blktap: Automatically start tapdisk-ioemu on demand

When a domain wants to use a tap:ioemu disk but has no device model,
start a tapdisk-ioemu instance as provider. Also, move the creation
and removal of communication pipes to xend so that qemu-dm doesn't
need the unwanted SIGHUP handler anymore.

Signed-off-by: Kevin Wolf <kwolf@suse.de>

tools/python/xen/xend/XendDomainInfo.py

@@ -1837,6 +1837,9 @@ class XendDomainInfo:
 
         @raise: VmError for invalid devices
         """
+        if self.image:
+            self.image.prepareEnvironment()
+
         ordered_refs = self.info.ordered_device_refs()
         for dev_uuid in ordered_refs:
             devclass, config = self.info['devices'][dev_uuid]

tools/python/xen/xend/image.py

@@ -185,6 +185,42 @@ class ImageHandler:
         """Build the domain. Define in subclass."""
         raise NotImplementedError()
 
+    def prepareEnvironment(self):
+        """Prepare the environment for the execution of the domain. This
+        method is called before any devices are set up."""
+        
+        domid = self.vm.getDomid()
+	
+        # Delete left-over pipes
+        try:
+            os.unlink('/var/run/tap/qemu-read-%d' % domid)
+            os.unlink('/var/run/tap/qemu-write-%d' % domid)
+        except:
+            pass
+
+        # No device model, don't create pipes
+        if self.device_model is None:
+            return
+
+        # If we use a device model, the pipes for communication between
+        # blktapctrl and ioemu must be present before the devices are 
+        # created (blktapctrl must access them for new block devices)
+
+        # mkdir throws an exception if the path already exists
+        try:
+            os.mkdir('/var/run/tap', 0755)
+        except:
+            pass
+
+        try:
+            os.mkfifo('/var/run/tap/qemu-read-%d' % domid, 0600)
+            os.mkfifo('/var/run/tap/qemu-write-%d' % domid, 0600)
+        except OSError, e:
+            log.warn('Could not create blktap pipes for domain %d' % domid)
+            log.exception(e)
+            pass
+
+
     # Return a list of cmd line args to the device models based on the
     # xm config file
     def parseDeviceModelArgs(self, vmConfig):
@@ -411,6 +447,12 @@ class ImageHandler:
             self.pid = None
             state = xstransact.Remove("/local/domain/0/device-model/%i"
                                       % self.vm.getDomid())
+            
+            try:
+                os.unlink('/var/run/tap/qemu-read-%d' % self.vm.getDomid())
+                os.unlink('/var/run/tap/qemu-write-%d' % self.vm.getDomid())
+            except:
+                pass
 
 
 class LinuxImageHandler(ImageHandler):