Transfer interface and plugin work in progress

Signed-off-by: Derek McGowan <derek@mcg.dev>

Transfer service implementation

Signed-off-by: Derek McGowan <derek@mcg.dev>

pkg/transfer/transfer.go

@@ -0,0 +1,89 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package transfer
+
+import (
+	"context"
+	"io"
+
+	"github.com/containerd/containerd/images"
+	"github.com/containerd/containerd/pkg/unpack"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+type Transferer interface {
+	Transfer(context.Context, interface{}, interface{}, ...Opt) error
+}
+
+type ImageResolver interface {
+	Resolve(ctx context.Context) (name string, desc ocispec.Descriptor, err error)
+
+	Fetcher(ctx context.Context, ref string) (Fetcher, error)
+}
+
+type Fetcher interface {
+	Fetch(ctx context.Context, desc ocispec.Descriptor) (io.ReadCloser, error)
+}
+
+// ImageFilterer is used to filter out child objects of an image
+type ImageFilterer interface {
+	ImageFilter(images.HandlerFunc) images.HandlerFunc
+}
+
+type ImageStorer interface {
+	Store(context.Context, ocispec.Descriptor) (images.Image, error)
+}
+
+type ImageUnpacker interface {
+	// TODO: Or unpack options?
+	UnpackPlatforms() []unpack.Platform
+}
+
+type TransferOpts struct {
+}
+
+type Opt func(*TransferOpts)
+
+func WithProgress() Opt {
+	return nil
+}
+
+type Progress struct {
+	Event    string
+	Name     string
+	Digest   string
+	Progress int64
+	Total    int64
+}
+
+/*
+// Distribution options
+// Stream handler
+// Progress rate
+// Unpack options
+// Remote options
+// Cases:
+//  Registry -> Content/ImageStore (pull)
+//  Registry -> Registry
+//  Content/ImageStore -> Registry (push)
+//  Content/ImageStore -> Content/ImageStore (tag)
+// Common fetch/push interface for registry, content/imagestore, OCI index
+// Always starts with string for source and destination, on client side, does not need to resolve
+//  Higher level implementation just takes strings and options
+//  Lower level implementation takes pusher/fetcher?
+
+*/