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

tools/telemetry/telemetry/core/backends/chrome/android_browser_backend.py

 # 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
 import time
 
 from telemetry.core import exceptions
@@ skipping 135 matching lines @@
         supports_tab_control=backend_settings.supports_tab_control,
         supports_extensions=False, browser_options=browser_options,
         output_profile_path=output_profile_path,
         extensions_to_load=extensions_to_load)
     if len(extensions_to_load) > 0:
       raise browser_backend.ExtensionsNotSupportedException(
           'Android browser does not support extensions.')
 
     # Initialize fields so that an explosion during init doesn't break in Close.
     self._backend_settings = backend_settings
-    self._saved_cmdline = ''
+    self._saved_cmdline = None
     self._target_arch = target_arch
     self._saved_sslflag = ''
 
     # TODO(tonyg): This is flaky because it doesn't reserve the port that it
     # allocates. Need to fix this.
     self._port = adb_commands.AllocateTestServerPort()
 
     # TODO(wuhu): Move to network controller backend.
     self._platform_backend.InstallTestCa()
 
@@ skipping 45 matching lines @@
       if values[0] in '"\'' and values[-1] == values[0]:
         return arg
       return '%s=%s' % (key, pipes.quote(values))
     args = [self._backend_settings.pseudo_exec_name]
     args.extend(self.GetBrowserStartupArgs())
     content = ' '.join(QuoteIfNeeded(arg) for arg in args)
     cmdline_file = self._backend_settings.GetCommandLineFile(
         self._adb.IsUserBuild())
 
     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_cmdline = ''.join(self._adb.device().ReadFile(cmdline_file))
+      # Save the current command line to restore later
+      self._saved_cmdline = self._adb.device().ReadFile(
+          cmdline_file, as_root=True)
+      # ... except if it appears to be a  Telemetry created one. This is to
+      # prevent common bug where --host-resolver-rules borks people's browsers
+      # if something goes wrong with Telemetry.
       if '--host-resolver-rules' in self._saved_cmdline:
-        self._saved_cmdline = ''
+        self._saved_cmdline = None
+    except device_errors.CommandFailedError: # file may not exist
+      self._saved_cmdline = None
+
+    try:
       self._adb.device().WriteFile(cmdline_file, content, as_root=True)
-    except device_errors.CommandFailedError:
-      logging.critical('Cannot set Chrome command line. '
-                       'Fix this by flashing to a userdebug build.')
+    except device_errors.CommandFailedError as exc:
+      logging.critical('Cannot set Chrome command line: %s', str(exc))
+      logging.critical('May be fixed by flashing to a userdebug build.')
       sys.exit(1)
 
   def _RestoreCommandLine(self):
     cmdline_file = self._backend_settings.GetCommandLineFile(
         self._adb.IsUserBuild())
-    self._adb.device().WriteFile(cmdline_file, self._saved_cmdline,
-                                 as_root=True)
+    if self._saved_cmdline is not None:
+      self._adb.device().WriteFile(cmdline_file, self._saved_cmdline,
+                                   as_root=True)
+    else:
+      self._adb.device().RunShellCommand(['rm', cmdline_file],
+                                         as_root=True)
 
   def Start(self):
     self._SetUpCommandLine()
 
     self._adb.device().RunShellCommand('logcat -c')
     if self.browser_options.startup_url:
       url = self.browser_options.startup_url
     elif self.browser_options.profile_dir:
       url = None
     else:
@@ skipping 91 matching lines @@
 
   def GetStandardOutput(self):
     return self._platform_backend.GetStandardOutput()
 
   def GetStackTrace(self):
     return self._platform_backend.GetStackTrace(self._target_arch)
 
   @property
   def should_ignore_certificate_errors(self):
     return not self._platform_backend.is_test_ca_installed