[systemd-devel] "udevadm settle" and "init"

[Scott James Remnant]

There is, of course, one use for "settle"; I like to have a "monitor"
running during boot that creates /var/log/udev - settle is as good a
command as any to kill monitor when it's done.

On Thu, Mar 31, 2011 at 11:35 AM, Kay Sievers <kay.sievers at vrfy.org> wrote:
> Hey,
>
> the underlying problem described below, is in no way specific to
> systemd, it applies to all modern systems and services, existing ones,
> or ones not even developed.
>
> The udev systemd service files for udev do no longer pull-in the
> barrier 'udevadm settle'. The udev coldplug run is executed fully
> asynchronously in the background along with many other services, and
> no other service will be blocked by default, waiting for udev to
> populate /dev.
>
> That means: non-hotplug-aware services are likely to break. Services
> which assume a static /dev, rely on scanning /dev and finding "all"
> devices at a certain point in time, are required to explicitly pull-in
> udev-settle.service to paper-over their inability to cope with a
> dynamic system. It will block the broken service until udev's coldplug
> run has finished and /dev is populated with all devices known at that
> time.
>
> It's 2011 and services should cope with hotplug, devices coming and
> going at any time, and a constantly changing and re-configuring
> environment. No service which wants survive the next years of Linux
> evolution should depend on the udev-settle.service barrier though,
> please fix your stuff, or the reality might go on without you. :)
>
> There is libudev, which connects your service to meaningful event for
> news devices, device changes, and allows you to enumerate all
> currently existing devices. Subscribing to events before enumeration
> is started will make sure not to miss anything, and to propagate later
> changes into the service.
>
> All that even works from python, java, mono, javascript, ... programs,
> so there is no excuse anymore to continue to do any of the
> much-too-simple things we all did in the past. Just stop making
> assumptions like: "booting has finished" or "all devices are there" --
> that never really was in the past, and it will never be true in the
> future.
>
> Thanks,
> Kay
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