init: transfer state across re-exec

Patch to fix this in 0.3.*, newly cleaned up and with test cases.

As discussed on IRC, this patch doesn't transfer nearly enough state to result in a correct re-exec; the event queue and pending D-Bus messages need to be transferred, as does connections to clients, etc.

Marking the 0.3 task as Won't Fix, 0.6 is the current stable release branch

I think this is a critical feature: working re-exec is a must for proper Upstart upgrades. Currently upgrade from older Upstart versions to 0.6 are fatal to the system: new initctl won't talk to the old and still running Upstart and even a simple reboot may be a problem. When upgrading one 0.6.x Upstart to other 0.6.x only the 'clients' are really upgraded, the old init will be running until the reboot. That is not a big problem for a desktop or embedded systems, but servers should not require reboot after upgrade.

That is why I have started an Upstart branch (lp:~jajcus-jajcus/upstart/state-save) to implement full state saving and re-exec with state transfer. I hope this can help making Upstart a usable (in all cases) replacement for SysVinit.

@Scott: do the connections to clients and pending D-Bus messages really need transfering? Re-exec would be an exceptional event anyway and the goal is to keep system running with the new init daemon, so upgrade can continue and, maybe, a clean shut-down can be done. Will anything disastrous happen if the pending D-Bus messages are just flushed (errors raised if the requests cannot be handled immediately) and connection are just closed before the re-exec?

On Thu, 2010-05-27 at 13:35 +0000, Jacek Konieczny wrote:

> That is why I have started an Upstart branch (lp:~jajcus-jajcus/upstart
> /state-save) to implement full state saving and re-exec with state
> transfer. I hope this can help making Upstart a usable (in all cases)
> replacement for SysVinit.
>
Awesome!

The biggest mental block I had was getting the state out of libdbus
about the state of connections to Upstart, state of messages, etc.

Restarting Upstart shouldn't drop in-progress commands.

Scott
--
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