Add support for layers from foreign sources

This will be used to support downloading Windows base layers from
Microsoft URLs.

Signed-off-by: John Starks <jostarks@microsoft.com>

docs/storage/schema2manifesthandler_test.go

@@ -0,0 +1,117 @@
+package storage
+
+import (
+	"testing"
+
+	"github.com/docker/distribution"
+	"github.com/docker/distribution/context"
+	"github.com/docker/distribution/manifest"
+	"github.com/docker/distribution/manifest/schema2"
+	"github.com/docker/distribution/registry/storage/driver/inmemory"
+)
+
+func TestVerifyManifestForeignLayer(t *testing.T) {
+	ctx := context.Background()
+	inmemoryDriver := inmemory.New()
+	registry := createRegistry(t, inmemoryDriver)
+	repo := makeRepository(t, registry, "test")
+	manifestService := makeManifestService(t, repo)
+
+	config, err := repo.Blobs(ctx).Put(ctx, schema2.MediaTypeConfig, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	layer, err := repo.Blobs(ctx).Put(ctx, schema2.MediaTypeLayer, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	foreignLayer := distribution.Descriptor{
+		Digest:    "sha256:463435349086340864309863409683460843608348608934092322395278926a",
+		Size:      6323,
+		MediaType: schema2.MediaTypeForeignLayer,
+	}
+
+	template := schema2.Manifest{
+		Versioned: manifest.Versioned{
+			SchemaVersion: 2,
+			MediaType:     schema2.MediaTypeManifest,
+		},
+		Config: config,
+	}
+
+	type testcase struct {
+		BaseLayer distribution.Descriptor
+		URLs      []string
+		Err       error
+	}
+
+	cases := []testcase{
+		{
+			foreignLayer,
+			nil,
+			errMissingURL,
+		},
+		{
+			layer,
+			[]string{"http://foo/bar"},
+			errUnexpectedURL,
+		},
+		{
+			foreignLayer,
+			[]string{"file:///local/file"},
+			errInvalidURL,
+		},
+		{
+			foreignLayer,
+			[]string{"http://foo/bar#baz"},
+			errInvalidURL,
+		},
+		{
+			foreignLayer,
+			[]string{""},
+			errInvalidURL,
+		},
+		{
+			foreignLayer,
+			[]string{"https://foo/bar", ""},
+			errInvalidURL,
+		},
+		{
+			foreignLayer,
+			[]string{"http://foo/bar"},
+			nil,
+		},
+		{
+			foreignLayer,
+			[]string{"https://foo/bar"},
+			nil,
+		},
+	}
+
+	for _, c := range cases {
+		m := template
+		l := c.BaseLayer
+		l.URLs = c.URLs
+		m.Layers = []distribution.Descriptor{l}
+		dm, err := schema2.FromStruct(m)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+
+		_, err = manifestService.Put(ctx, dm)
+		if verr, ok := err.(distribution.ErrManifestVerification); ok {
+			// Extract the first error
+			if len(verr) == 2 {
+				if _, ok = verr[1].(distribution.ErrManifestBlobUnknown); ok {
+					err = verr[0]
+				}
+			}
+		}
+		if err != c.Err {
+			t.Errorf("%#v: expected %v, got %v", l, c.Err, err)
+		}
+	}
+}