Add One-shot Install/Uninstall Hook

[Matt Middleton]

It would be nice to have a hook that is executed once on install / uninstall ( and possibly upgrade ) and that is guaranteed to run before any daemons are launched. This would allow for configs, databases, etc to be properly configured for daemons that require it. I'm currently snapping a daemon that I'm unable to modify and would benefit very much from this addition and I assume this would also benefit people that need to snap daemons that are not rewritten exclusively for the snap environment as well as 'classic mode' daemons that need to setup other parts of the system, users, etc..

[Mark Shuttleworth (sabdfl)]

Yes, I've run into this challenge making snaps too.

The only question in my mind, though, is whether you don't actually want
to run a sanity check of config etc on every launch, anyway, to deal
with cases of corruption. What I did was sanity check config and restore
if needed (or recreate from a template) when the daemon is being
launched. That would slow down launch slightly but provide a very high
level of certainty on every launch.

Mark

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Kyle Fazzari (kyrofa)]

I'd like this as well. The only way to do things like this today is to wrap the services that require configs (or databases, whatever) in a script and make that script the daemon. It'd be great if that was an optional step (e.g. if I wanted to verify a config before launching it, as Mark mentioned) and the heavy lifting of generating things needed in order to run was handled in a dedicated place. Having wrappers for all my daemons makes my snap difficult to navigate and maintain.

[Kyle Fazzari (kyrofa)]

@Matt, I'd like to hear about your use-case for an uninstall hook. I typically think of this as a "clean up behind yourself" hook, but since when removing a snap the snap itself and all its writable areas are blown away, I don't have a clear picture for how an uninstall hook would be useful.

[Matt Middleton (mmiddleton)]

@Kyle - currently I'm making a snap for a daemon I cannot modify that runs in classic confinement and specifically requires a user to be added to the system. In the short term, it may also be necessary to add udev rules, systemctl configs, network manager dispatch scripts, etc. This may be abusing the very nice encapsulating aspect of snaps but that seems to be implied by classic confinement and I think is likely that people porting legacy software to snap distribution may face similar challenges.

> On 3/02/2017, at 5:46 AM, Kyle Fazzari <email address hidden> wrote:
>
> @Matt, I'd like to hear about your use-case for an uninstall hook. I
> typically think of this as a "clean up behind yourself" hook, but since
> when removing a snap the snap itself and all its writable areas are
> blown away, I don't have a clear picture for how an uninstall hook would
> be useful.
>
> --
> You received this bug notification because you are subscribed to the bug
> report.
> https://bugs.launchpad.net/bugs/1661126
>
> Title:
> Add One-shot Install/Uninstall Hook
>
> Status in snapd package in Ubuntu:
> Confirmed
>
> Bug description:
> It would be nice to have a hook that is executed once on install /
> uninstall ( and possibly upgrade ) and that is guaranteed to run
> before any daemons are launched. This would allow for configs,
> databases, etc to be properly configured for daemons that require it.
> I'm currently snapping a daemon that I'm unable to modify and would
> benefit very much from this addition and I assume this would also
> benefit people that need to snap daemons that are not rewritten
> exclusively for the snap environment as well as 'classic mode' daemons
> that need to setup other parts of the system, users, etc..
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/ubuntu/+source/snapd/+bug/1661126/+subscriptions

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[Kyle Fazzari (kyrofa)]

@Matt that seems like a valid use-case to me, thanks!

[Brian Murray (brian-murray)]

My use-case for an uninstall hook is modifying items in "/proc/sys" back to their original state e.g. apport would modify "/proc/sys/kernel/core_pattern".

[James Jesudason (jamesj)]

The install hook would be useful in a snap that I'm working on that includes postgresql embedded in the snap. That has an additional challenge as the database initialization needs to be done as a non-root user, so having a hooks that run as the logged-in user and the root user (since daemons run as root) would be useful.

[Nish Aravamudan (nacc)]

Just an FYI as an also interested party, the install/uninstall hook landed upstream: https://github.com/snapcore/snapd/pull/3498. I have no idea how to figure out if this is in any releases.

[Boris Rybalkin (ribalkin)]

To me it is half done as only fixes config generation on install (https://github.com/snapcore/snapd/blob/master/overlord/snapstate/snapstate.go#L173), but how about upgrade when I bring fixes with config templates? Should there be an upgrade hook as well?

My simple version of it contains pre-startup hook which runs always on install or upgrade (https://github.com/syncloud/snapd/blob/master/overlord/snapstate/snapstate.go#L171)

[Paweł Stołowski (stolowski)]

Refresh (upgrade) hook is also in the review queue, we just try to keep changes minimal and land them separetly.

[Michael Vogt (mvo)]

This is fixed with https://github.com/snapcore/snapd/pull/3498