[systemd-devel] PIDFile creation logic

[Andrei Borzenkov]

On 18.10.2021 23:08, Silvio Knizek wrote:
> Am Montag, dem 18.10.2021 um 12:43 -0700 schrieb Kenneth Porter:
>> I just installed the new-to-EPEL ndppd service and am seeing this in my log:
>>
>> Oct 17 21:10:08 saruman systemd: Can't open PID file
>> /var/run/ndppd/ndppd.pid (yet?) after start: No such file or directory
>>

That is just an information. May be it could be downgraded to debug, at
least initially.

>> Examining the source, I see that the pidfile is created by the child
>> process, not the parent. I'm guessing that systemd is expecting the pidfile
>> to exist when the parent exits? (I want to file the issue on the upstream
>> program and want to make sure I understand how this should work.)
>>
>> Source in question. Note how daemonize() forks and exits and main() invokes
>> daemonize and then creates the pidfile. I'm thinking that daemonize()
>> should create the pidfile before it invokes exit().
>>
>> <https://github.com/DanielAdolfsson/ndppd/blob/master/src/ndppd.cc>
>>
> Hi,
> 
> just don't demonize and don't use a PIDFile= at all. systemd is
> actually quite apt in figuring out which process is the right main one.

It is not about "main process". It is about notifying systemd that your
service is ready and systemd can proceed with After dependencies.
Without PIDFile your service is "ready" as soon as it forked. This most
likely is not correct.

Whether service developers are actually careful to only create PID file
at the right time is unknown.

If nothing depends on service availability Type=simple is easier.

> Also, the ndppd 0.2.5 release is 10 years old and doesn't look
> maintained anymore.
> OTOH, systemd-networkd itself has inbuilt NDPProxy capabilities.
> 
> BR
> Silvio
>