Reland of Migrate DeviceUtils.ReadFile to adb_wrapper (try 3)

build/android/pylib/flag_changer.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.
 
 import logging
 
 import pylib.android_commands
 import pylib.device.device_utils
 
+from pylib.device import device_errors
+
 
 class FlagChanger(object):
   """Changes the flags Chrome runs with.
 
   There are two different use cases for this file:
   * Flags are permanently set by calling Set().
   * Flags can be temporarily set for a particular set of unit tests.  These
     tests should call Restore() to revert the flags to their original state
     once the tests have completed.
   """
 
   def __init__(self, device, cmdline_file):
     """Initializes the FlagChanger and records the original arguments.
 
     Args:
       device: A DeviceUtils instance.
       cmdline_file: Path to the command line file on the device.
     """
     # TODO(jbudorick) Remove once telemetry switches over.
     if isinstance(device, pylib.android_commands.AndroidCommands):
       device = pylib.device.device_utils.DeviceUtils(device)
     self._device = device
     self._cmdline_file = cmdline_file
 
     # Save the original flags.
-    self._orig_line = self._device.ReadFile(self._cmdline_file)
-    if self._orig_line:
-      self._orig_line = self._orig_line[0].strip()
+    try:
+      self._orig_line = self._device.ReadFile(self._cmdline_file).strip()
+    except device_errors.CommandFailedError:
+      self._orig_line = ''
 
     # Parse out the flags into a list to facilitate adding and removing flags.
     self._current_flags = self._TokenizeFlags(self._orig_line)
 
   def Get(self):
     """Returns list of current flags."""
     return self._current_flags
 
   def Set(self, flags):
     """Replaces all flags on the current command line with the flags given.
@@ skipping 49 matching lines @@
     logging.info('Current flags: %s', self._current_flags)
     # Root is not required to write to /data/local/tmp/.
     use_root = '/data/local/tmp/' not in self._cmdline_file
     if self._current_flags:
       # The first command line argument doesn't matter as we are not actually
       # launching the chrome executable using this command line.
       cmd_line = ' '.join(['_'] + self._current_flags)
       self._device.WriteFile(
           self._cmdline_file, cmd_line, as_root=use_root)
       file_contents = self._device.ReadFile(
-          self._cmdline_file, as_root=use_root)
-      assert len(file_contents) == 1 and file_contents[0] == cmd_line, (
+          self._cmdline_file, as_root=use_root).rstrip()
+      assert file_contents == cmd_line, (
           'Failed to set the command line file at %s' % self._cmdline_file)
     else:
       self._device.RunShellCommand('rm ' + self._cmdline_file,
                                    as_root=use_root)
       assert not self._device.FileExists(self._cmdline_file), (
           'Failed to remove the command line file at %s' % self._cmdline_file)
 
   @staticmethod
   def _TokenizeFlags(line):
     """Changes the string containing the command line into a list of flags.
@@ skipping 35 matching lines @@
 
     # Tack on the last flag.
     if not current_flag:
       if within_quotations:
         logging.warn('Unterminated quoted argument: ' + line)
     else:
       tokenized_flags.append(current_flag)
 
     # Return everything but the program name.
     return tokenized_flags[1:]