[PATCH]: Fix xm block-detach

[Chris Lalancette <clalance@redhat.com>]

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All,
     The recent changes to where xenstore stores data about domains (c/s 18562,
in particular) seem to have broken xm block-detach.  What happens is that
attaching a disk works just fine, but detaching a disk ends with:

Error: 51776 not connected
Usage: xm block-detach <Domain> <DevId> [-f|--force]

Destroy a domain's virtual block device.

The problem basically boils down to where the new code is placing the xenstore
entries.  Previously, it was storing them in /local/domain/<number>/device/vbd;
it is now storing them in /vm/UUID/device/tap.  The tap is wrong; tap describes
the backend, not the frontend, which should always be vbd.  The attached patch
fixes this by overriding deviceRoot() in the BlktapController class, similar to
how frontendRoot() is done, and then changing devicePath() to use deviceRoot().
 There is also a small fix for destroyDevice, to make sure we remove the proper
/vm/UUID/device/vbd entry on removal.
    With the patch in place, I was able to successfully block-attach and
block-detach disks again.  Note that I only tested this on RHEL-5 xend, which
has diverged quite a bit from upstream.  However, a quick glance at the code in
xen-unstable shows that this is probably a problem there as well; only testing
will tell for sure.

Signed-off-by: Chris Lalancette <clalance@redhat.com>

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diff -urp xen-3.1.0-src.orig/tools/python/xen/xend/server/BlktapController.py xen-3.1.0-src/tools/python/xen/xend/server/BlktapController.py
--- xen-3.1.0-src.orig/tools/python/xen/xend/server/BlktapController.py	2008-12-02 13:07:04.000000000 +0100
+++ xen-3.1.0-src/tools/python/xen/xend/server/BlktapController.py	2008-12-02 13:08:32.000000000 +0100
@@ -16,6 +16,11 @@ class BlktapController(BlkifController):
     def __init__(self, vm):
         BlkifController.__init__(self, vm)
         
+    def deviceRoot(self):
+        """@see DevController#deviceRoot"""
+
+        return "%s/device/vbd" % self.vm.vmpath
+
     def frontendRoot(self):
         """@see DevController#frontendRoot"""
         
diff -urp xen-3.1.0-src.orig/tools/python/xen/xend/server/DevController.py xen-3.1.0-src/tools/python/xen/xend/server/DevController.py
--- xen-3.1.0-src.orig/tools/python/xen/xend/server/DevController.py	2008-12-02 13:07:04.000000000 +0100
+++ xen-3.1.0-src/tools/python/xen/xend/server/DevController.py	2008-12-02 13:09:25.000000000 +0100
@@ -221,6 +221,8 @@ class DevController:
         # drivers, so this ordering avoids a race).
         self.writeBackend(devid, 'online', "0")
         self.writeBackend(devid, 'state', str(xenbusState['Closing']))
+        devpath = self.devicePath(devid)
+        xstransact.Remove(devpath)
 
         if force:
             frontpath = self.frontendPath(devid)
@@ -487,8 +489,7 @@ class DevController:
     def devicePath(self, devid):
         """Return the /device entry of the given VM. We use it to store
         backend/frontend locations"""
-        return "%s/device/%s/%s" % (self.vm.vmpath,
-                                    self.deviceClass, devid)
+        return "%s/%s" % (self.deviceRoot(), devid)
 
 def hotplugStatusCallback(statusPath, ev, result):
     log.debug("hotplugStatusCallback %s.", statusPath)

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