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..94a755b7b9ac25c7080f419a1b096376a08413cd
--- /dev/null
+++ b/build/android/pylib/system_properties.py
@@ -0,0 +1,38 @@
+# 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 expose by adb shell getprop/setprop.

[frankf, 2013/12/03 00:20:27]

exposed

[tonyg, 2013/12/03 01:02:02]

Done.

+
+  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):
+    self._adb.SendShellCommand('setprop %s "%s"' % (key, value), retry_count=3)

[frankf, 2013/12/03 00:20:27]

We're trying be more defensive wrt android commands. So be mindful of how things
could silently fail.

One case I can think of is when device is not root. setprop will succeed. So
perhaps, you want to verify the value is actually set.

[tonyg, 2013/12/03 01:02:02]

Done.

+
+  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()