support dry run

Signed-off-by: Shiwei Zhang <shizh@microsoft.com>

cmd/oras/push.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"path/filepath"
 
+	iresolver "github.com/deislabs/oras/internal/resolver"
 	"github.com/deislabs/oras/pkg/content"
 	ctxo "github.com/deislabs/oras/pkg/context"
 	"github.com/deislabs/oras/pkg/oras"
@@ -33,6 +34,7 @@ type pushOptions struct {
 	artifactRefs           []string
 	pathValidationDisabled bool
 	verbose                bool
+	dryRun                 bool
 
 	debug     bool
 	configs   []string
@@ -88,6 +90,7 @@ Example - Push file to the HTTP registry:
 	cmd.Flags().StringVarP(&opts.password, "password", "p", "", "registry password")
 	cmd.Flags().BoolVarP(&opts.insecure, "insecure", "", false, "allow connections to SSL registry without certs")
 	cmd.Flags().BoolVarP(&opts.plainHTTP, "plain-http", "", false, "use plain http and not https")
+	cmd.Flags().BoolVarP(&opts.dryRun, "dry-run", "", false, "push to a dummy registry instead of the real remote")
 	return cmd
 }
 
@@ -99,9 +102,16 @@ func runPush(opts pushOptions) error {
 		ctx = ctxo.WithLoggerDiscarded(ctx)
 	}
 
+	// specify resolver
+	var resolver remotes.Resolver
+	if opts.dryRun {
+		resolver = iresolver.Dummy()
+	} else {
+		resolver = newResolver(opts.username, opts.password, opts.insecure, opts.plainHTTP, opts.configs...)
+	}
+
 	// bake artifact
 	var pushOpts []oras.PushOpt
-	resolver := newResolver(opts.username, opts.password, opts.insecure, opts.plainHTTP, opts.configs...)
 	if opts.artifactType != "" {
 		manifests, err := loadReferences(ctx, resolver, opts.artifactRefs)
 		if err != nil {

internal/resolver/dummy.go

@@ -0,0 +1,103 @@
+package resolver
+
+import (
+	"context"
+	"io"
+	"time"
+
+	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/remotes"
+	artifactspec "github.com/notaryproject/artifacts/specs-go/v2"
+	"github.com/opencontainers/go-digest"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"github.com/pkg/errors"
+)
+
+// dummyResolver is a dummy resolver, which resolves nothing.
+// It accepts any pushes but ignores them.
+type dummyResolver struct{}
+
+var dummyResolverInstance = &dummyResolver{}
+
+// Dummy creates a new dummy resolver
+func Dummy() remotes.Resolver {
+	return dummyResolverInstance
+}
+
+func (r *dummyResolver) Resolve(ctx context.Context, ref string) (name string, desc ocispec.Descriptor, err error) {
+	return "", ocispec.Descriptor{}, errors.Wrap(errdefs.ErrNotFound, "dummy resolver")
+}
+
+func (r *dummyResolver) Fetcher(ctx context.Context, ref string) (remotes.Fetcher, error) {
+	return remotes.FetcherFunc(func(ctx context.Context, desc ocispec.Descriptor) (io.ReadCloser, error) {
+		return nil, errors.Wrap(errdefs.ErrNotFound, "dummy resolver")
+	}), nil
+}
+
+// Pusher returns a new pusher for the provided reference
+func (r *dummyResolver) Pusher(ctx context.Context, ref string) (remotes.Pusher, error) {
+	return remotes.PusherFunc(func(ctx context.Context, desc ocispec.Descriptor) (content.Writer, error) {
+		now := time.Now()
+		return &dummyWriter{
+			actual: digest.Canonical.Digester(),
+			status: content.Status{
+				Ref:       ref,
+				Total:     desc.Size,
+				Expected:  desc.Digest,
+				StartedAt: now,
+				UpdatedAt: now,
+			},
+		}, nil
+	}), nil
+}
+
+// Discoverer returns a new discoverer for the provided reference
+func (r *dummyResolver) Discoverer(ctx context.Context, ref string) (remotes.Discoverer, error) {
+	return remotes.DiscovererFunc(func(ctx context.Context, desc ocispec.Descriptor, artifactType string) ([]artifactspec.Artifact, error) {
+		return nil, errors.Wrap(errdefs.ErrNotFound, "dummy resolver")
+	}), nil
+}
+
+type dummyWriter struct {
+	actual digest.Digester
+	status content.Status
+}
+
+func (dw *dummyWriter) Write(p []byte) (n int, err error) {
+	n, err = dw.actual.Hash().Write(p)
+	dw.status.Offset += int64(n)
+	dw.status.UpdatedAt = time.Now()
+	return
+}
+
+func (dw *dummyWriter) Close() error {
+	return nil
+}
+
+func (dw *dummyWriter) Status() (content.Status, error) {
+	return dw.status, nil
+}
+
+func (dw *dummyWriter) Digest() digest.Digest {
+	return dw.status.Expected
+}
+
+func (dw *dummyWriter) Commit(ctx context.Context, size int64, expected digest.Digest, opts ...content.Opt) error {
+	if size > 0 && size != dw.status.Offset {
+		return errors.Errorf("unexpected size %d, expected %d", dw.status.Offset, size)
+	}
+	if expected == "" {
+		expected = dw.status.Expected
+	}
+
+	actual := dw.actual.Digest()
+	if actual != expected {
+		return errors.Errorf("got digest %s, expected %s", actual, expected)
+	}
+	return nil
+}
+
+func (dw *dummyWriter) Truncate(size int64) error {
+	return errors.New("cannot truncate dummy upload")
+}