Android gtest runner: Only query device for test list when necessary

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 85af2bc2c3365e59388f37a85f4980f2fbe208bf..81d85fda484730d9fa8e8d100dc1e847e5142ae9 100644
--- a/build/android/pylib/local/device/local_device_gtest_run.py
+++ b/build/android/pylib/local/device/local_device_gtest_run.py
@@ -52,6 +52,32 @@ def PullAppFilesImpl(device, package, files, directory):
         break
     device.PullFile(device_file, host_file)
 
+
+def _ExtractTestsFromFilter(gtest_filter):
+  """Returns the list of tests specified by the given filter.
+
+  Returns:
+    None if the device should be queried for the test list instead.
+  """
+  # Empty means all tests, - means exclude filter.
+  if not gtest_filter or '-' in gtest_filter:
+    return None
+
+  patterns = gtest_filter.split(':')
+  # For a single pattern, allow it even if it has a wildcard so long as the
+  # wildcard comes at the end and there is at least one . to prove the scope is
+  # not too large.
+  # This heuristic is not necessarily faster, but normally is.
+  if len(patterns) == 1 and patterns[0].endswith('*'):
+    no_suffix = patterns[0].rstrip('*')
+    if '*' not in no_suffix and '.' in no_suffix:
+      return patterns
+
+  if '*' in gtest_filter:
+    return None
+  return patterns
+
+
 class _ApkDelegate(object):
   def __init__(self, test_instance):
     self._activity = test_instance.activity
@@ -221,6 +247,16 @@ class LocalDeviceGtestRun(local_device_test_run.LocalDeviceTestRun):
 
   #override
   def _GetTests(self):
+    # When the exact list of tests to run is given via command-line (e.g. when
+    # locally iterating on a specific test), skip querying the device (which
+    # takes ~3 seconds).
+    tests = _ExtractTestsFromFilter(self._test_instance.gtest_filter)
+    if tests:
+      return tests
+
+    # Even when there's only one device, it still makes sense to retrieve the
+    # test list so that tests can be split up and run in batches rather than all
+    # at once (since test output is not streamed).
     @local_device_test_run.handle_shard_failures_with(
         on_failure=self._env.BlacklistDevice)
     def list_tests(dev):
@@ -230,9 +266,11 @@ class LocalDeviceGtestRun(local_device_test_run.LocalDeviceTestRun):
       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)
-    tests = list(sorted(set().union(*[set(tl) for tl in test_lists if tl])))
-    return tests
+    # TODO(agrieve): Make this fail rather than return an empty list when
+    #     all devices fail.
+    return list(sorted(set().union(*[set(tl) for tl in test_lists if tl])))
 
   #override
   def _RunTest(self, device, test):