Fix race condition between screen locker and dpms in power_Backlight test.

client/site_tests/power_Backlight/power_Backlight.py

 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import logging, os, time, utils
 from autotest_lib.client.bin import test
 from autotest_lib.client.common_lib import error, site_power_status, site_ui, \
                                            utils
 
 class power_Backlight(test.test):
     version = 1
 
 
     def run_once(self, delay=60, seconds=10, tries=20):
-        # disable screen locker and screen blanking
+        # disable screen locker
+        os.system('stop screen-locker')
+
+        # disable screen blanking. Stopping screen-locker isn't
+        # synchronous :(. Add a sleep for now, till powerd comes around
+        # and fixes all this for us.
         # TODO(davidjames): Power manager should support this feature directly
-        os.system('stop screen-locker')
+        time.sleep(5)
+        site_ui.xsystem('xset s off')
+        site_ui.xsystem('xset dpms 0 0 0')
         site_ui.xsystem('xset -dpms')
 
         status = site_power_status.get_status()
         if status.linepower[0].online:
             raise error.TestFail('Machine must be unplugged')
 
         cmd = 'backlight-tool --get_max_brightness'
         max_brightness = int(utils.system_output(cmd).rstrip())
         if max_brightness < 4:
             raise error.TestFail('Must have at least 5 backlight levels')
@@ skipping 14 matching lines @@
             keyvals['w_bl_%d_rate' % i] = rate
             rates.append(rate)
         self.write_perf_keyval(keyvals)
         for i in range(1, len(levels)):
             if rates[i] <= rates[i-1]:
                 raise error.TestFail('Turning up the backlight ' \
                                      'should increase energy consumption')
 
 
     def cleanup(self):
-        # re-enable screen locker and screen blanking
+        # Re-enable screen locker. This also re-enables dpms.
         utils.system('start screen-locker')
-        site_ui.xsystem('xset +dpms')