[devil] Add system app replacement wrapper script.

devil/devil/android/tools/system_app_test.py

Index: devil/devil/android/tools/system_app_test.py
diff --git a/devil/devil/android/tools/system_app_test.py b/devil/devil/android/tools/system_app_test.py
new file mode 100644
index 0000000000000000000000000000000000000000..c20f9a88ca57d94c5a39147c30e431ec41877a6d
--- /dev/null
+++ b/devil/devil/android/tools/system_app_test.py
@@ -0,0 +1,66 @@
+#!/usr/bin/env python
+# Copyright 2017 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 os
+import sys
+import unittest
+
+if __name__ == '__main__':
+  sys.path.append(os.path.abspath(
+      os.path.join(os.path.dirname(__file__), '..', '..', '..')))
+
+from devil import devil_env
+from devil.android import device_utils
+from devil.android.sdk import adb_wrapper
+from devil.android.tools import system_app
+
+with devil_env.SysPath(devil_env.PYMOCK_PATH):
+  import mock
+
+
+class SystemAppTest(unittest.TestCase):
+
+  def testDoubleEnableModification(self):
+    """Ensures that system app modification logic isn't repeated.
+
+    If EnableSystemAppModification uses are nested, inner calls should
+    not need to perform any of the expensive modification logic.
+    """
+    # pylint: disable=no-self-use,protected-access
+    mock_device = mock.Mock(spec=device_utils.DeviceUtils)
+    mock_device.adb = mock.Mock(spec=adb_wrapper.AdbWrapper)
+
+    system_props = {}
+
+    def dict_setprop(prop_name, value):
+      system_props[prop_name] = value
+
+    def dict_getprop(prop_name):
+      return system_props.get(prop_name, '')
+
+    mock_device.SetProp.side_effect = dict_setprop
+    mock_device.GetProp.side_effect = dict_getprop
+
+    with system_app.EnableSystemAppModification(mock_device):
+      mock_device.EnableRoot.assert_called_once()
+      mock_device.GetProp.assert_called_once_with(
+          system_app._ENABLE_MODIFICATION_PROP)
+      mock_device.SetProp.assert_called_once_with(
+          system_app._ENABLE_MODIFICATION_PROP, '1')
+      mock_device.reset_mock()
+
+      with system_app.EnableSystemAppModification(mock_device):
+        mock_device.EnableRoot.assert_not_called()
+        mock_device.GetProp.assert_called_once_with(
+            system_app._ENABLE_MODIFICATION_PROP)
+        mock_device.SetProp.assert_not_called()
+        mock_device.reset_mock()
+
+    mock_device.SetProp.assert_called_once_with(
+        system_app._ENABLE_MODIFICATION_PROP, '0')
+
+
+if __name__ == '__main__':
+  unittest.main()