leases: support resource management

Add three methods for lease service so that the client can use it to
manage the resource by lease, not just gc.root label. With the following
methods, it is easy for client to maintain their own cache system.

```
 - AddResource(context.Context, Lease, Resource) error
 - RemoveResource(context.Context, Lease, Resource) error
 - ListResources(context.Context, Lease) ([]Resource, error)
```

And the resource is to be

```golang
type Resource {
  ID   string
  Type string
}
```

For the snapshots, the Type field will be formatted by
snapshots/%{type}, like snapshots/overlayfs.

fix: #3295

Signed-off-by: Wei Fu <fuweid89@gmail.com>

services/leases/local.go

@@ -107,3 +107,27 @@ func (l *local) List(ctx context.Context, filters ...string) ([]leases.Lease, er
 	}
 	return ll, nil
 }
+
+func (l *local) AddResource(ctx context.Context, lease leases.Lease, r leases.Resource) error {
+	return l.db.Update(func(tx *bolt.Tx) error {
+		return metadata.NewLeaseManager(tx).AddResource(ctx, lease, r)
+	})
+}
+
+func (l *local) DeleteResource(ctx context.Context, lease leases.Lease, r leases.Resource) error {
+	return l.db.Update(func(tx *bolt.Tx) error {
+		return metadata.NewLeaseManager(tx).DeleteResource(ctx, lease, r)
+	})
+}
+
+func (l *local) ListResources(ctx context.Context, lease leases.Lease) ([]leases.Resource, error) {
+	var rs []leases.Resource
+	if err := l.db.View(func(tx *bolt.Tx) error {
+		var err error
+		rs, err = metadata.NewLeaseManager(tx).ListResources(ctx, lease)
+		return err
+	}); err != nil {
+		return nil, err
+	}
+	return rs, nil
+}

services/leases/service.go

@@ -113,6 +113,56 @@ func (s *service) List(ctx context.Context, r *api.ListRequest) (*api.ListRespon
 	}, nil
 }
 
+func (s *service) AddResource(ctx context.Context, r *api.AddResourceRequest) (*ptypes.Empty, error) {
+	lease := leases.Lease{
+		ID: r.ID,
+	}
+
+	if err := s.lm.AddResource(ctx, lease, leases.Resource{
+		ID:   r.Resource.ID,
+		Type: r.Resource.Type,
+	}); err != nil {
+		return nil, errdefs.ToGRPC(err)
+	}
+	return &ptypes.Empty{}, nil
+}
+
+func (s *service) DeleteResource(ctx context.Context, r *api.DeleteResourceRequest) (*ptypes.Empty, error) {
+	lease := leases.Lease{
+		ID: r.ID,
+	}
+
+	if err := s.lm.DeleteResource(ctx, lease, leases.Resource{
+		ID:   r.Resource.ID,
+		Type: r.Resource.Type,
+	}); err != nil {
+		return nil, errdefs.ToGRPC(err)
+	}
+	return &ptypes.Empty{}, nil
+}
+
+func (s *service) ListResources(ctx context.Context, r *api.ListResourcesRequest) (*api.ListResourcesResponse, error) {
+	lease := leases.Lease{
+		ID: r.ID,
+	}
+
+	rs, err := s.lm.ListResources(ctx, lease)
+	if err != nil {
+		return nil, errdefs.ToGRPC(err)
+	}
+
+	apiResources := make([]api.Resource, 0, len(rs))
+	for _, i := range rs {
+		apiResources = append(apiResources, api.Resource{
+			ID:   i.ID,
+			Type: i.Type,
+		})
+	}
+	return &api.ListResourcesResponse{
+		Resources: apiResources,
+	}, nil
+}
+
 func leaseToGRPC(l leases.Lease) *api.Lease {
 	return &api.Lease{
 		ID:        l.ID,