bug#47253: network-manager shepherd services does not wait to be online

[Mark H Weaver <mhw@netris.org>]

Hi,

Earlier, I wrote:
>> How about leaving "networking" as it is now, and instead adding a new
>> service called "network-online" or similar, that requires "networking"
>> and then waits until a network connection is established?

I withdraw my proposal for a separate "network-online" service.  It was
a half-baked idea made in haste.  Now that I've looked, I see that
almost every service in Guix that requires 'networking' should
arguably[*] wait until the network comes up before starting up.
Moreover, now that I think about it, I'm not sure what the use case
would be for requiring 'networking', if not to wait for the network to
come up.

My immediate concern here is to avoid blocking the startup of a typical
Guix desktop or laptop system for 30 seconds if there's no network
connection, and more generally to keep Guix working well for users like
myself who are not "always online".

I haven't yet looked into the details, but at first glance, I'm inclined
to agree with you that the right place to fix this is in Shepherd.
Somehow, it ought to be possible to delay the startup of services that
require 'networking', without delaying anything else.

       Mark


[*] I'll note, however, that merely waiting up to 30 seconds (or
whatever timeout you choose) is not, in itself, a robust solution.  What
happens if the network is down for more than 30 seconds?  What if it
goes down after 'nm-online' checks, but before the dependent service has
finished starting?  Also, if a service fails to handle lack of network
when it starts, it makes me wonder whether it properly handles a
prolonged network failure while its running.  It seems to me that the
only fully satisfactory solution is for each service to robustly handle
network failures at any time, although I acknowledge that workarounds
are needed in the meantime.