[bug#36952] [PATCH v2] machine: Implement 'roll-back-machine'.

[Ricardo Wurmus <rekado@elephly.net>]

Hi Jakob,

> +(define (roll-back-managed-host machine)
> +  "Internal implementation of 'roll-back-machine' for MACHINE instances with
> +an environment type of 'managed-host."
> +  (define remote-exp
> +    (with-extensions (list guile-gcrypt)
> +      (with-imported-modules (source-module-closure '((guix config)
> +                                                      (guix profiles)))
> +        #~(begin
> +            (use-modules (guix config)
> +                         (guix profiles))
> +
> +            (define %system-profile
> +              (string-append %state-directory "/profiles/system"))
> +
> +            (define target-generation
> +              (relative-generation-spec->number %system-profile "-1"))

Can we use “relative-generation” or “previous-generation-number” here?
I think the stringified “-1” is kinda ugly, and the “*-spec” procedure
only exists to handle user input, which is provided as a string.

> +  (mlet* %store-monad ((boot-parameters (machine-boot-parameters machine))
> +                       (_ -> (if (< (length boot-parameters) 2)
> +                                 (raise roll-back-failure)))
> +                       (entries -> (map boot-parameters->menu-entry
> +                                        (list (second boot-parameters))))
> +                       (old-entries -> (map boot-parameters->menu-entry
> +                                            (drop boot-parameters 2)))
> +                       (bootloader -> (operating-system-bootloader
> +                                       (machine-operating-system machine)))
> +                       (bootcfg (lower-object
> +                                 ((bootloader-configuration-file-generator
> +                                   (bootloader-configuration-bootloader
> +                                    bootloader))
> +                                  bootloader entries
> +                                  #:old-entries old-entries)))
> +                       (eval -> (cut machine-remote-eval machine <>))
> +                       (remote-result (machine-remote-eval machine
> +
> remote-exp)))

Is it on purpose that you aren’t using the previously defined “eval”
here?

--
Ricardo