[skarnet]

There is no good solution for servers that, mistakenly, background themselves; the practice of backgrounding itself comes from a time where supervision was not as widely known as it is today, and nowadays there is no excuse for a daemon to fail to at least provide an option to avoid backgrounding itself.

The best thing to do is to contact the daemon's authors and insist they change that. In nginx's case, they think it's ok if they perform their own supervision, but it would be nice if they also could integrate with existing supervision systems.

In the meantime, as a workaround, the accepted practice for supervising forking daemons is known as "fghack": maintaining a process keeping track of the forking daemons via a few pipes that close when the daemons die. s6, for instance, provides such a tool: https://skarnet.org/software/s6/s6-fghack.html . AFAIK, so does nosh.