Rethinking how service extensions work

Hi all,

The current situation with services in Guix is that service extensions do not care about dependencies.

This can result in cryptic errors as seen in [1].

In [1], the issue arises from using activation-service-type to create files/directories for services

when these should be either (1) shepherd one-shot services or moved into the 'start' procedure of the service.

'activation-service-type' should only be used for doing things "listed on its label", that is, performing

actions at boot-time or after a system reconfigure.

But both solutions (1) and (2) are still not enough as the directories themselves might not yet

be available and the services must be aware of this fact and wait for them to be ready. One example

would be a network dependent mount or a simple service that mounts a volume such as:

This example also means that it's untenable to just look into the file-systems field entries and attempt

to intelligently discover which paths are required for the services and add them as dependencies (another hole to this idea

is that overlayfs and some fuse filesystems can mount over the same path).

I've proposed in [2] for the service procedure to accept optional arguments, these could be of interest in solving this problem.

Another place we should look at is how systemd manages its service dependencies, with the 'Wants', 'After', 'Before', 'RequiresMountsFor', etc. [3]

directives. These could potentially be implemented and used alongside [2].

Such changes might also imply that a UI change in herd is required to handle the structured information or to avoid cluttering it with too

much "noise".

[1]: https://issues.guix.gnu.org/57589#12

[2]: https://lists.gnu.org/archive/html/guix-devel/2022-12/msg00292.html

[3]: https://www.freedesktop.org/software/systemd/man/systemd.unit.html#%5BUnit%5D%20Section%20Options

On 2023-01-08 12:31, Bruno Victal wrote:

Idea:

Instead of moving these procedures into the start procedure from shepherd-service and end up with a very

large start constructor, we could augment <shepherd-service> with a 'pre-start' field that is responsible for

setting up the initial conditions for the service. That is, we move most of the code in the activation-service-type extensions

into this 'pre-start' field. We could also consider if it would make sense adding post-start, pre-stop and post-stop fields.

Cheers,

Bruno

Hi Bruno,

Bruno Victal <mirai@makinata.eu> skribis:

This is the result of “services” being unrelated to “Shepherd services”,

as noted in the manual (info "(guix) Services").

Right.

As we once discussed on IRC, the conclusion to me is that some of the

code currently implemented as activation snippets should rather be

implemented either as part of the ‘start’ method of the corresponding

Shepherd service, or as a one-shot Shepherd service that the main

service would depend on.

Note that this should prolly be declared as a ‘file-system’ rather than

as a custom service. That way, it would get a “standard” Shepherd

service.

There are cases where we add explicit dependencies on

‘file-system-/media/foo’ or similar. <file-system> has a ‘dependencies’

field specifically for this purpose (info "(guix) File Systems").

Would that work for you?

HTH,

Ludo’.

Hi Ludo’,

On 2023-02-25 17:46, Ludovic Courtès wrote:

I think moving them into the ‘start’ method is the best course of action.

I'm considering the following changes:

I expect that mkdir-p/perms is going to be used frequently enough, using the number of activation-service

extensions in use as a rough estimate.

There's one issue I'm somewhat concerned about, consider the following snippet:

This is somewhat pitfall prone since you most likely don't want to chown /var/log to a non-root user.

I'm unsure what's the best course to take here, would a simple file-exist? check before mkdir-p/perms be sufficient?

In either case, with or without refactoring this issue is already present (but in activation-service extensions)

so it's no worse than the status quo.

Unfortunately OverlayFS is filtered out from fstab by Guix (reported #60246) and the dependencies field IMO is too restrictive,

there should be a (sane) way to pass shepherd service symbols too. (for cases where a file system depends on 'networking or

depends on a particular interface e.g. NFS mount that uses a IPv6 link-local address)

Cheers,

Bruno

Am Dienstag, dem 09.05.2023 um 20:12 +0100 schrieb Bruno Victal:

I think this question highlights perfectly why one-shot services (or

perhaps an as-of yet unknown type of services) are the way to go: With

clearly named services for the creation of directories, you don't need

to worry about creating some file with the wrong permissions as the

owner is already predetermined. You also don't need mkdir-p; you

simply depend on the mkdir-#$(dirname my-directory) service.

Cheers

Hi Bruno,

Bruno Victal <mirai@makinata.eu> skribis:

[...]

OK. Cosmetic considerations: how about adding a ‘pre-start’ field in

<shepherd-service>? That would allow us to keep the “setup” bit

visually separate from the actual ‘start’ method, even if under the hood

they get “merged” together:

(shepherd-service

;; …

(pre-start #~(mkdir-p "/whatever"))

(start #~(make-forkexec-constructor …)))

We ensure /var/log exists before anything else—see ‘directives’ in (gnu

build install).

If we want an extra safety, we can add a real activation snippet that

does (mkdir-p "/var/log"), with the understanding that it would notably

run at boot time before shepherd is started.

Right.

Sure, we could make these changes. Let’s discuss it separately?

Thanks,

Ludo’.