Please make sure there's a way to run this in the old way. systemd automatically manages daemons so the old way of daemonizing processes isn't required on systemd-based systems.

I'm generally OK with this but it will break existing installs (by changing how aphlict is invoked) so I'd like to hold it until the next notification phase is a little further along and we can hopefully just do all the breaks at once with supporting documentation and such.

@hach-que, do you use systemd for the normal daemons (vs aphlict)? This is very similar to phd now. Is there something missing that allows phd to work but not aphlict?

At the moment phd is configured under systemd to have Type=forking, which is a fully supported mode and still works fine. Obviously having daemons not double fork is better architecturally and makes the daemon code much more simple (most normal services I configure under systemd I use a foregrounding option if they have it). Since all major Linux distributions are either already using systemd or are planning to move to it, I'm not sure how much sense it makes to add double fork logic / self management of PID files, etc these days.

In phd's case it has to fork multiple processes anyway, so it mostly makes sense as something that forks anyway (as an aside, there is a way that phd could use systemd to manage the daemons it spawns, and probably simplify a bunch of Phabricator's daemon management by doing so).

Can you just use Type=forking for this, too, then?

Managing our own daemons currently seems to work well for most users (the only complaint I'm aware of is that there's no standard way to get them started on system startup). It's also relatively easy to support, because you can't really misconfigure phd, I know how phd works, commands like phd status can print out exactly the right information, having control over the workflow lets us do things like freeing leases on phd start (D9256), we can manage which daemons start based on configuration, we can adjust how daemons start easily (e.g., with debug flags), and detect hung processes with a well-known heartbeat signal. I think these kinds of things would become far more challenging and complex if we relied on systemd instead, and it would only really solve one relatively small problem (daemon startup on machine start).

I can add Type=forking for this as well.

Okay, cool.

I do want to hold this for a bit so we don't have to hit users with two "a bunch of stuff changed, go read the documentation again" issues in short succession, and AFAIK this doesn't unblock anything on its own, but let's plan to move forward with this once T4324 is far enough along to justify a doc update.

If we wanted to land this earlier, we could just (temprarily) restore the existing functionality by adding something like this to aphlict_launcher.php:

if (!$argv) {
  $argv = array('start');
} 
if ($argv == array('--foreground')) { // or similar
  $argv = arraay('debug');
}