Factor out a system_properties interface for interacting with getprop/setprop.

build/android/pylib/system_properties.py

Index: build/android/pylib/system_properties.py
diff --git a/build/android/pylib/system_properties.py b/build/android/pylib/system_properties.py
new file mode 100644
index 0000000000000000000000000000000000000000..1cede5647c2d5c6a050f3642b44331136f1b23d1
--- /dev/null
+++ b/build/android/pylib/system_properties.py
@@ -0,0 +1,42 @@
+# Copyright (c) 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.
+
+
+class SystemProperties(dict):
+
+  """A dict interface to interact with device system properties.
+
+  System properties are key/value pairs as exposed by adb shell getprop/setprop.
+
+  This implementation minimizes interaction with the physical device. It is
+  valid for the lifetime of a boot.
+  """
+
+  def __init__(self, android_commands):
+    super(SystemProperties, self).__init__()
+    self._adb = android_commands
+    self._cached_static_properties = {}
+
+  def __getitem__(self, key):
+    if self._IsStatic(key):
+      if key not in self._cached_static_properties:
+        self._cached_static_properties[key] = self._GetProperty(key)
+      return self._cached_static_properties[key]
+    return self._GetProperty(key)
+
+  def __setitem__(self, key, value):
+    status = self._adb.SendShellCommand(
+        'setprop %s "%s"; echo $?' % (key, value), retry_count=3).strip()

[frankf, 2013/12/03 01:27:28]

I tried on KRT16M and setprop returns 0 even if doesn't set the value because
the device is not rooted. In fact it seems to always return 0. E.g. 

$ adb shell setprop blah; echo $?

usage: setprop <key> <value>
0

+    if status != '0':
+      raise RuntimeError(
+          'Failed to setprop %s to %s. Exit status=%s' % (key, value, status))
+
+  def _IsStatic(self, key):
+    # TODO(tonyg): This list is conservative and could be expanded as needed.
+    return (key.startswith('ro.boot.') or
+            key.startswith('ro.build.') or
+            key.startswith('ro.product.'))
+
+  def _GetProperty(self, key):
+    return self._adb.SendShellCommand('getprop %s' % key, retry_count=3).strip()