[systemd-devel] Antw: Re: Unexplainable unit restart ("Start request repeated too quickly")

[Michael Chapman]

On Tue, 4 Jun 2019, Ulrich Windl wrote:
> >>> Michael Chapman <mike at very.puzzling.org> schrieb am 03.06.2019 um 13:14 in
> Nachricht <alpine.LFD.2.21.1906032106200.3180 at beren.home>:
> [...]
> > 
> > Um, OK. I don't think we're any closer to solving your problem though. :-)
> 
> Actually I am!
> The root of the problem is that any oneshot service without 
> RemainAfterExit=true is listed as "inactive (dead)" after being started. 
> I think the manual page should be more clear on that fact! Then if you 
> have a dependency like this (best viewed with a monospaced font):
>   B
>  / \
> A-C-E
>  \ /
>   D
> 
> where A is a target that wants B,C,D, and each of those in turn wants E 
> (which is the oneshot service), the following happens: When starting A, 
> E is NOT started ONCE, but it is started THREE TIMES, while my 
> expectation was it will be started once only. As I had set a burst limit 
> of 1, an unexpected fauilure was reported.
> 
> What I'm unsure: Does systemd wait for each "start of E" to terminate 
> before calling the next one (i.e. serialized), or are the starts being 
> attempted in parallel?
> 
> The obvious deficit in systemd is that it does not no a proper 2-pass 
> dependency analysis before executing: If it would do a dependency 
> analysis for starting A, it would see that E has to be started ONCE 
> before B,C,D. In stead it seems to do a 1-pass analysis firing the start 
> of E every time.

This doesn't seem right to me, as systemd _does_ have the concept of 
transactions to which multiple jobs can be attached, and the set of jobs 
does appear to be determined in one go based upon the configured 
requirement dependencies.

So I thought I'd set up a test case. I created a set of Type=oneshot units 
with Wants= dependencies as in your diagram. There were no ordering 
dependencies (Before= or After=). Each service just echoed its own name.

Then:

  $ systemctl --user daemon-reload
  $ systemctl --user start A.service

yielded:

  systemd[2312]: Reloading.
  systemd[2312]: Starting B...
  systemd[2312]: Starting D...
  echo[21672]: B
  systemd[2312]: Starting E...
  echo[21673]: D
  systemd[2312]: Starting A...
  echo[21674]: E
  systemd[2312]: Starting C...
  echo[21675]: A
  systemd[2312]: Started B.
  systemd[2312]: Started D.
  echo[21676]: C
  systemd[2312]: Started E.
  systemd[2312]: Started A.
  systemd[2312]: Started C.

As you can see, even E.service was only started once.

Are you sure you were actually doing everything in one transaction?