Update scripts/block for blkback's new pause/resume ops.

The kernel can emit uevents when quiescing a VBD's I/O queue. It won't
write pause-done, but signal state changes through a new key:
queue-state. The basic idea is to let storage-level code hook into
pause/resume transitions where desirable. This script implements a
default handler.

The original device.ml protocol remains as is, but the implementation
differs:

 * ACTION=add:    request queue-state change notifications.
 * ACTION=change: translate queue-state to pause-done.

Also adds a bit more verbosity to the daemon.log.

Signed-off-by: Daniel Stodden <daniel.stodden@citrix.com>
diff -r ba079d4e6516 scripts/block--- a/scripts/block	Mon May 24 20:01:19 2010 -0700
+++ b/scripts/block	Tue May 25 00:30:01 2010 -0700
@@ -4,13 +4,59 @@
 DOMID=`echo ${XENBUS_PATH} | cut -f 3 -d '/'`
 DEVID=`echo ${XENBUS_PATH} | cut -f 4 -d '/'`

+DEVNAME=$TYPE-$DOMID-$DEVID
+
+SYSFS_PATH=/sys/devices/xen-backend/$DEVNAME
+
 XAPI=/xapi/${DOMID}/hotplug/${TYPE}/${DEVID}

-case "$1" in
+HOTPLUG="${XAPI}/hotplug"
+PAUSE="${XENBUS_PATH}/pause"
+PAUSE_DONE="${XENBUS_PATH}/pause-done"
+
+syslog ()
+{
+	logger -pdaemon.info -tscripts-block -- "$DEVNAME[$ACTION]: $*"
+}
+
+case "$ACTION" in
 add)
-        xenstore-write "${XAPI}/hotplug" "online"
-        ;;
+	syslog "writing $HOTPLUG = online"
+	xenstore write "$HOTPLUG" "online"
+
+	echo $(((1<<1) | (1<<3))) > $SYSFS_PATH/queue_events # running|paused
+	;;
+
+change)
+	state=$(xenstore read "${XENBUS_PATH}/queue-state")
+	case "$state" in
+		1) state=running;;
+		3) state=paused;;
+		*) state="unexpected-$state";;
+	esac
+
+	syslog "queue-state = $state"
+
+	case "$state" in
+		paused)
+			if xenstore exists "$PAUSE"
+			then
+				syslog "writing $PAUSE_DONE"
+				xenstore write "$PAUSE_DONE" ""
+			fi
+			;;
+		running)
+			if xenstore exists "$PAUSE_DONE"
+			then
+				syslog "removing $PAUSE_DONE"
+				xenstore rm "$PAUSE_DONE"
+			fi
+			;;
+	esac
+	;;
+
 remove)
-        xenstore-rm "${XAPI}/hotplug"
-        ;;
+	syslog "removing $HOTPLUG"
+	xenstore rm "${XAPI}/hotplug"
+	;;
 esac