Enhance chromium_tests.configure_build for generalized trybot mirroring

scripts/slave/recipe_modules/chromium_tests/bot_config_and_test_db.py

 # Copyright 2016 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 BotConfig(object):
+  """Wrapper that allows combining several compatible bot configs."""
+
+  def __init__(self, bots_dict, bot_ids):
+    self._bots_dict = bots_dict
+
+    assert len(bot_ids) >= 1
+    self._bot_ids = bot_ids
+
+  def _consistent_get(self, getter, name, default=None):
+    result = getter(self._bot_ids[0], name, default)
+    for bot_id in self._bot_ids:
+      other_result = getter(bot_id, name, default)
+      assert result == other_result, (
+          'Inconsistent value for %r: bot %r has %r, '
+          'but bot %r has %r' % (
+              name, self._bot_ids[0], result, bot_id, other_result))
+    return result
+
+  def _get(self, bot_id, name, default=None):
+    return self._bots_dict.get(bot_id['mastername'], {}).get(
+        'builders', {}).get(bot_id['buildername'], {}).get(name, default)
+
+  def get(self, name, default=None):
+    return self._consistent_get(self._get, name, default)

[Ken Russell (switch to Gerrit), 2016/01/12 18:06:21]

This will need to be relaxed or generalized in the future. Properties like
chromium_apply_config and gclient_apply_config have to be computed as the union
of all of those on all of the contained bots.

[Ken Russell (switch to Gerrit), 2016/01/12 18:20:35]

Pawel and I talked offline about ensuring that these are actually consistent
across all of the builders that are mirrored by a given tryserver. After looking
into this more deeply, I think that's feasible, so this isn't a hard
requirement.

+
+  def _get_master_setting(self, bot_id, name, default=None):
+    return self._bots_dict.get(bot_id['mastername'], {}).get(
+        'settings', {}).get(name, default)
+
+  def get_master_setting(self, name, default=None):
+    return self._consistent_get(self._get_master_setting, name, default)
+
+
 class BotConfigAndTestDB(object):
   """An immutable database of bot configurations and test specifications.
   Holds the data for potentially multiple waterfalls (masternames). Most
   queries against this database are made with (mastername, buildername)
   pairs.
   """
 
   def __init__(self):
     # Indexed by mastername. Each entry contains a master_dict and a
     # test_spec.
@@ skipping 27 matching lines @@
     """A generator of all the (buildername, bot_config) tuples whose
     parent_buildername is the passed one on the given master.
     """
     for buildername, bot_config in self._db[mastername]['master_dict'].get(
         'builders', {}).iteritems():
       if bot_config.get('parent_buildername') == parent_buildername:
         yield buildername, bot_config
 
   def get_test_spec(self, mastername, buildername):
     return self._db[mastername]['test_spec'].get(buildername, {})