Re: Expanding service procedure to accept additional arguments

[Ryan Sundberg]

Hi Bruno,

I have written some useful macros for transforming services as you describe 
here. You may find them useful:

https://lab.arctype.co/poseidon/poseidon-os/-/blob/master/poseidon/services/base.scm

Sincerely,
Ryan Sundberg

Dec 28, 2022 8:48:15 AM mirai <mirai@makinata.eu>:

> It is occasionally desired to have a service depend on additional shepherd 
> services
> than the defaults listed in their definition.
> 
> Examples where this can be seen is the shepherd-requirement field provided by
> nginx-configuration and opensmtpd-configuration, but these fields are 
> record-type
> specific and not available for the other service types (unless patched in).
> 
> An alternative to patching the original record-type is to define a custom 
> service-type
> record-type and inheriting it which is somewhat clunky for what amounts to a 
> setup-specific
> single line change:
> 
> --8<---------------cut here---------------start------------->8---
>   (shepherd-service
>     ...
> -   (requirement `(...))
> +   (requirement `(... ,@shepherd-requirement))
> --8<---------------cut here---------------end--------------->8---
> 
> Another type of extensibility that should be considered is the capability to 
> add extra
> service-extensions to existing service-type records.
> These can be useful for setup-specific (i.e. not applicable in general to be 
> included with Guix) scenarios,
> for example, to add an extra activation-service-type extension or have a 
> special-files-service-type
> extension that the service-type usually does not have.
> 
> The current approach that I'm aware of to achieve something similar is through
> simple-service, drawbacks being extra boilerplate code and a "disjoint" 
> service
> whose connection to the "parent service" is not immediately apparent.
> 
> --8<---------------cut here---------------start------------->8---
> ;; Ideally this should go in radicale-service-type
>           (simple-service 'chmod-radicale shepherd-root-service-type
>                           (list (shepherd-service (requirement '(user-homes))
>                                                   (provision '(radicale-home))
>                                                   (documentation "chmod g+rwx 
> to user 'radicale' home.")
>                                                   (one-shot? #t)
>                                                   (start #~(lambda _
>                                                              (chmod 
> (passwd:dir (getpw "radicale")) #o770)
>                                                              #t)))))
> --8<---------------cut here---------------end--------------->8---
> 
> 
> Instead, what if the service procedure could be changed to accept optional 
> keyword arguments?
> Then one could express the extensions and dependencies as:
> 
> --8<---------------cut here---------------start------------->8---
> (services
>   (service foo-service-type
>            #:requirements '(networking nscd smtpd)
>            #:extensions (list (service-extension bar-service-type (lambda _ 
> ...))
>                               (service-extension activation-service-type 
> (lambda _ ...))))
> --8<---------------cut here---------------end--------------->8---
> 
> Thoughts?
> 
> 
> Regards,
> Bruno