Create AndroidCommandLineBackend to handle setting up cmdline args

tools/telemetry/telemetry/core/backends/android_command_line_backend.py

Index: tools/telemetry/telemetry/core/backends/android_command_line_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/android_command_line_backend.py b/tools/telemetry/telemetry/core/backends/android_command_line_backend.py
new file mode 100644
index 0000000000000000000000000000000000000000..e2c3e1a1d4b6983732bbe1226638ac6601ad0e66
--- /dev/null
+++ b/tools/telemetry/telemetry/core/backends/android_command_line_backend.py
@@ -0,0 +1,98 @@
+# Copyright 2013 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 pipes
+import sys
+
+from telemetry.core import util
+
+util.AddDirToPythonPath(util.GetChromiumSrcDir(), 'build', 'android')
+from pylib.device import device_errors  # pylint: disable=F0401
+
+
+def _QuoteIfNeeded(arg):
+  # Properly escape "key=valueA valueB" to "key='valueA valueB'"
+  # Values without spaces, or that seem to be quoted are left untouched.
+  # This is required so CommandLine.java can parse valueB correctly rather
+  # than as a separate switch.
+  params = arg.split('=', 1)
+  if len(params) != 2:
+    return arg
+  key, values = params
+  if ' ' not in values:
+    return arg
+  if values[0] in '"\'' and values[-1] == values[0]:
+    return arg
+  return '%s=%s' % (key, pipes.quote(values))
+
+
+class SetUpCommandLineFlags(object):
+  """A context manager for setting up the android command line flags.
+
+  This provides a readable way of using the android command line backend class.
+  Example usage:
+
+      with android_command_line_backend.SetUpCommandLineFlags(
+          adb, backend_settings, startup_args):
+        # Something to run while the command line flags are set appropriately.
+  """
+  def __init__(self, adb, backend_settings, startup_args):
+    self._android_command_line_backend = _AndroidCommandLineBackend(
+        adb, backend_settings, startup_args)
+
+  def __enter__(self):
+    self._android_command_line_backend.SetUpCommandLineFlags()
+
+  def __exit__(self, *args):
+    self._android_command_line_backend.RestoreCommandLineFlags()
+
+
+class _AndroidCommandLineBackend(object):
+  """The backend for providing command line flags on android.
+
+  There are command line flags that Chromium accept in order to enable
+  particular features or modify otherwise default functionality. To set the
+  flags for Chrome on Android, specific files on the device must be updated
+  with the flags to enable. This class provides a wrapper around this
+  functionality.
+  """
+
+  def __init__(self, adb, backend_settings, startup_args):
+    self._adb = adb
+    self._backend_settings = backend_settings
+    self._startup_args = startup_args
+    self._saved_command_line_file_contents = ''
+
+  @property
+  def command_line_file(self):
+    return self._backend_settings.GetCommandLineFile(self._adb.IsUserBuild())
+
+  def SetUpCommandLineFlags(self):
+    args = [self._backend_settings.pseudo_exec_name]
+    args.extend(self._startup_args)
+    content = ' '.join(_QuoteIfNeeded(arg) for arg in args)
+
+    try:
+      # Save the current command line to restore later, except if it appears to
+      # be a  Telemetry created one. This is to prevent a common bug where
+      # --host-resolver-rules borks people's browsers if something goes wrong
+      # with Telemetry.
+      self._saved_command_line_file_contents = ''.join(self._ReadFile())
+      if '--host-resolver-rules' in self._saved_command_line_file_contents:
+        self._saved_command_line_file_contents = ''
+      self._WriteFile(content)
+    except device_errors.CommandFailedError:
+      logging.critical('Cannot set Chrome command line. '
+                       'Fix this by flashing to a userdebug build.')
+      sys.exit(1)
+
+  def RestoreCommandLineFlags(self):
+    self._WriteFile(self._saved_command_line_file_contents)
+
+  def _ReadFile(self):
+    return self._adb.device().ReadFile(self.command_line_file)
+
+  def _WriteFile(self, contents):
+    self._adb.device().WriteFile(self.command_line_file, contents, as_root=True)