telemetry: Fix runaway subprocess in monsoon_power_monitor.py

tools/telemetry/telemetry/core/platform/power_monitor/monsoon_power_monitor.py

 # Copyright 2014 The Chromium 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 json
 import multiprocessing
 import tempfile
 import time
 
 from telemetry.core import exceptions
@@ skipping 55 matching lines @@
     assert not self._powermonitor_process, (
         'Must call StopMonitoringPower().')
     super(MonsoonPowerMonitor, self).StartMonitoringPower(browser)
     self._powermonitor_output_file = tempfile.TemporaryFile()
     self._is_collecting = multiprocessing.Event()
     self._powermonitor_process = multiprocessing.Process(
         target=_MonitorPower,
         args=(self._monsoon,
               self._is_collecting,
               self._powermonitor_output_file))
+    # Ensure child is not left behind: parent kills daemonic children on exit.
+    self._powermonitor_process.daemon = True
     self._powermonitor_process.start()
     if not self._is_collecting.wait(timeout=0.5):
       self._powermonitor_process.terminate()
       raise exceptions.ProfilingException('Failed to start data collection.')
 
   def StopMonitoringPower(self):
     assert self._powermonitor_process, (
         'StartMonitoringPower() not called.')
     try:
       cpu_stats = super(MonsoonPowerMonitor, self).StopMonitoringPower()
@@ skipping 29 matching lines @@
         energy_consumption_mw = current_a * voltage_v * 10**3
         total_energy_consumption_mwh += energy_consumption_mw * timedelta_h
         power_samples.append(energy_consumption_mw)
 
     out_dict = {}
     out_dict['identifier'] = 'monsoon'
     out_dict['power_samples_mw'] = power_samples
     out_dict['energy_consumption_mwh'] = total_energy_consumption_mwh
 
     return out_dict