add shepherd requirement to an existing service?

add shepherd requirement to an existing service?

[Remco van 't Veer]

Hi,

I'd like to add a shepherd requirement to an existing service, as
provided by guix, to prevent it from starting before some other service
starts.

For exampple: I have NAS which is very slow to start after a power
failure so I created a simple service to keep trying to mount an NFS
share until it succeeds.  This share contains files shared through
syncthing, so I want syncthing to start after the NFS service is
started.

Is there any way to do this apart from defining my own
syncthing-shepherd-service which includes the extra shepherd
requirement?

Kind regards,
Remco

Re: add shepherd requirement to an existing service?

[Oleg Pykhalov]

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Hi,

Remco van 't Veer <remco@remworks.net> writes:

> I'd like to add a shepherd requirement to an existing service, as
> provided by guix, to prevent it from starting before some other service
> starts.
>
> For exampple: I have NAS which is very slow to start after a power
> failure so I created a simple service to keep trying to mount an NFS
> share until it succeeds.  This share contains files shared through
> syncthing, so I want syncthing to start after the NFS service is
> started.

How do you mount the NFS share? From my understanding, system services
typically wait for all mounts listed in 'file-systems' to be mounted
unless 'mount?' is specifically set to false.

> Is there any way to do this apart from defining my own
> syncthing-shepherd-service which includes the extra shepherd
> requirement?

To my knowledge, unless the service definition explicitly allows for it,
there may not be an alternative approach aside from creating a custom
syncthing-shepherd service that incorporates the additional shepherd
requirement.

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Re: add shepherd requirement to an existing service?

[Remco van 't Veer]

Hi Oleg,

2024/03/11, Oleg Pykhalov:

>> I'd like to add a shepherd requirement to an existing service, as
>> provided by guix, to prevent it from starting before some other service
>> starts.
>>
>> For exampple: I have NAS which is very slow to start after a power
>> failure so I created a simple service to keep trying to mount an NFS
>> share until it succeeds.  This share contains files shared through
>> syncthing, so I want syncthing to start after the NFS service is
>> started.
>
> How do you mount the NFS share? From my understanding, system services
> typically wait for all mounts listed in 'file-systems' to be mounted
> unless 'mount?' is specifically set to false.

I tried the file-systems declaration about a year ago but found it
didn't work all the time when automatically booting after a power
failure.  There's also routers and stuff booting at the same time so
the order of things may have been an issue.  Anyway, a simple service
trying to mount until it succeeds works pretty reliably.

>> Is there any way to do this apart from defining my own
>> syncthing-shepherd-service which includes the extra shepherd
>> requirement?
>
> To my knowledge, unless the service definition explicitly allows for it,
> there may not be an alternative approach aside from creating a custom
> syncthing-shepherd service that incorporates the additional shepherd
> requirement.

I was afraid so.  There may be other situations where I'd like some
home grown service to startup before another pre-existing one, so maybe
this should be a feature request, if others are also running into this.

Re: add shepherd requirement to an existing service?

[Richard Sent]

If this winds up being something you do frequently, it may be possible
to write a function to modify the service type. See [1] as an example. I
suspect your code would be smaller, something like

--8<---------------cut here---------------start------------->8---
(define (depend-on type new-requirement)
  "Returns a @code{service-type} record derived from @var{type} with any
  shepherd extensions additionally depending on @var{new-requirement}."
  (service-type
   (inherit type)
   (extensions
    (map (lambda (extension)
           (let ((target (service-extension-target extension))
                 (compute (service-extension-compute extension)))
             (if (eq? target shepherd-root-service-type)
                 (service-extension shepherd-root-service-type
                                    (lambda (config)
                                      (map (lambda (service)
                                             (shepherd-service
                                              (inherit service)
                                              (requirement
                                               (cons*
                                                new-requirement
                                                (shepherd-service-requirement config)))))
                                           (compute config))))
                 extension)))
         (service-type-extensions type)))))
--8<---------------cut here---------------end--------------->8---

This code might not be it exactly, but something similar to this
/should/ work.

[1] https://git.sr.ht/~freakingpenguin/rsent/tree/master/item/channel/rsent/utils/services.scm


-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.

Re: add shepherd requirement to an existing service?

[Remco van 't Veer]

That's exactly what I need!  Thanks!