nixos:nixos-rebuild: allow switching to a specific generation number #105910

miallo · 2020-12-04T19:49:14Z

Motivation for this change

If the user wants to rollback to any of the system generations but the last, this is currently not easily doable (see #24374).
In order to unify the interface with nix-env a bit more, I propose to add a flag ``nixos-rebuild --generation` to easily go to a specific generation.

I had an issue, where I didn't notice a package breaking ~10 generations ago and while fixing it, I had to go back and forth multiple times. Since I didn't know about

sudo nix-env --switch-generation 12345 -p /nix/var/nix/profiles/system
sudo /nix/var/nix/profiles/system/bin/switch-to-configuration switch

I repeated the rollback n times.

This implementation would make it possible to run

sudo nixos-rebuild switch --generation 12345

Maintainers

@nbp added --rollback
@worldofpeace did the last edits
And I guess @edolstra ?

Things done

Tested manually on local system
Ensured that relevant documentation is up to date
Fits CONTRIBUTING.md.

nixos-discourse · 2020-12-05T20:53:13Z

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/394

miallo · 2021-01-03T14:10:41Z

I am not sure about having list-generations in nixos-rebuild, because it isn't strictly related to rebuilding. But on the other hand, having a seperate nixos-list-generations command feels a bit overkill as well, looking at the short list of existing nixos-* commands... On the other hand one could add parameters for formatting and so on, but I am not sure if that is really needed...

Any opinions @edolstra or others?

miallo · 2021-01-03T14:30:14Z

Many ideas for the list-generations come from the script that builds the grub-menu. Maybe it would be worth, having one script that produces a description for a given generation so that grub, as well as the list-generations can utilize it?

edolstra · 2021-01-04T12:30:30Z

Maybe this should be its own subcommand? I.e. nixos-rebuild switch-generation rather than switch --switch-generation, which seems a bit superfluous.

Or alternatively, nixos-rebuild [switch|test] --generation.

miallo · 2021-01-04T14:42:10Z

I totally agree with your suggestions and I was a bit unhappy with the doubling of the "switch" as well. The reason I chose it nevertheless is that I took the same flag as nix-env...
So far I only used switch and I don't know if others would want to test old generations. But given the fact that --rollback supports both, I guess having a similar interface would be good. I think the nixos-rebuild [switch|test] --generation sounds nice.

miallo · 2021-01-04T14:45:51Z

Any opinions about the list-generations subcommand? I added it to this PR, because with the option to switch to a specific generation I thought it would be nice to have an overview about the main things. I guess a (current) tag as in nix-env --list-generations might be usefull as well.

edolstra · 2021-01-04T15:19:13Z

I think it's okay to add it to nixos-rebuild for now. There is an issue somewhere about replacing nixos-rebuild with a nixos command that would resolve the fact that it's not related to building.

nixos/modules/installer/tools/nixos-rebuild.sh

miallo · 2021-01-06T17:50:41Z

I prepared a branch with two clean commits instead of this mess and I could use those instead. I guess it would be easier to read that way

miallo · 2021-01-15T08:17:13Z

Anyone wants to take a look and review this?

nixos/modules/installer/tools/nixos-rebuild.sh

nixos-discourse · 2023-09-26T16:37:19Z

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/2687

mairs8 · 2023-11-04T10:52:43Z

i thought that switching to specific generation is acheived by: /nix/var/nix/profiles/system-N-link/bin/switch-to-configuration switch where N is the generation to switch to.

Is this thread about adding a more convenient syntax, for example to switch to generation 5: nixos-rebuild switch --rollback 5 ?

miallo · 2023-11-05T01:34:41Z

i thought that switching to specific generation is acheived by: /nix/var/nix/profiles/system-N-link/bin/switch-to-configuration switch where N is the generation to switch to.

Yes, I think so. This PR is equivalent to

sudo nix-env --switch-generation N -p /nix/var/nix/profiles/system
sudo /nix/var/nix/profiles/system/bin/switch-to-configuration switch

because that is also the way it was implemented for --rollback. I am not 100% sure about the implications - it might just be the use of a couple of programs from the later (potentially broken) generation if I am not mistaken. But maybe this is indeed just too complex for this and the reason it was used for --rollback was not to duplicate the logic of "How to find the previous generation?" in nixos-rebuild and nix-env 🤔 Someone with a better understanding of the details should probably take a close look at it...

