[systemd-devel] Revisiting the "ExecRestart" issue

[Brandon Black]

On Sat, Mar 29, 2014 at 9:09 PM, Michael Scherer <misc at zarb.org> wrote:

> Le vendredi 28 mars 2014 à 12:12 -0500, Brandon Black a écrit :
> > 4) Socket Activation! I know this is what some will scream when they
> > skim the above, but it's not a realistic solution in this case for a
> > few reasons:
> >     a) The startup delay, in some cases, can be many whole wallclock
> > seconds.  This is necessary and acceptable in the general sense (this
> > is network service that people use with large server-side
> > installations, not a desktop thing).
>
> It only occurs on the first start, no ?


No, these delays (well, for configurations large enough to involve
substantial delays) will happen on every fresh start, include "restart"
starts.  This means the sequential stop->start that systemd wants to do is
always going to give an availability gap where no daemon is processing
requests for a while.  Socket activation would keep the sockets open during
that window, but the buffers would just overflow anyways and/or the
eventual responses would be way too late to matter.  The command I want to
execute for ExecRestart doesn't have this issue because it knows how to
coordinate with itself for overlapping, so that the expensive "start"
operations happen before "stop".


> > socket "label" that could be accessed by the daemon via sd_* APIs to
> > distinguish would be useful here?
>
> You can use getsockopt to get some information, and match the port/type
> to the appropriate structure.
> See https://trac.torproject.org/projects/tor/ticket/8908 for a patch
> doing that kind of thing for tor.
>

What I was trying to say (perhaps very unclearly): there might be
distinctions between the many sockets which getsockname() does not capture.
 For a generic example: the daemon may allow the user to configure 0->N TCP
sockets for HTTP and 0->M other TCP sockets for HTTPS.  The user gets to
choose arbitrary port numbers for them all.  getsockname() is going to show
me M+N TCP sockets on arbitrary ports, but how does the information about
which was meant for which role get from user -> service unit -> actual
daemon code?
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