Raise exception when all devices fail to list tests.

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

Index: build/android/pylib/local/device/local_device_gtest_run.py
diff --git a/build/android/pylib/local/device/local_device_gtest_run.py b/build/android/pylib/local/device/local_device_gtest_run.py
index e740f564a23cdb8cb8c52aac216b99421a1d977d..5f8dd59542fa3f89236df239ead36585328d44c0 100644
--- a/build/android/pylib/local/device/local_device_gtest_run.py
+++ b/build/android/pylib/local/device/local_device_gtest_run.py
@@ -315,8 +315,12 @@ class LocalDeviceGtestRun(local_device_test_run.LocalDeviceTestRun):
 
     # 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.

[mikecase (-- gone --), 2016/02/19 23:41:53]

super nit: s/list_tests/list tests

[BigBossZhiling, 2016/02/24 22:51:14]

Done.

+    if all([tests == None for tests in test_lists]):

[mikecase (-- gone --), 2016/02/19 23:41:54]

super nit: To match the variable naming below...
s/tests/tl

Also super nit:
"tests is None" is probably more pythonic than "==" None

+      # Now we know that all devices failed to do list_tests.

[mikecase (-- gone --), 2016/02/19 23:41:53]

super nit: Comment is probably unnecessary given you have a comment above
explaining what you are doing.

+      raise device_errors.CommandFailedError(
+          "Failed to list_tests on all devices")

[mikecase (-- gone --), 2016/02/19 23:41:53]

super nit: s/list_tests/list tests
super nit: single quotes (style is single quotes in python unless you have a
single quote in the string)

Also, while I'm being annoying with all my nits. Should probably read (to make
it more clear)...

'Failed to list tests on any device.'
OR
'All devices failed to list tests.'

     return list(sorted(set().union(*[set(tl) for tl in test_lists if tl])))
 
   #override