code: @Override
      public Promise<Void> truncate(String path, long len) {
          return adapter.toPromise(handler -> vertx.fileSystem().truncate(path, len, handler));
      }
docs: Truncate the file represented by {@code path} to length {@code len} in bytes, asynchronously.
  <p>
  The operation will fail if the file does not exist or {@code len} is less than {@code zero}.
  
  @param path the path to the file
  @param len  the length to truncate it to
  @return a promise for completion