[systemd-devel] Using systemd to as a FD Store to provide service while package upgrade

[Pathangi Janardhanan]

Hi,

I had a previous thread on some issues in using sd_pid_notify_with_fds, but
started this as a separate thread, as this is not just about that call,
which anyway seems to be because I am using a older version.

 I am trying to provide continuous service to connected clients (over TCP
socket) while my service package is being upgraded.

 In regard to that I had a few questions on what is the best way to handle
this:

1. When I initiate the upgrade, I have my service store the fds into
systemd using the call sd_pid_notify_with_fds.

    Currently in my package scripts I am doing a service stop and start.
But when I need to do an upgrade, if I do a service stop, systemd clears
all the fds. So would that mean that my package scripts would have to:

 a. In case of upgrade, do not call stop, but ensure that the fds are
saved, and then after the upgrade, ensure that we call a service restart?
Is it safe to remove/change the service files while it is executing?

 b. In case the service package is being removed, do a normal stop in the
scripts

   Is this approach o.k., or are there other recommendations?

2. I had also indicated in the other thread, that systemd does not seem to
clear the fds, even if the fds are actually closed. So to repeat, the
sequence is

   a. First invocation of the service, start servicing the clients, and
then store the fds in systemd
    b. Do a restart, the new instance gets these fds, and continues to
service the clients. things work o.k.
    c. the client closes the connection, the service also closes all its
state and connection about this client.
    d. I do another restart and since at this time there is no client fds,
the service does not store anything with  systemd
    e. In the new invocation of my service, in sd_listen_fds, I still get
the older fd that I had stored.
    f. currently in my service, this is not a problem, since when my
service tries to do a select on these fds, it gets a notification and then
closes out the fd.

    But this behaviour could cause some issues later on, and also since
systemd does not seem to ever let go of the fds, If i continue to do many
restarts, would the set of fds build up to a large set in systemd?

    What is the correct approach to handle this situation?

3. Is Systemd intended for this type of fd store for upgrade handling, and
if others have any other approaches or limitations with this approach, that
would be welcome?

Thanks
Jana
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