[Libguestfs] Proposed changes for OpenStack

[Richard W.M. Jones]

On Wed, Dec 14, 2011 at 11:06:24AM +0000, Matthew Booth wrote:
> On 12/14/2011 10:36 AM, Richard W.M. Jones wrote:
> >On Wed, Dec 14, 2011 at 09:33:35AM +0000, Matthew Booth wrote:
> >>On 12/14/2011 08:44 AM, Richard W.M. Jones wrote:
> >>>[These two patches are for discussion only]
> >>>
> >>>Allow FUSE support to be used directly through the API.
> >>>This is the second commit.
> >>>
> >>>In order to make this usable from guestfish, we have to
> >>>also bind the events API in guestfish.  This is the first
> >>>commit.
> >>
> >>There's not really enough information here to evaluate the proposed
> >>changes. Can you give an example of what you're trying to do?
> >
> >There's a very specific need for this in OpenStack.  We want to be
> >able to inject files and make other filesystem changes -- and there is
> >already common code which assumes the guest filesystem is mounted
> >somewhere.  But also we want to call libguestfs APIs such as
> >guestfs_tune2fs.  Currently we can do the mounting bit by calling
> >guestmount, and we can do the other libguestfs API calls, but we need
> >to launch libguestfs twice to do this.
> >
> >Here is Padraig's current OpenStack patch:
> >
> >https://review.openstack.org/#change,1993
> >https://review.openstack.org/#change,1994
> 
> Ok, it's an efficiency thing. However, I still think it's the wrong
> solution. What you really want to do is use 1 appliance for 2
> things. We already have a mechanism to do that: ATTACH_METHOD_UNIX.
> Why not update guestmount to use it?
> 
> >>In general, though, this feels wrong for a couple of reasons.
> >>Firstly, a callback mechanism implemented with shell scripts is
> >>simply hideous. Imagine trying to use that from another language.
> >
> >This is only for guestfish (which is a shell script tool).  How would
> >you want to capture events in guestfish?
> 
> Not quite so hideous, but I remain unconvinced. Use another
> language? That's not a flippant suggestion, btw. There comes a point
> in shell scripting when you're exceeding what it was designed for.

Let me explain this again ...

The guestfish change is only for us to do testing.  OpenStack is
written in Python and uses the libguestfs-python API.

What Padraig's current patch is doing is essentially assembling the
command line for guestmount and calling out to it.  This is
reminiscent of the bad old days when inspection was part of a separate
set of Perl tools, and we had to coordinate between virt-inspector and
guestfish by assembling command lines.

This would work much better if Padraig was able to do:

 g = guestfs.GuestFS ()
 g.add_drive_opts (...)
 g.launch ()

 # Locate the filesystems:
 try:
   roots = g.inspect_os ()
   # mount some stuff
 except:
   # Inspection failed, backup method:
   g.list_filesystems ()
   # mount some stuff

 g.set_event_callback (callback, guestfs.EVENT_MOUNTED)
 g.mount_local ("/tmp/dir")
 # In the event callback, start a thread which:
 # - injects files
 # - modifies network config
 # - finally calls g.umount_local ()

 # Finally, tune the filesystem
 g.tune2fs (...)

You can see this is much cleaner, as well as more efficient, than
constructing command lines and shelling out to guestmount.

(BTW I am making some untested assumptions about libfuse here.
I don't know that this will really work)

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org