[systemd-devel] Services with multiple pre-requisites

[freedesktop at priatel.co.uk]

Perhaps I'm missing something, but that's still not doing what I expect.

Here's what I have...

#----- /etc/systemd/system/first.target -----#
[Unit]
Description=Started first
Wants=third.service

#----- /etc/systemd/system/second.target -----#
[Unit]
Description=Started second
Wants=third.service

#----- /etc/systemd/system/third.service -----#
[Unit]
Description=Third service
Requisite=first.target second.target

[Service]
Type=simple
ExecStartPre=/usr/bin/logger -t "FooBar" -p daemon.notice "%P is starting"
ExecStart=/tmp/third.sh
Restart=always
ExecStop=/usr/bin/logger -t "FooBar" -p daemon.notice "%P has stopped"


#----- /tmp/third.sh -----#
#!/bin/bash
count=1
while (( count > 0 ))
do
  /usr/bin/logger -i -t "FooBar" -p daemon.notice "Counter: $count"
  (( count++ ))
  sleep 5
done

And here's what I observe happening. When I start *either* first.target or
second.target, the third.service is started. That's not what I expect or
want though. The third.service should be requisite on **both** first.target
and second.target.

localhost# systemctl start first.target

2020-12-21T17:03:51.039814+00:00 localhost systemd[1]: Starting Third
service...
2020-12-21T17:03:51.041827+00:00 localhost FooBar: Third service is starting
2020-12-21T17:03:51.042451+00:00 localhost systemd[1]: Started third
service.
2020-12-21T17:03:51.045033+00:00 localhost systemd[1]: Reached target
Started first.
2020-12-21T17:03:51.045114+00:00 localhost systemd[1]: Started second is not
active.
2020-12-21T17:03:51.045843+00:00 localhost FooBar[29048]: Counter: 1
2020-12-21T17:03:56.048463+00:00 localhost FooBar[644]: Counter: 2
2020-12-21T17:04:01.050951+00:00 localhost FooBar[4881]: Counter: 3

localhost# systemctl stop first.target
...
localhost# systemctl start second.target

2020-12-21T17:04:47.544840+00:00 localhost systemd[1]: Starting Third
service...
2020-12-21T17:04:47.546885+00:00 localhost FooBar: Third Service is starting
2020-12-21T17:04:47.555459+00:00 localhost systemd[1]: Started third
service.
2020-12-21T17:04:47.555556+00:00 localhost FooBar[3938]: Counter: 1
2020-12-21T17:04:47.555666+00:00 localhost systemd[1]: Started first is not
active.
2020-12-21T17:04:47.555730+00:00 localhost systemd[1]: Reached target
Started second.
2020-12-21T17:04:52.553867+00:00 localhost FooBar[8255]: Counter: 2
2020-12-21T17:04:57.554741+00:00 localhost FooBar[12157]: Counter: 3

localhost# systemctl stop second.target

Note particularly the `systemd` logs:

2020-12-21T17:03:51.045114+00:00 localhost systemd[1]: Started second is not
active.
2020-12-21T17:04:47.555666+00:00 localhost systemd[1]: Started first is not
active.

It has noticed and logged that one of the Requisite targets for the Third
service isn't active, but it starts it anyway :-(

That seems to go directly against the documented behaviour (at
https://www.freedesktop.org/software/systemd/man/systemd.unit.html#Requisite
=):
  | Requisite=
  | 
  |     Similar to Requires=. However, if the units listed here are not
  |     started already, they will not be started and the starting of
  |     this unit will fail immediately.

Is this a bug, or am I still missing something?

Martin

-----Original Message-----
From: Lennart Poettering <lennart at poettering.net> 
Sent: 21 December 2020 15:32
To: freedesktop at priatel.co.uk
Cc: systemd-devel at lists.freedesktop.org
Subject: Re: [systemd-devel] Services with multiple pre-requisites

On Mo, 21.12.20 14:43, freedesktop at priatel.co.uk (freedesktop at priatel.co.uk)
wrote:

> Hi
>
> I have two "primary" services running on a Linux server with `systemd` 
> that may be started at approximately the same time, or could just as 
> easily be started days apart. When started, they take a few seconds to
reach "ready"
> state, and can signal that readiness however I choose.
>
> I have a third (and fourth, fifth, etc...) service which:
>   * must not start before both Service 1 & Service 2 are ready
>   * must start as soon as possible once both Service 1 & Service 2 are
ready
>   * must be stopped as soon as possible if either Service 1 or Service 
> 2 terminates

Did I get this right: service 3 is not supposed to pull in services 1
+ 2, but just be activated as side-effect of those services being
activated? i.e. the activation of service 1 and 2 need to happen first and
they are the trigger for service 3, but besides that noone ever asks for
service 3?

If so, it appears to me a Wants= dep from services 1 and 2 on 3 would make
sense, plus a Requisite= dep in the other direction.

Lennart

--
Lennart Poettering, Berlin