code: @Override
    public void set(final String name, final UserCredentials hostUser) throws IOException {
  
      assert this.proxyUGI == null;
      assert hostUser instanceof YarnProxyUser;
  
      LOG.log(Level.FINE, "UGI: user {0} copy from: {1}", new Object[] {name, hostUser});
  
      final UserGroupInformation hostUGI = ((YarnProxyUser) hostUser).get();
      final Collection<Token<? extends TokenIdentifier>> tokens = hostUGI.getCredentials().getAllTokens();
  
      this.set(name, hostUGI, tokens.toArray(new Token[tokens.size()]));
    }
docs: Set YARN user. This method can be called only once per class instance.
  @param name Name of the new proxy user.
  @param hostUser User credentials to copy. Must be an instance of YarnProxyUser.