Add layer store

Layer store manages read-only and read-write layers on a union file system.
Read only layers are always referenced by content addresses.
Read-write layer identifiers are handled by the caller but upon registering
its difference, the committed read-only layer will be referenced by content
hash.

Signed-off-by: Derek McGowan <[email protected]> (github: dmcgowan)
Signed-off-by: Tonis Tiigi <[email protected]>

layer/empty.go

@@ -0,0 +1,47 @@
+package layer
+
+import (
+	"archive/tar"
+	"bytes"
+	"io"
+)
+
+// DigestSHA256EmptyTar is the canonical sha256 digest of empty tar file -
+// (1024 NULL bytes)
+const DigestSHA256EmptyTar = DiffID("sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef")
+
+type emptyLayer struct{}
+
+// EmptyLayer is a layer that corresponds to empty tar.
+var EmptyLayer = &emptyLayer{}
+
+func (el *emptyLayer) TarStream() (io.Reader, error) {
+	buf := new(bytes.Buffer)
+	tarWriter := tar.NewWriter(buf)
+	tarWriter.Close()
+	return buf, nil
+}
+
+func (el *emptyLayer) ChainID() ChainID {
+	return ChainID(DigestSHA256EmptyTar)
+}
+
+func (el *emptyLayer) DiffID() DiffID {
+	return DigestSHA256EmptyTar
+}
+
+func (el *emptyLayer) Parent() Layer {
+	return nil
+}
+
+func (el *emptyLayer) Size() (size int64, err error) {
+	return 0, nil
+}
+
+func (el *emptyLayer) DiffSize() (size int64, err error) {
+	return 0, nil
+}
+
+func (el *emptyLayer) Metadata() (map[string]string, error) {
+	return make(map[string]string), nil
+}

layer/empty_test.go

@@ -0,0 +1,46 @@
+package layer
+
+import (
+	"io"
+	"testing"
+
+	"github.com/docker/distribution/digest"
+)
+
+func TestEmptyLayer(t *testing.T) {
+	if EmptyLayer.ChainID() != ChainID(DigestSHA256EmptyTar) {
+		t.Fatal("wrong ID for empty layer")
+	}
+
+	if EmptyLayer.DiffID() != DigestSHA256EmptyTar {
+		t.Fatal("wrong DiffID for empty layer")
+	}
+
+	if EmptyLayer.Parent() != nil {
+		t.Fatal("expected no parent for empty layer")
+	}
+
+	if size, err := EmptyLayer.Size(); err != nil || size != 0 {
+		t.Fatal("expected zero size for empty layer")
+	}
+
+	if diffSize, err := EmptyLayer.DiffSize(); err != nil || diffSize != 0 {
+		t.Fatal("expected zero diffsize for empty layer")
+	}
+
+	tarStream, err := EmptyLayer.TarStream()
+	if err != nil {
+		t.Fatalf("error streaming tar for empty layer: %v", err)
+	}
+
+	digester := digest.Canonical.New()
+	_, err = io.Copy(digester.Hash(), tarStream)
+
+	if err != nil {
+		t.Fatalf("error hashing empty tar layer: %v", err)
+	}
+
+	if digester.Digest() != digest.Digest(DigestSHA256EmptyTar) {
+		t.Fatal("empty layer tar stream hashes to wrong value")
+	}
+}