Update from https://crrev.com/304121

build/android/pylib/forwarder.py

 # Copyright (c) 2012 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.
 
 # pylint: disable=W0212
 
 import fcntl
 import logging
 import os
 import psutil
@@ skipping 252 matching lines @@
       pid_with_start_time = pid_file.readline()
       if pid_with_start_time:
         (pid, process_start_time) = pid_with_start_time.split(':')
         if pid == str(pid_for_lock):
           if process_start_time == str(_GetProcessStartTime(pid_for_lock)):
             return
       self._KillHostLocked()
       pid_file.seek(0)
       pid_file.write(
           '%s:%s' % (pid_for_lock, str(_GetProcessStartTime(pid_for_lock))))
+      pid_file.truncate()
 
   def _InitDeviceLocked(self, device, tool):
     """Initializes the device_forwarder daemon for a specific device (once).
 
     Note that the global lock must be acquired before calling this method. This
     method kills any existing device_forwarder daemon on the device that could
     be stale, pushes the latest version of the daemon (to the device) and starts
     it.
 
     Args:
@@ skipping 44 matching lines @@
     """
     logging.info('Killing device_forwarder.')
     Forwarder._instance._initialized_devices.discard(str(device))
     if not device.FileExists(Forwarder._DEVICE_FORWARDER_PATH):
       return
 
     cmd = '%s %s --kill-server' % (tool.GetUtilWrapper(),
                                    Forwarder._DEVICE_FORWARDER_PATH)
     device.old_interface.GetAndroidToolStatusAndOutput(
         cmd, lib_path=Forwarder._DEVICE_FORWARDER_FOLDER)