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Re: Seeking guidance regarding system roll-back and switch-generation

From: Ludovic Courtès
Subject: Re: Seeking guidance regarding system roll-back and switch-generation
Date: Fri, 22 Jul 2016 17:49:18 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)


Chris Marusich <address@hidden> skribis:

> Thank you very much for taking the time to respond!  Now I have a better
> idea of how to proceed.

Sorry for the delay!  That’s what you get for asking difficult
questions.  ;-)

> address@hidden (Ludovic Courtès) writes:
>> Chris Marusich <address@hidden> skribis:
>>> I've noticed that the GuixSD mechanism is different from the NixOS
>>> mechanism.  In particular, NixOS uses an "install-grub" script (which is
>>> specific to each system generation) to install grub, but GuixSD does
>>> not.  Is this difference intentional?
>> Looking at
>> <>,
>> part of it seems to be concerned with the generation of grub.cfg, which
>> is what (gnu system grub) does.
>> It also does a couple more things, such as providing proper EFI support,
>> and avoiding reinstalling GRUB when possible (whereas ‘guix system
>> reconfigure’ currently reruns ‘grub-install’ each time, even when it’s
>> not strictly needed.)
>> So I don’t think it’s very different, after all.  Or am I missing
>> something?
> Yes, they both install grub, but the mechanism seems different.  NixOS
> generates a script for each system generation, which installs exactly
> the right grub for that generation.  Unless I'm mistaken, GuixSD does
> not do that.  Instead, the existing GuixSD mechanism seems to require an
> operating system configuration file.  GuixSD uses that to determine the
> correct grub, which it then installs.

Sure, but it seems to be equivalent in the end.

>> Switching to a generations primarily means: (1) running the target’s
>> activation script, (2) updating Shepherd services, and (3) updating
>> grub.cfg.
> In addition to (3), don't we also need to install the grub software
> itself?

In general yes (there are cases where this is not needed, but let’s
ignore them.)

>> Of these (1) and (3) are currently easy to do on GuixSD.  (Right? :-))
> For both (1) and (3), the current GuixSD mechanism appears to require an
> operating system configuration file.

The output of ‘guix system build’ contains the ‘parameters’ file, which
is enough to generate grub.cfg (see ‘previous-grub-entries’ in (guix
scripts system)).

However, the activation script is indeed missing.  We can add it to the
output of ‘guix system build’ by extending ‘system-service-type’:

diff --git a/gnu/services.scm b/gnu/services.scm
index 5479bfa..fc3e17e 100644
--- a/gnu/services.scm
+++ b/gnu/services.scm
@@ -352,11 +352,18 @@ ACTIVATION-SCRIPT-TYPE."
 (define (second-argument a b) b)
+(define (gexps->activation-system-entry gexps)
+  "Return a directory entry to add to the result of the 'system' derivation."
+  (mlet %store-monad ((script (activation-script gexps)))
+    (return `(("activate" ,script)))))
 (define activation-service-type
   (service-type (name 'activate)
                  (list (service-extension boot-service-type
-                                          gexps->activation-gexp)))
+                                          gexps->activation-gexp)
+                       (service-extension system-service-type
+                                          gexps->activation-system-entry)))
                 (compose append)
                 (extend second-argument)))
This way we have direct access to each generation’s activation script
and we should be fine with (3).


> Yes, I think that makes sense.  I need to learn more about GuixSD's
> activation mechanism, but it makes sense that a procedural definition
> (e.g., like in NixOS's switch-to-configuration script) is equivalent to
> a declarative definition.  I think a declarative definition of services
> would be fine.  Concretely speaking, is there a specific format that is
> convenient to use for such a "declarative definition of services"?

I would suggest an sexp as discussed at:

> What about the grub installation?

The ‘parameters’ file should be enough, as you wrote.



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