On newer systems, the upper hard limit for open file descriptors (see
`ulimit -H -n`) was increased from 4096 to 524288. Due to how python-daemon
closes potentially open file descriptors (basically stores them in a set,
removes those excluded by config, and loops through all of them), the updown
script was either killed immediately (by the OOM killer) or not ready yet
when updown events occurred.
Tobias Brunner