[systemd-devel] GuessMainPID=no required to make daemon reload work

[Lennart Poettering]

On Mon, 05.05.14 10:00, Gerd v. Egidy (lists at egidy.de) wrote:

> > systemd will behave as expected: once your main process terminates it will
> > re-read PID
> > from this file (assuming that before dying your old process writes its
> > child's PID) and set it as MAINPID for your service.
> 
> Hmm. Currently it is done like this: the old daemon releases the lock on the 
> pidfile and terminates itself. The new daemon detects this and then writes it's 
> own pid to the pidfile and locks it.
> 
> So there is a short time where the old daemon is already dead and the new one 
> hasn't written it's pid yet. Probably this is the problem. But I have to think 
> about a way to get the locking stuff right as I can't easily transfer the lock 
> over an exec.

Generally, it's not a good idea to keep file locks for a longer period
of time... YOu should really just take them while you write a file, and
then release them, but not keep them forever...

systemd expects that binaries forked off with ExecStart= (if you use
Type=forking) or ExecReload= return only after the new daemon is fully
up and the PID file is written. THis isn't actually any different from
sysv except that sysv traditionally has been a bit more forgiving here,
since daemons where never monitored...

You need to have some synchronization logic in place that lets the
ExecStart= or ExecReload= binary know when the daemon process is done
with what it wants to do, and only then exits. A pipe of some kinda
should work fine, or a unix socket, or something like that...

Lennart

-- 
Lennart Poettering, Red Hat