[systemd-devel] Difference between restart versus stop-start

[Lennart Poettering]

On Fri, 15.01.16 03:41, Pathangi Janardhanan (path.jana at gmail.com) wrote:

> Hi,
> 
>  I am using sd_notify_pid_with_fds and subsequently sd_notify_fds to store
> and receive fds from systemd.
> 
>  Systemd differentiates between
> 
>  systemctl restart <service>  -> Where it stops and starts the service, but
> maintains the fd stored above
> 
>  whereas if I do systemctl stop <service> and then start, the fds are
> cleaned out.
> 
>  This works well for me, but my question is that apart from the fds, the
> service also stores local state and tries to restore them on start, and i
> need the same logic for that data also. i.e. if the service is being
> restarted than it stores that data otherwise it deletes and flsuhes them
> out.
> 
>  So my question is, is there any way for the service to know when it is
> being stopped that it is done for a restart versus stop command?
> 
>  Without that it seems difficult for the service to make the determination
> to maintain/restore state or flush/init state?

Hmm, one option could be to serialize that data into a deleted
temporary file, and pass the fd of that deleted file to PID 1. Then,
if PID 1 closes the file the file will go on disk too, but if not,
then your process will get the fd back and cna read it.

Never tried this, but I think this should work. Could that work for you?

Lennart

-- 
Lennart Poettering, Red Hat