[devil] Add a constant for forwarder device dynamic port allocation.

devil/devil/android/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
 
 from devil import base_error
 from devil import devil_env
 from devil.android import device_errors
 from devil.android.constants import file_system
 from devil.android.valgrind_tools import base_tool
 from devil.utils import cmd_helper
 
 logger = logging.getLogger(__name__)
 
+# If passed as the device port, this will tell the forwarder to allocate
+# a dynamic port on the device. The actual port can then be retrieved with
+# Forwarder.DevicePortForHostPort.
+DYNAMIC_DEVICE_PORT = 0
+
 
 def _GetProcessStartTime(pid):
   return psutil.Process(pid).create_time
 
 
 def _LogMapFailureDiagnostics(device):
   # The host forwarder daemon logs to /tmp/host_forwarder_log, so print the end
   # of that.
   try:
     with open('/tmp/host_forwarder_log') as host_forwarder_log:
@@ skipping 413 matching lines @@
     logger.info('Killing device_forwarder.')
     self._initialized_devices.discard(device.serial)
     if not device.FileExists(Forwarder._DEVICE_FORWARDER_PATH):
       return
 
     cmd = '%s %s --kill-server' % (tool.GetUtilWrapper(),
                                    Forwarder._DEVICE_FORWARDER_PATH)
     device.RunShellCommand(
         cmd, env={'LD_LIBRARY_PATH': Forwarder._DEVICE_FORWARDER_FOLDER},
         check_return=True)