&gt;null() == true</code>. If <code>N != target_payload_blob.size()</code>, then no further bytes can currently be read without blocking and should be tried later if desired. (Informally: In that case consider <code>Peer_socket::async_wait()</code> followed by retrying the present function, in that case.)</li>
<li>If non-null <code>err_code</code>, then <code>N == 0</code> is returned; and <code>*err_code == E</code> is set to the triggering problem. If null, then <code>flow::error::Runtime_error</code> is 