watchdog and sd_notify between host and container systemd etc inits

[<Mikko.Rapeli@xxxxxx>]

Hi,

I'd like to run systemd as init on host but run various containers and some of them
with their own container side systemd init.

Then I'd like to have sd_notify and watchdog available to check the health of the
systemd init in the container. I trust the init in the container to check the health
of all the services and processes running there.

If systemd init in the container fails to respond to watchdog, then I'd like to
restart only the container, not the whole host system.

For the container systemd watchdog, I've proposed patch:

https://github.com/systemd/systemd/pull/13643

Comments to the PR mention that sd_notify support would be better, but AFAIK it uses
the PID of processes and thus doesn't work with another systemd init as PID 0 in
the container PID namespace.

Thus we inveted a simple fifo between host init and container init where
container writes MACHINE and HOSTNAME as watchdog ping. This works well with a
custom watchdog manager on host and systemd init in an LXC container.

These don't seem to fit very well to systemd, and we'd also like to know sd_notify type
things like when is the container in running state, which systemd nspawn does
provide, but I have use cases also for LXC containers...

So, could you provide some ideas and/or feedback how this kind of functionality
could/should be implemented with systemd?

Cheers,

-Mikko
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