Defer layer download until unpack

Moves the content fetching into the unpack process
and defers the download until the snapshot needs it
and is ready to apply. As soon as a layer is reached
which requires fetching, all remaining layers are
fetched.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

pull.go

@@ -70,11 +70,6 @@ func (c *Client) Pull(ctx context.Context, ref string, opts ...RemoteOpt) (_ Ima
 		}
 		unpackWrapper, eg := u.handlerWrapper(ctx, &unpacks)
 		defer func() {
-			if retErr != nil {
-				// Forcibly stop the unpacker if there is
-				// an error.
-				eg.Cancel()
-			}
 			if err := eg.Wait(); err != nil {
 				if retErr == nil {
 					retErr = errors.Wrap(err, "unpack")