Raise exception when 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 301 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)
-
     # If all devices failed to list tests, raise an exception.
-    if all([tl is None for tl in test_lists]):
+    # Check that tl is not None and is not empty.
+    if all([(tl is None or not tl) for tl in test_lists]):

[BigBossZhiling, 2016/04/25 20:07:15]

But in this case, if a suite indeed has an empty list of test cases, we will
raise an exception, instead of passing 0 test cases. I think that is fine
because we will never have an empty test suite, or what's the point of running
an empty test suite? I guess it's ok to raise an exception when the suite has 0
test case inside it.

[jbudorick, 2016/04/25 20:11:38]

Yeah, I think I'm ok with this in general. However, this should be

  if all(not t1 for t1 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(
@@ skipping 20 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)