Raise exception when all devices fail to list tests.

build/android/pylib/local/device/local_device_gtest_run.py

 # Copyright 2014 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 collections
 import itertools
 import os
 import posixpath
 
 from devil.android import device_errors
@@ skipping 297 matching lines @@
         on_failure=self._env.BlacklistDevice)
     def list_tests(dev):
       tests = self._delegate.Run(
           None, dev, flags='--gtest_list_tests', timeout=20)
       tests = gtest_test_instance.ParseGTestListTests(tests)
       tests = self._test_instance.FilterTests(tests)
       return tests
 
     # Query all devices in case one fails.
     test_lists = self._env.parallel_devices.pMap(list_tests).pGet(None)
-    # TODO(agrieve): Make this fail rather than return an empty list when
-    #     all devices fail.
+
+    # If all devices failed to list tests, raise an exception.
+    if all([tl is None for tl in test_lists]):
+      raise device_errors.CommandFailedError(
+          'Failed to list tests on any device')
     return list(sorted(set().union(*[set(tl) for tl in test_lists if tl])))
 
   #override
   def _RunTest(self, device, test):
     # Run the test.
     timeout = (self._test_instance.shard_timeout
                * self.GetTool(device).GetTimeoutScale())
     output = self._delegate.Run(
         test, device, flags=self._test_instance.test_arguments,
         timeout=timeout, retries=0)
@@ skipping 14 matching lines @@
   def TearDown(self):
     @local_device_test_run.handle_shard_failures
     def individual_device_tear_down(dev):
       for s in self._servers.get(str(dev), []):
         s.TearDown()
 
       tool = self.GetTool(dev)
       tool.CleanUpEnvironment()
 
     self._env.parallel_devices.pMap(individual_device_tear_down)