func() error { defer aLayer.Release(); return blob.Close() }), nil
+		}
+	}
+
 	tsfile, err := os.Open(r.tspath(to))
 	if err != nil {
 		if !os.IsNotExist(err) {
@@ -1733,3 +1779,16 @@ func (r *layerStore) ReloadIfChanged() error {
 	}
 	return nil
 }
+
+func closeAll(closes ...func() error) (rErr error) {
+	for _, f := range closes {
+		if err := f(); err != nil {
+			if rErr == nil {
+				rErr = errors.Wrapf(err, "close error")
+				continue
+			}
+			rErr = errors.Wrapf(rErr, "%v", err)
+		}
+	}
+	return
+}