autofs - automatic unmounting / syncing. how does HAL help?

[David Zeuthen]

On Thu, 2004-09-16 at 18:13 +0100, Luke Kenneth Casson Leighton wrote:
> okay.
> 
> the reason why i am abandoning my hacked-up version of usbmount, most
> of which i moved into an autofs script /etc/auto.usb.sh, is because
> of integration with the desktop.  and i was getting into a muddle.
> 

For some background of HAL see the spec in the doc/spec directory.
There's a recent version here

 http://freedesktop.org/~david/hal-spec/hal-spec.html

> the issue(s) are:
> 
> - usb floppies and usb card readers tend to get left plugged in
>   permanently, whilst the media on them is arbitrarily removed.

HAL does this; we poll the devices (but have a blacklist for problematic
devices such as pcmcia-cs, Zip drives etc.).

> - it's NOT reasonable to expect users to cope with "oo you have
>    to run umount" - they just won't care.
> 

Of course users won't care. Now, the hal daemon does a 'umount -l' and
emits what we call a DeviceCondition so the desktop may choose to deal
with it to e.g. put up a dialog telling off the user. 

(GNOME Volume Manager doesn't do this right now; not sure about KDE
Volume Manager)

> - some usb pendrives, some usb floppies, and also some usb card
>   readers, will either have a partition on them (/dev/sda) or
>   they may have a partition table (/dev/sda[1-4]).

HAL detects this and deals with it.

> - usbmount _was_ smart enough to read /sys in order to determine
>   partition information should it be there (so i was able to
>   create an fstab-equivalent for the autofs.usb.sh script to
>   parse and understand...
> 
> - ... but of course that doesn't help you for a usb removable
>   media which might happen to have a partition on it, it might
>   happen to have a partition table, you won't know until the
>   media is plugged in and tested.
> 

We probe the media in HAL for filesystems; both partitions and the main
block device. We also retrieve labels and UUID if applicable.

>   usbmount made the assumption that there might be one or
>   the other, but only one partition (ultimately) and used
>   "/sbin/fdisk" to check (!)  you ended up with either /dev/sda
>   or /dev/sda1 (but never /dev/sda[1-4]).
> 
> so.
> 
> i understand that HAL captures hotplug events from usb card reader
> "media insert" events, is that correct?
> 

There are no media insert events - you will have to poll; no question
about it. In general USB storage devices are cheap junk that doesn't
have the logic to even emit a media change event so the kernel has no
chance to capture it. See the linux-hotplug archives from Jan-May 2004
for some discussion.

> because if so (and usbmount certainly couldn't cope with that)
> then part of the problem is solved: media in, redo entries,
> notify KVM, off we go.
> 

This is basically what HAL does.

> this still leaves two issues outstanding:
> 
> 1) how can HAL help with automatic unmounting?
> 

Well, we do the umount -l in the hal daemon. We really have to (even
though it may smell like policy) because the hardware/media is indeed
gone.

>    should this be hacked into KDE?
> 

Someone is already working on the KDE Volume Manager. One explicit goal
of HAL is to get wide adoption and get all the key desktop projects
(KDE, GNOME, OO.o, Mozilla, Java, Mono, ...) to use it. That's in part
why HAL is hosted on freedesktop.org.

Presently I'm working on a libhal-storage library so you can say 

 name = hal_drive_get_displayable_name(hal_drive)

such that e.g. GNOME and KDE displays the same name of the drive. Share
policy and stuff.

>    should i continue to investigate autofs?
> 

Well, HAL pretty much does all that autofs does as well as a quite a few
more things like emitting signals over D-BUS, it's well-documented,
provides an abstract view of the hardware (also other hardware like
networking, input devices, multimedia devices etc.)

Also, we're always looking for more hackers :-)

>    should i write an /etc/hal/device.d/autofs-update.hal
>    script that mirrors what fstab-update.hal does?
> 

I assume you mean fstab-sync that is invoked as a hal callout.

> 2) usb floppies: these definitely don't generate usb hotplug
>    notify events (mine doesn't... just checking.... nope!)
> 

We'll hal handles these (modulo bugs in hal)

> help help i need to get something working - fast!
> 

Well, HAL is pretty much designed to cope with all this and it works
pretty well most of the time. It's a pretty young project (I started it
about one year ago) but distributions like Fedora and Debian have
already picked it up. When it doesn't work we usually fix the bugs
pretty fast.

Cheers,
David

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