Add push object

Split resolver to only return a name with separate methods
for getting a fetcher and pusher. Add implementation for
push.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

remotes/docker/fetcher.go

@@ -0,0 +1,78 @@
+package docker
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"path"
+	"strings"
+
+	"github.com/Sirupsen/logrus"
+	"github.com/containerd/containerd/images"
+	"github.com/containerd/containerd/log"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"github.com/pkg/errors"
+)
+
+type dockerFetcher struct {
+	*dockerBase
+}
+
+func (r dockerFetcher) Fetch(ctx context.Context, desc ocispec.Descriptor) (io.ReadCloser, error) {
+	ctx = log.WithLogger(ctx, log.G(ctx).WithFields(
+		logrus.Fields{
+			"base":   r.base.String(),
+			"digest": desc.Digest,
+		},
+	))
+
+	paths, err := getV2URLPaths(desc)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, path := range paths {
+		u := r.url(path)
+
+		req, err := http.NewRequest(http.MethodGet, u, nil)
+		if err != nil {
+			return nil, err
+		}
+
+		req.Header.Set("Accept", strings.Join([]string{desc.MediaType, `*`}, ", "))
+		resp, err := r.doRequestWithRetries(ctx, req, nil)
+		if err != nil {
+			return nil, err
+		}
+
+		if resp.StatusCode > 299 {
+			if resp.StatusCode == http.StatusNotFound {
+				continue // try one of the other urls.
+			}
+			resp.Body.Close()
+			return nil, errors.Errorf("unexpected status code %v: %v", u, resp.Status)
+		}
+
+		return resp.Body, nil
+	}
+
+	return nil, errors.New("not found")
+}
+
+// getV2URLPaths generates the candidate urls paths for the object based on the
+// set of hints and the provided object id. URLs are returned in the order of
+// most to least likely succeed.
+func getV2URLPaths(desc ocispec.Descriptor) ([]string, error) {
+	var urls []string
+
+	switch desc.MediaType {
+	case images.MediaTypeDockerSchema2Manifest, images.MediaTypeDockerSchema2ManifestList,
+		ocispec.MediaTypeImageManifest, ocispec.MediaTypeImageIndex:
+		urls = append(urls, path.Join("manifests", desc.Digest.String()))
+	}
+
+	// always fallback to attempting to get the object out of the blobs store.
+	urls = append(urls, path.Join("blobs", desc.Digest.String()))
+
+	return urls, nil
+}