[RFC] mdev: support reading events from stdin, add features to mdev -s

[Isaac Dunham]

On Wed, Apr 15, 2015 at 06:46:08PM +0200, Denys Vlasenko wrote:
> On Mon, Apr 13, 2015 at 7:15 PM, Isaac Dunham <ibid.ag at gmail.com> wrote:
> > Here's a patch series to add mdev -i and make mdev -s a little more
> > featureful, so rules like this one:
> > $MODALIAS=.*    root:root 660 @modprobe "$MODALIAS"
> > will work even with coldplug.
> >
> > mdev -i reads hotplug events from stdin, in a format proposed by Laurent Bercot
> > wherein each "line" is terminated with '\0' (rather than '\n') and each event
> > is terminated by an additional '\0'.
> > Internally, each event is loaded into the environment (without copying) at
> > the start of handle_event(), then the environment is cleared before
> > handle_event() returns.
> > This is done instead of using a new key lookup method because helper
> > scripts rely extensively on environment variables; if we did not load
> > the events into the environment at some point, autoloading modules and
> > other important features would no longer work.
> >
> > The new mdev -s (FEATURE_MDEV_NEWSCAN) uses the same codepath as mdev -i
> > instead of calling make_device() directly.
> >
> > Both mdev -i and the new mdev -s are compile-time options, separately
> > selectable.
> 
> How about the following applet, which would listen to netlink and spawn a helper
> for every new message?
> 
> So far not tested with "mdev".

Your proposed applet is essentially interchangeable with nldev and s6-devd.
Yes, it's a nice concept.

But it doesn't address the number of forks that happen if you use mdev as
a hotplugger during system startup, which can overwhelm some systems
(depending on the hardware and the kernel configuration) or cause
relatively slow boots when you provide an mdev.conf for all possible
hardware (as is required for a distro package), since every instance of
the hotplugger has to reparse the config file.

To solve these problems, Harald Becker, Laurent, and Natanael Copa had
proposed to allow mdev to serve as a longer-running process that would
read a textual dump of events from stdin ("mdev -p"/ "mdev -i").

Harald had a much larger set of changes in mind, including turning the
hotplug helper into something that forwards events to the reader and
making mdev mount the needed filesystems, which was the cause of the big
argument about mdev.
(That was the thread "RFD: Rework/extending functionality of mdev").

Laurent Bercot and Natanael thought that letting mdev read from a pipe
sounded good, and was the only part of the extra functionality that was
needed. There was some disagreement about whether to let mdev timeout
and exit when no hotplug events happen in a specified duration.
A bit of discussion related to this is in the thread 
"[RFC] Proof-of-concept for netlink listener for mdev -i".
Eventually, Nataneal made nldev work like he wanted, then wrote a minimal
patch to implement mdev -i (with timeout), requesting that someone else
pick up where he left off (these are the first two patches).
The third and fourth patches in the series are my try at finishing it,
and I ended up implementing a version of mdev -s that's equivalent to
hotplugging in features (because the refactoring made it easy).

As a note for review, the second patch has a few issues with it that are
addressed in the third patch; I was considering squashing them together,
but left them separate to acknowledge Natanael Copa's part in writing it.

Thanks,
Isaac Dunham