cache: add filelist methods for file to layer mapping

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

cache/filelist.go

@@ -0,0 +1,90 @@
+package cache
+
+import (
+	"archive/tar"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"path"
+	"sort"
+
+	cdcompression "github.com/containerd/containerd/archive/compression"
+	"github.com/moby/buildkit/session"
+)
+
+const keyFileList = "filelist"
+
+// FileList returns an ordered list of files present in the cache record that were
+// changed compared to the parent. The paths of the files are in same format as they
+// are in the tar stream (AUFS whiteout format). If the reference does not have a
+// a blob associated with it, the list is empty.
+func (sr *immutableRef) FileList(ctx context.Context, s session.Group) ([]string, error) {
+	res, err := g.Do(ctx, fmt.Sprintf("filelist-%s", sr.ID()), func(ctx context.Context) (interface{}, error) {
+		dt, err := sr.GetExternal(keyFileList)
+		if err == nil && dt != nil {
+			var files []string
+			if err := json.Unmarshal(dt, &files); err != nil {
+				return nil, err
+			}
+			return files, nil
+		}
+
+		if sr.getBlob() == "" {
+			return nil, nil
+		}
+
+		// lazy blobs need to be pulled first
+		if err := sr.Extract(ctx, s); err != nil {
+			return nil, err
+		}
+
+		desc, err := sr.ociDesc(ctx, sr.descHandlers, false)
+		if err != nil {
+			return nil, err
+		}
+
+		ra, err := sr.cm.ContentStore.ReaderAt(ctx, desc)
+		if err != nil {
+			return nil, err
+		}
+
+		r, err := cdcompression.DecompressStream(io.NewSectionReader(ra, 0, ra.Size()))
+		if err != nil {
+			return nil, err
+		}
+		defer r.Close()
+
+		var files []string
+
+		rdr := tar.NewReader(r)
+		for {
+			hdr, err := rdr.Next()
+			if err == io.EOF {
+				break
+			}
+			if err != nil {
+				return nil, err
+			}
+			name := path.Clean(hdr.Name)
+			files = append(files, name)
+		}
+		sort.Strings(files)
+
+		dt, err = json.Marshal(files)
+		if err != nil {
+			return nil, err
+		}
+		if err := sr.SetExternal(keyFileList, dt); err != nil {
+			return nil, err
+		}
+		return files, nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	if res == nil {
+		return nil, nil
+	}
+	return res.([]string), nil
+}

cache/refs.go

@@ -57,6 +57,7 @@ type ImmutableRef interface {
 	Extract(ctx context.Context, s session.Group) error // +progress
 	GetRemotes(ctx context.Context, createIfNeeded bool, cfg config.RefConfig, all bool, s session.Group) ([]*solver.Remote, error)
 	LayerChain() RefList
+	FileList(ctx context.Context, s session.Group) ([]string, error)
 }
 
 type MutableRef interface {