func diffFile(sctx *wsync.Context, dctx *DiffContext, blockLibrary *wsync.BlockLibrary, reader io.Reader, opsWriter wsync.OperationWriter, preferredFileIndex int64, errs chan error, done chan bool) {
err := sctx.ComputeDiff(reader, blockLibrary, opsWriter, preferredFileIndex)
if err != nil {
errs <- errors.Wrap(err, 1)
}
done <- true
}
