bug#56441: guix time-machine broken by profiles speed-up

[Ricardo Wurmus <rekado@elephly.net>]

Ricardo Wurmus <rekado@elephly.net> writes:

> zimoun <zimon.toutoune@gmail.com> writes:
>
>> Bug#55499 [1] is fixed by 4ff12d1de7cd617b791996ee7ca1240660b4c20e.
>> However, because the manifest version is going from 3 to 4 with new
>> fields, the new Guix cannot builds the old Guix.
>>
>> Commit 4ff12d1de7cd617b791996ee7ca1240660b4c20e is not able to go to its
>> parent 9b8c442b254b82196fe2492142b3c3bbbd891a1b.
>>
>> $ git rev-parse 4ff12d1de7^
>> 9b8c442b254b82196fe2492142b3c3bbbd891a1b
>>
>> $ guix time-machine --commit=4ff12d1de7 -- time-machine --commit=9b8c442b25 -- help
>> Updating channel 'guix' from Git repository at 'https://git.savannah.gnu.org/git/guix.git'...
>> Computing Guix derivation for 'x86_64-linux'... \
>> The following derivation will be built:
>>   /gnu/store/r5qk23fibxn5ryd2k7b8qkbryqv4m3ds-profile.drv
>>
>> building package cache...
>> |builder for
>> `/gnu/store/19nk2x26s0dp68r7d36ifbg0ck0q3xps-guix-package-cache.drv'
>> failed to produce output path
>> `/gnu/store/axqgrls563slnp76x60dqlv7sdwcm2ly-guix-package-cache'
>> build of /gnu/store/19nk2x26s0dp68r7d36ifbg0ck0q3xps-guix-package-cache.drv failed
>> View build log at '/var/log/guix/drvs/19/nk2x26s0dp68r7d36ifbg0ck0q3xps-guix-package-cache.drv.gz'.
>> cannot build derivation `/gnu/store/r5qk23fibxn5ryd2k7b8qkbryqv4m3ds-profile.drv': 1 dependencies couldn't be built
>> guix time-machine: error: build of `/gnu/store/r5qk23fibxn5ryd2k7b8qkbryqv4m3ds-profile.drv' failed
>
> IIUC the problem here is that PACKAGE-CACHE-FILE, a profile hook running
> inside an inferior (= with an older Guix), is operating on a manifest
> that uses the new version 4 format.  That manifest was built with the
> *current* version of Guix that understands the version 4 format.
>
> PACKAGE-CACHE-FILE uses GEXP->DERIVATION-IN-INFERIOR and the inferior is
> a PROFILE that’s made from a given MANIFEST value.  That PROFILE is
> *not* built inside the inferior, so it doesn’t use the old manifest
> format.
>
> The cache generation is happening in
> /gnu/store/17v5781w8kl1snp826jl6z40z5lbbw1y-inferior-script.scm.drv, which — as the name indicates — is
> run inside the inferior, i.e. the older Guix.

It’s worse than that: an older Guix living in a new profile (i.e a
profile with a version 4 manifest) cannot describe itself, because it
cannot read the manifest.

I took the same profile that was generated by the time machine and
contains the old Guix.  Here’s the manifest:

--8<---------------cut here---------------start------------->8---
$ head /gnu/store/mwbgfyl0zzipyac1lbgss2gcji67fp4s-profile/manifest
;; This file was automatically generated and is for internal use only.
;; It cannot be passed to the '--manifest' option.
;; Run 'guix package --export-manifest' if you want to export a file
;; suitable for '--manifest'.

(manifest
  (version 4)
  (packages
    (("guix"
      "9b8c442"
--8<---------------cut here---------------end--------------->8---

And then:

--8<---------------cut here---------------start------------->8---
$ /gnu/store/mwbgfyl0zzipyac1lbgss2gcji67fp4s-profile/bin/guix describe
guix describe: error: unsupported manifest format
--8<---------------cut here---------------end--------------->8---

This old Guix lives in a new profile.  We must prevent this.  But how?

How about this:

Build Guix with only the 'guix channel (no any other custom channels
allowed), and then build the channel’s profile (with all requested
channels) inside an inferior of that plain Guix.  Since the inferior
Guix can only build profiles that it supports the resulting *profile*
will be compatible with the same Guix (and any additional channels).

-- 
Ricardo