From Murillo Fernandes Bernardes <mfb@br.ibm.com>:

The problem is: There is no mechanism to detect block device setup failure

Network devices have the same problem, and are fixed with this too.

I handling this problem in the way suggested by aliguori:
- hotplug scripts write a "hotplug-status" node on store
- Xend DevController.createDevice() check verify this node and return success
or throw an exception on failure.
- If no changes in "hotplug-status" node after DEVICE_CREATE_TIMEOUT seconds
Xend throw an exception showing the problem with hotplug scripts.

Signed-off-by: Ewan Mellor <ewan@xensource.com>

tools/python/xen/xend/server/DevController.py

@@ -16,12 +16,18 @@
 # Copyright (C) 2005 XenSource Ltd
 #============================================================================
 
+from threading import Event
 
 from xen.xend import sxp
 from xen.xend.XendError import VmError
 from xen.xend.XendLogging import log
-from xen.xend.xenstore.xstransact import xstransact
 
+from xen.xend.xenstore.xstransact import xstransact
+from xen.xend.xenstore.xswatch import xswatch
+
+DEVICE_CREATE_TIMEOUT = 120
+HOTPLUG_STATUS_NODE = "hotplug-status"
+HOTPLUG_STATUS_ERROR = "error"
 
 class DevController:
     """Abstract base class for a device controller.  Device controllers create
@@ -54,6 +60,18 @@ class DevController:
 
         self.writeDetails(config, devid, back, front)
 
+        status, fn_ret = self.waitForBackend(devid)
+        if status:
+            self.destroyDevice(devid)
+            raise VmError( ("Device %s (%s) could not be connected. "
+                            "Hotplug scripts not working") 
+                            % (devid, self.deviceClass))
+
+        elif fn_ret == HOTPLUG_STATUS_ERROR:
+            self.destroyDevice(devid)
+            raise VmError( ("Device %s (%s) could not be connected. "
+                            "Backend device not found!") 
+                            % (devid, self.deviceClass))
         return devid
 
 
@@ -242,6 +260,29 @@ class DevController:
         xstransact.Write(frontpath, frontDetails)
         xstransact.Write(backpath, backDetails)
 
+    def waitForBackend(self,devid):
+        ev = Event()
+
+        def hotplugStatus():
+            status = self.readBackend(devid, HOTPLUG_STATUS_NODE)
+            if status is not None:
+                watch.xs.unwatch(backpath, watch)
+                hotplugStatus.value = status
+                ev.set()
+
+        hotplugStatus.value = None
+        frontpath = self.frontendPath(devid)
+        backpath = xstransact.Read(frontpath, "backend")
+
+        watch = xswatch(backpath, hotplugStatus)
+
+        ev.wait(DEVICE_CREATE_TIMEOUT)
+        if ev.isSet():
+            return (0, hotplugStatus.value)
+        else:
+            return (-1, hotplugStatus.value)
+
+
 
     def backendPath(self, backdom, devid):
         """@param backdom [XendDomainInfo] The backend domain info."""