Re: RFC: subcommand to pause/resume builds

[Bengt Richter]

Hi all,

On +2020-11-03 14:53:07 +0100, Ludovic Courtès wrote:
> Hi,
> 
> John Soo <jsoo1@asu.edu> skribis:
> 
> > I was looking to pause a long build today and asked on IRC how to
> > accomplish pause/resume.  It seems this is possible already with the
> > following:
> >
> > kill --signal SIGSTOP|SIGCONT {pids-of-build-process-tree}
> >
> > There is already a command to list the processes associated to guix
> > commands: guix processes.  Perhaps pause/resume can be a subcommand or
> > set of flags to guix processes. The following is the first thing that
> > comes to mind:
> >
> > guix processes --pause package-name ... --resume package-name ...
> >
> > What do you think?
> 
> First, note that the daemon is unaware of “packages”, it only knows
> about “derivations”.
>

What if you turned the problem inside out, and made the derivation
volunteer to be paused (at sensible places in its progress)?

I.e., if you defined part of a given derivation 
"hash-prefixed-derivation-file-name-in-question.drv"
to do a check for the existence of

    /var/guix/daemon-ctl/hash-prefixed-derivation-file-name-in-question.drv.sfx
    (.sfx appended to signify special effects :) 

ISTM that could open the door for some easy hacks, (and probably some dangers 
to watch for :)
E.g. a proof of concept might be just to sleep 6 seconds (say) and repeat 
sleep/check
until the file disappears.

IWG this should not change anything for non-volunteering derivations other than 
the load-relief
of not running the sleeping process(es).

Then the person wanting to pause the derivation 
"hash-prefixed-derivation-file-name-in-question.drv"
could do so simply by
    touch  
/var/guix/daemon-ctl/hash-prefixed-derivation-file-name-in-question.drv.sfx
and deleting it when wanting to allow it to continue.

Later, if that works, .sfx files could have content, for as yet unimagined 
purposes ;)

[...]
> 
> Conclusion: I don’t think we can implement this reliably.
>

IDK from the outside, but inside-out, WDYT?

> HTH!
> 
> Ludo’.
> 

-- 
Regards,
Bengt Richter