Is this thread about adding a more convenient syntax

Yes, I think having an easier syntax would be better than the current way (and also more discoverable, especially as soon as you add things like different profiles/specialisations).

for example to switch to generation 5: nixos-rebuild switch --rollback 5 ?

I considered your suggested syntax, but I think it is a bit ambiguous if you want to rollback to a specific generation (as is the intention here) or a relative amount (so in your case e.g. from generation 100 to 95). That is why I thought a different syntax is more precise and it is also the one Eelco suggested: #105910 (comment)

bzm3r

Everything here makes sense/looks good to me. I too would be interested in helping @miallo pushing this forward.

miallo · 2023-11-19T19:27:49Z

Everything here makes sense/looks good to me. I too would be interested in helping @miallo pushing this forward.

That would be nice!

I just pushed a rebase where the only change I made was the update of the nixos-rebuild list-generations output in nixos/doc/manual/administration/rollback.section.md since the format has changed between when I implemented it and wrote this doc and now...

ThinkChaos · 2023-11-27T04:25:39Z

I'd love to see this merged, the current manpage mentions switch --generation n which is not yet available so the situation is quite confusing!

miallo · 2023-11-27T09:59:42Z

the current manpage mentions switch --generation n which is not yet available so the situation is quite confusing!

Oh damn... -.- That one is totally on me. I extracted the list-generations feature into a separate MR and even though I was certain that I fixed the man-page in that MR at some point not to include that reference I must have messed it up...
I don't have a clue on how to push this MR, so in order to avoid confusion I'll remove that sentence in a different MR...

This is a hint that mentions the feature NixOS#105910 but that is not yet merged. This is very confusing and since it there is no progress with that merge we should remove the reference

Run `nixos-rebuild switch --generation 12345` to roll back to generation `12345`. This makes rolling back to an older generation easier Also updated the release notes.

miallo · 2023-12-04T16:29:17Z

I just added some tests. This was the first time I wrote any nixos tests and so this is just an adaptation of tests/nixos-rebuild-specialisations.nix - but I don't really know if this can be simplified any more...

@Mic92 @SuperSandro2000 What else do you think is necessary for this PR?

@SuperSandro2000 you mentioned a few things you wanted to do in #105910 (review)

SuperSandro2000

lets only use one style of test at least in the changed code

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

Previously there was a mixture of `[` and `[[`. That is a bit unnecessary and even if there is a mixture in the rest of the code, this PR should stick to one of the versions... Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

SuperSandro2000 · 2023-12-04T22:31:45Z

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

+      --generation)
+        if [ $# -eq 0 ]; then
+            log "Must provide a generation number"
+            log "Usage: \`$(basename "$0") ${action:-switch} --generation <NUMBER>'"


Why do I need to give an action when I just want to switch to a generation?

We don't know if you want to switch, boot or test said generation, so you have to tell us what you want to do with it...

but if I want to switch to an older generation, why does it start to build the current config file from disk which might be newer than the current generation?

That is interesting - I don't think this happened to me before 🤔 Where did you see this behavior? If it is in the tests: the first three tests don't actually do the --generation, but they create the generations, so there it is expected that they build the config files...

I know it isn't a good answer, but: I don't really know what I am doing (at least not 3 years after I first wrote this patch). I just adapted the code for the --rollback since I thought they would be more or less identical... I have forgotten the few things I knew about the nix-env stuff and the rest on how this is activated 🙈 If we just wanted to activate the generation and don't care about the action I guess we could also just run /nix/var/nix/profiles/system-$generation-link/activate...

I tried to add a test for the boot --generation, but something is strange (maybe the QEMU runs some kind of tempfs - at least after the machine.reboot() and it doesn't have nix-channels or actually the old generations any more on the disk) and so I could only check that the current generation was still active...

