In [[ https://secure.phabricator.com/diffusion/PHU/browse/master/src/daemon/PhutilDaemonOverseer.php;cd5e097be808b65267e293ade3c13ed45ca183f7$294 | PhutilDaemonOverseer.php ]] `STDOUT` and `STDERR` are flushed and closed explicitly. This errors when they are already closed.
There are no "good" ways of detecting them, however. The simplest fix is prepending `@` which is ugly.