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>

services/transfer/service.go

@@ -0,0 +1,84 @@
+/*
+   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"
+
+	transferapi "github.com/containerd/containerd/api/services/transfer/v1"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/pkg/transfer"
+	"github.com/containerd/containerd/plugin"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+	"google.golang.org/protobuf/types/known/emptypb"
+)
+
+func init() {
+	plugin.Register(&plugin.Registration{
+		Type: plugin.GRPCPlugin,
+		ID:   "transfer",
+		Requires: []plugin.Type{
+			plugin.TransferPlugin,
+		},
+		InitFn: newService,
+	})
+}
+
+type service struct {
+	transferers []transfer.Transferer
+	transferapi.UnimplementedTransferServer
+}
+
+func newService(ic *plugin.InitContext) (interface{}, error) {
+	plugins, err := ic.GetByType(plugin.TransferPlugin)
+	if err != nil {
+		return nil, err
+	}
+	// TODO: how to determine order?
+	t := make([]transfer.Transferer, 0, len(plugins))
+	for _, p := range plugins {
+		i, err := p.Instance()
+		if err != nil {
+			return nil, err
+		}
+		t = append(t, i.(transfer.Transferer))
+	}
+	return &service{
+		transferers: t,
+	}, nil
+}
+
+func (s *service) Register(gs *grpc.Server) error {
+	transferapi.RegisterTransferServer(gs, s)
+	return nil
+}
+
+func (s *service) Transfer(ctx context.Context, req *transferapi.TransferRequest) (*emptypb.Empty, error) {
+	// TODO: Optionally proxy
+
+	// TODO: Convert options
+	for _, t := range s.transferers {
+		if err := t.Transfer(ctx, req.Source, req.Destination); err == nil {
+			return nil, nil
+		} else if !errdefs.IsNotImplemented(err) {
+			return nil, errdefs.ToGRPC(err)
+		}
+	}
+	return nil, status.Errorf(codes.Unimplemented, "method Transfer not implemented for %s to %s", req.Source.GetTypeUrl(), req.Destination.GetTypeUrl())
+}