I could not reproduce the building of the current configuration.nix in the tests, as this passes, which I think is what you mean:

diff --git a/nixos/tests/nixos-rebuild-switch.nix b/nixos/tests/nixos-rebuild-switch.nix index b9ed2954299a..e5f3e378ad73 100644 --- a/nixos/tests/nixos-rebuild-switch.nix +++ b/nixos/tests/nixos-rebuild-switch.nix @@ -84,12 +84,15 @@ import ./make-test-python.nix ({ pkgs, ... }: { with subtest("Create generation 3"): ${createConfig "3"} - machine.succeed("nixos-rebuild switch") + out = machine.succeed("nixos-rebuild switch 2>&1") + assert "building the system configuration..." in out ${testForBootGeneration "3"} ${testForActiveGeneration "3"} with subtest("must switch to `--generation 1`"): - machine.succeed("nixos-rebuild switch --generation 1") + ${createConfig "4"} + out = machine.succeed("nixos-rebuild switch --generation 1 2>&1") + assert "building the system configuration..." not in out ${testForBootGeneration "1"} ${testForActiveGeneration "1"}

Did I misunderstand you or do you have a reproduction by any chance?

well the script (re)executes itself https://github.com/NixOS/nixpkgs/blob/1860f2ab371659d417f30e9f32e242c6ce5928a5/pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh#L405C68-L405C75, so maybe that is why it starts building?

If we just wanted to activate the generation and don't care about the action I guess we could also just run /nix/var/nix/profiles/system-$generation-link/activate...

Yes, this is basically what I wanted to do.

Did I misunderstand you or do you have a reproduction by any chance?

I think this is what I did, yes.

well the script (re)executes itself

I am usually passing --fast to it.

If we just wanted to activate the generation and don't care about the action I guess we could also just run /nix/var/nix/profiles/system-$generation-link/activate...

Yes, this is basically what I wanted to do.

I just implemented it with "feature parity" to the --rollback. I personally mostly use switch as an action, but maybe e.g. someone could want to set a "known good generation" as a default after a reboot when testing a server update and on error they can just do a reboot and everything should be back to normal. Could something like this be a use case (and if not, just out of curiosity: why would it be one for rollback)?

Did I misunderstand you or do you have a reproduction by any chance?

I think this is what I did, yes.

Very interesting... This should not happen (https://github.com/NixOS/nixpkgs/pull/105910/files#diff-fc23c722779993fcffc932965cd143b265aa196d577ca10173b63e5bdcbc0001R608-R609) and also the test that I wrote above passes... 🤔 I really don't have a clue how this could have happened... Anyone has any idea (or even better can add a failing test case)?

…tely

ofborg bot added 6.topic: nixos 8.has: module (update) 10.rebuild-darwin: 0 10.rebuild-linux: 1-10 labels Dec 4, 2020

miallo changed the title ~~WIP Allow nixos-rebuild --switch-generation~~ WIP: Allow nixos-rebuild --switch-generation Dec 4, 2020

miallo changed the title ~~WIP: Allow nixos-rebuild --switch-generation~~ nixos:nixos-rebuild: [WIP] Allow nixos-rebuild --switch-generation Dec 4, 2020

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miallo changed the title ~~nixos:nixos-rebuild: Allow nixos-rebuild --switch-generation~~ nixos:nixos-rebuild: Add --switch-generation flag Dec 12, 2020

miallo mentioned this pull request Jan 3, 2021

List-generations and rollback to any configuration #24374

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miallo changed the title ~~nixos:nixos-rebuild: Add --switch-generation flag~~ nixos:nixos-rebuild: Add --generation flag Jan 4, 2021

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nixos-rebuild: remove manpage hint on feature that is not merged #270403

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miallo and others added 2 commits December 4, 2023 15:56

nixos-rebuild: Add --generation flag

1d78922

Run `nixos-rebuild switch --generation 12345` to roll back to generation `12345`. This makes rolling back to an older generation easier Also updated the release notes.

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add test for nixos-rebuild-switch

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SuperSandro2000 requested review from samueldr and K900 December 4, 2023 21:00

Unify test syntax

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Previously there was a mixture of `[` and `[[`. That is a bit unnecessary and even if there is a mixture in the rest of the code, this PR should stick to one of the versions... Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

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nixos:nixos-rebuild: allow switching to a specific generation number #105910

nixos:nixos-rebuild: allow switching to a specific generation number #105910

miallo commented Dec 4, 2020 •

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miallo commented Dec 4, 2020 • edited

Motivation for this change

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