Add docs for env var, use upstream constant

README.md

@@ -26,6 +26,15 @@ There are a number of packages for reading/writing these interfaces from/to vari
 The main focus has been registry interactions (hence the name) via the [`remote`](pkg/v1/remote) package,
 but we have implemented other formats as we needed them to interoperate with various tools.
 
+### Experiments
+
+Over time, we will add new functionality under experimental environment variables listed here.
+
+| Env Var | Value(s) | What is does |
+---------------------------------------
+| `GGCR_EXPERIMENT_ESTARGZ` | `"1"` | When enabled this experiment will direct `tarball.LayerFromOpener` to emit [estargz](https://github.com/opencontainers/image-spec/issues/815) compatible layers, which enable them to be lazily loaded by an appropriately configured containerd. |
+
+
 ### `v1.Image`
 
 #### Sources

pkg/v1/tarball/layer.go

@@ -188,7 +188,7 @@ func LayerFromOpener(opener Opener, opts ...LayerOption) (v1.Layer, error) {
 			if err != nil {
 				return nil, err
 			}
-			layer.annotations[gestargz.TOCDigestKey] = h.String()
+			layer.annotations[estargz.TOCJSONDigestAnnotation] = h.String()
 			return &and.ReadCloser{
 				Reader: rc,
 				CloseFunc: func() error {

pkg/v1/tarball/layer_test.go

@@ -24,7 +24,6 @@ import (
 
 	"github.com/containerd/stargz-snapshotter/estargz"
 	"github.com/google/go-containerregistry/pkg/internal/compare"
-	gestargz "github.com/google/go-containerregistry/pkg/v1/internal/estargz"
 	"github.com/google/go-containerregistry/pkg/v1/validate"
 )
 
@@ -130,10 +129,10 @@ func TestLayerFromFileEstargz(t *testing.T) {
 		t.Errorf("expected digests to differ: %s", defaultDigest.String())
 	}
 
-	if descriptorDefaultCompression.Annotations[gestargz.TOCDigestKey] == descriptorSpeedCompression.Annotations[gestargz.TOCDigestKey] {
+	if descriptorDefaultCompression.Annotations[estargz.TOCJSONDigestAnnotation] == descriptorSpeedCompression.Annotations[estargz.TOCJSONDigestAnnotation] {
 		t.Errorf("wanted different toc digests got default: %s, speed: %s",
-			descriptorDefaultCompression.Annotations[gestargz.TOCDigestKey],
-			descriptorSpeedCompression.Annotations[gestargz.TOCDigestKey])
+			descriptorDefaultCompression.Annotations[estargz.TOCJSONDigestAnnotation],
+			descriptorSpeedCompression.Annotations[estargz.TOCJSONDigestAnnotation])
 	}
 
 	tarLayerPrioritizedFiles, err := LayerFromFile("testdata/content.tar",
@@ -162,10 +161,10 @@ func TestLayerFromFileEstargz(t *testing.T) {
 		t.Errorf("expected digests to differ: %s", defaultDigest.String())
 	}
 
-	if descriptorDefaultCompression.Annotations[gestargz.TOCDigestKey] == descriptorPrioritizedFiles.Annotations[gestargz.TOCDigestKey] {
+	if descriptorDefaultCompression.Annotations[estargz.TOCJSONDigestAnnotation] == descriptorPrioritizedFiles.Annotations[estargz.TOCJSONDigestAnnotation] {
 		t.Errorf("wanted different toc digests got default: %s, prioritized: %s",
-			descriptorDefaultCompression.Annotations[gestargz.TOCDigestKey],
-			descriptorPrioritizedFiles.Annotations[gestargz.TOCDigestKey])
+			descriptorDefaultCompression.Annotations[estargz.TOCJSONDigestAnnotation],
+			descriptorPrioritizedFiles.Annotations[estargz.TOCJSONDigestAnnotation])
 	}
 }