Process lifecycle on snap refresh

That’s still arguing about how to keep applications running when everything under them is significantly changing. Yes, we can make it work, in the sense of the application not crashing immediately, but very often this leads to serious bugs, non-determinism, and bad user experience – a system which is all over the place in non-obvious ways.

So, can we please spend some time on playing with the idea of how to map these needs into a world where each snap may restart its processes on updates, but different snaps may keep the workloads running?

I will give it some thought.

I just keep bumping into scenarios that make it hard to do - so it would be helpful for other people to help architect this.

Let’s forget about qemu and libvirt for a minute. Say, we are talking about lxd or docker which face the same issue.

LXD has a daemon process and monitor processes:

lxd       3175     1  3174  3174  0 апр24 ?     00:00:01   dnsmasq --strict-order --bind-interfaces --pid-file=/var/lib/lxd/networks/lxdbr0/ --except-interface=lo --interface=lxdb
r0 --listen-address= --dhcp-no-override --dhcp-authoritative --dhcp-leasefile=/var/lib/lxd/networks/lxdbr0/dnsmasq.leases --dhcp-hostsfile=/var/lib/lxd/networks/lxdbr0/dnsmasq.hos
ts --dhcp-range,,1h -s lxd -S /lxd/ -u lxd
root      3216     1  3216  3216  0 апр24 ?     00:00:00   [lxc monitor] /var/lib/lxd/containers juju-58445f-0
165536    3248  3216  3248  3248  0 апр24 ?     00:00:01     /sbin/init
165536    3362  3248  3362  3362  0 апр24 ?     00:00:00       /lib/systemd/systemd-journald
165536    3377  3248  3377  3377  0 апр24 ?     00:00:00       /lib/systemd/systemd-udevd

You can restart a daemon without touching monitor processes.

And this is the same package - so I cannot just take those out to a separate snap. There is probably not even a separate binary for that.

Same with docker:

We’re conflating two different things here I think. Restarting a daemon without killing everything is of course fine and a good idea. It’s the refresh case that has a number of associated displacements that is problematic to not have a complete halt while it’s taking place, as far as the processes depending on the old state are concerned.

In fact, the Docker documentation agrees with me, right there on that page:

If you skip releases during an upgrade, the daemon may not restore its connection to the containers. If the daemon is unable to restore the connection, it ignores the running containers and you must manage them manually.

If you kill -9 PostgreSQL, rather than wait for it to shut down gracefully, the database will go into recovery mode which could take it out of operation for a long period. PostgreSQL is well written software, and as such takes great care to shutdown gracefully and minimize downtime. On larger deployments this can take some time.

Can SIGKILL behaviour be configurable? Terminating PostgreSQL with SIGKILL is against best practice and unacceptable in most production deploys, where it is better to leave the system running until the problem can be manually dealt with. The recommended systemd service files for PostgreSQL disable using SIGKILL for this reason.

This sort of policy is now generally encoded in systemd service files, so maybe snapd should expose more systemd configuration options and rely on systemd to restart… eventually, maybe even after the system is rebooted. As a user and sysadmin, I would prefer snapd to be blocked than have it overrule my policies and terminate my user facing services.

but under classic confinement this is not a problem right?

Okay, after may conversations on the topic in various meetings, we’d like to revisit this and push something forward.

Here is a new proposal that takes into account those conversations:

Problem statement

It is reasonable for some workloads to remain running across refreshes. The typical case is managers of containers or virtual machines that want to remain alive while the software that enables them to run gets updated.


We’ll introduce a new option that can be used under the application definition scope in snap.yaml and snapcraft.yaml:

        command: ...
        daemon: ...
        refresh-mode: [ restart / endure ] 

The default for this option is restart for daemons and endure for other applications. Initially only changing daemons will be accepted, but we plan to support changing that for other applications too.

