Move transfer and unpack packages

Packages related to transfer and unpacking provide core interfaces which
use other core interfaces and part of common functionality.

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

core/transfer/plugins/plugins.go

@@ -0,0 +1,63 @@
+/*
+   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 plugins
+
+import (
+	"fmt"
+	"reflect"
+	"sync"
+
+	"github.com/containerd/errdefs"
+	"github.com/containerd/typeurl/v2"
+)
+
+var register = struct {
+	sync.RWMutex
+	r map[string]reflect.Type
+}{}
+
+func Register(apiObject, transferObject interface{}) {
+	url, err := typeurl.TypeURL(apiObject)
+	if err != nil {
+		panic(err)
+	}
+	// Lock
+	register.Lock()
+	defer register.Unlock()
+	if register.r == nil {
+		register.r = map[string]reflect.Type{}
+	}
+	if _, ok := register.r[url]; ok {
+		panic(fmt.Sprintf("url already registered: %v", url))
+	}
+	t := reflect.TypeOf(transferObject)
+	if t.Kind() == reflect.Ptr {
+		t = t.Elem()
+	}
+	register.r[url] = t
+}
+
+func ResolveType(any typeurl.Any) (interface{}, error) {
+	register.RLock()
+	defer register.RUnlock()
+	if register.r != nil {
+		if t, ok := register.r[any.GetTypeUrl()]; ok {
+			return reflect.New(t).Interface(), nil
+		}
+	}
+	return nil, fmt.Errorf("%v not registered: %w", any.GetTypeUrl(), errdefs.ErrNotFound)
+}