Re: [PATCH] vm: Have qemu-image generate derivations instead.

[ludo@gnu.org (Ludovic Courtès)]

Jookia <166291@gmail.com> skribis:

> This small refactor should simplify some duplicated effort across functions and
> allow smarter qemu-image to do smarter things based on the operating system
> configuration rather than having each function that uses qemu-image pass
> selective parameters whenever new information is needed.

Damn, it’s been more than a month, please accept my apologies!

Overall this sounds like a good idea.  However…

> * gnu/system/vm.scm (qemu-image): Replace os-derivation, grub-configuration and
>   inputs parameters with os-configuration, base-inputs and extra-inputs.
>   (qemu-image): Based on base-inputs, generate grub.cfg and os-drv.
>   (system-disk-image, system-qemu-image, system-qemu-image/shared-store):
>   Pass in the operating system configuration and base-inputs to qemu-image
>   instead of derivations.

One of the reasons ‘os-drv’ and ‘grub.cfg’ are passed around is that
recomputing them is relatively costly.

There’s a solution to that in 4c2ade20c65e94c41dc8c65db73dd128343a0ad5
(in ‘wip-build-systems-gexp’; it memoizes ‘operating-system-derivation’
and others), so we could almost consider it solved.

Nevertheless it’d be nice to make sure performance remains reasonable
even in the absence of the above commit.

> +                     (base-inputs (list 'grub.cfg 'system))

[...]

> +  (mlet* %store-monad ((os-drv   (operating-system-derivation os-configuration))
> +                       (grub.cfg (operating-system-grub.cfg os-configuration))
> +                       (inputs -> (append
> +                                    (if (member 'grub.cfg base-inputs)
> +                                      `(("grub.cfg" ,grub.cfg)) '())
> +                                    (if (member 'system base-inputs)
> +                                      `(("system" ,os-drv)) '())

Use of “magic” values like 'grub.cfg here is undesirable IMO, because it
introduces singularities in the API, and generally makes the interface
non-obvious.

So I think I’d leave it up to the caller to pass

  #:inputs `(("grub.cfg" ,grub.cfg)) 

Same for "system".

All in all, I sympathize with the desire to avoid passing OS-DRV and
GRUB.CFG around, but I’m not convinced that this approach can improve
the situation.

WDYT?

Thank you!

Ludo’.