[9fans] advertised mounts in Plan 9

[9fans] advertised mounts in Plan 9

[Fco.J.Ballesteros]

As part of my schedule for this semester, I'm trying to apply things
I learned with Plan B to Plan 9 (which is still my Plan A ☺).

Right now I have advertised mounts almost working, but would like
to hear comments from you about the idea, since this change affects the
design of the system.

My point is that although plan 9 is quite good at making it easy to
use network resources, I think it's main weakness is that it does not
adapt well to changes in the environment.  For example, I'd like aplay
to use different /dev/audio devices depending on the set of devices at
hand (for me, the Ipaq and a desktop machines).  That set of devices
changes during a computing session (I turn on the ipaq, etc.)  But
once I choose one and mount it, it's the one used by the program
during its whole life.  The same happens for other resources.

One of the things I do in Plan B is that each server that exports a
resource sends ads to the network about it.  The mount tables admit
import requests (mounts, in Plan 9) that instruct the
system to add a mount entry not at the time of the call, but whenever
the system sees an ad for the resource.

This is an example in the changed version of our local Plan 9:

# At the client machine
# would like audio devices reached through etherN from dat.escet.urjc.es
# (-A is for `resouce Ads')
mount -A net /dev/audio !ether!dat.escet.urjc.es!/dev/audio!9fs

# The server machines run this server to send ads,
# and export the files through 9fs
adsrv !!dat.escet.urjc.es!/dev/audio!9fs

When the client later gets connected to the net where the server
exports another audio device, it sees that the ad matches the pattern
given to mount, and mounts it in the place determined at mount time.

Further opens would reach now my preferred audio device of those available.

The x!y!x expression is of the form
	arch!net!domain!/files/exported!machine!service[!spec]
but that's a convention and can be changed at will.


Comments? I think it's worth, but do you agree?
Any better way? Drawbacks? Kills?