As implied, when a daemon is set to endure it won’t be restarted during a refresh. Additionally:

  • It will lose write access to its per-revision data ($SNAP_DATA)
  • It won’t lose access to the data common across revisions ($SNAP_COMMON)
  • The current symlink will be moved to the new revision as usual
  • Pre and post refresh hooks will work as usual
  • The security profiles will be updated (see caveats below)


In the first implementation of this feature the security profiles for the application will be updated during the refresh, meaning the old process will get the same permissions as the new process. This is not ideal, but it’s a reasonable compromise to get something working sooner, and can be addressed in a future release without breakages.


How does that sound? Anything we should watch out for or consider further around this feature?


Just to clarify, this particular item will not apply to classically-confined snaps, correct?

I think you mean only write access here. read has always been permitted and should be across refresh.

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Classic snaps aren’t strictly confined and the thing the makes snaps lose write access to SNAP_DATA is AppArmor, so you are correct, this would not apply to classic snaps.

does it mean only “snap refresh” command will work this way?

also in the case of restarting the daemon in order for update, is there anyway to avoid killing the entire cgroup, just like we set “killMode=process” in systemd service config?

Yes, that’s the idea. Is that an issue for a case you’re interested on?

One of the side effects of changing kill mode in systemd is that the behavior of service termination is altered altogether, and it makes no distinction of what the reason for stopping is or what the actual processes involved in the operation are.

In other words, in general when we say processes that should not be terminated, we have a very specific use case in mind, but over the life time of a non-trivial application usually multiple processes are spawned with particular purposes. In a well-behaving system, when a snap is removed or disabled altogether that snap should take out with it any left over applications that are still in the system, otherwise the administrator loses control of what is running and why.

The case of refreshes is different, though, because the intention wasn’t really to stop or disable anything, but rather to bring an update into it. In those cases it sounds reasonable to keep specific workloads running that are aware of the constraints at play.

Also note that if the proposed refresh-mode: endure option is used, the snap is still free to do whatever it wishes with its own daemon processes. For example, it’d be trivial to call kill -TERM $SOME_PID inside the post-refresh hook, and get the same effect in refreshes of the KillMode=process systemd option.

@sherman Does that sound reasonble?

we’d like “snap install local_snap.snap” to work this way as well.

I agree that this really applies to our specific use case, but falling back to kill mode default is also “altered the termination altogether” because it kills all. IMO snap is making the kill mode stiffer, while systemd is already not fine-grained on this. I didn’t see service kill mode can anyhow negatively impact snapd even if a snap’s orphan process tracking older “$SNAP_DATA” or “$SNAP_COMMON”, and if that orphan is tracking global data like classic confinement, and guarantee to exit at a reasonable time, it’s a safe move.

yes, but the daemon “process” itself cannot be “kill -TERM $SOME_PID” of that “SOME_PID” right? As I understand this proposal, the daemon process, which is spawn from the apps “command”, will never be touched throughout “snap refresh”, nor can it be killed/restarted from post-refresh hook right? Or is there anything I missed out here?

That’s reasonable, and we’ll make sure it does respect the restart settings.

When installing a local snap over an existing one, that’s more of a refresh operation than an install one. We should really clarify that in the API.

Per note above, the issue with changing kill mode altogether is that this affects removals as well. Being a package manager, we want to make sure the machine is as clean of side effects as possible when a remove operation takes place.

That said, I we very much want to handle your use case too, and these are not in conflict.

Here is a proposal extending the original idea above:

        command: ...
        daemon: ...
        refresh-mode: [ endure / restart / sigterm[-all] / sighup[-all] / sigusr1[-all] / sigusr2[-all] ] 

That means, for example, that if the snap defines refresh-mode: sigterm on refreshes and local installs over an existing snap, the main process will receive a SIGTERM signal, which I think is what you want.

Does that address your needs?

yes, exactly, that’ll make the behavior identical to our existing systemd processes

Fantastic, thanks for confirming. We’ll implement it and ping you again once we have something ready for testing, if that’s okay. Expect something early next week the latest.

@sherman Sorry for the lack of feedback here. This feature has been in edge for a while, and ready for testing. The design is per the discussion above, so refresh-mode: sigterm is supposed to do what you want.

Please let us know how it goes.

Thank you for the changes made. I wanted to confirm if there is any change that would be needed to be made in the config for this to work?