[android] Fail if all devices drop offline while running tests.

build/android/pylib/base/test_dispatcher.py

Index: build/android/pylib/base/test_dispatcher.py
diff --git a/build/android/pylib/base/test_dispatcher.py b/build/android/pylib/base/test_dispatcher.py
index 31fa0784dc94cd1249ee289eefc04672c8e7299b..39cc0817702a8092ec28305a1c0c790b84161219 100644
--- a/build/android/pylib/base/test_dispatcher.py
+++ b/build/android/pylib/base/test_dispatcher.py
@@ -126,6 +126,10 @@ class _TestCollection(object):
         break
       yield r
 
+  def __len__(self):
+    """Return the number of tests currently in the collection."""
+    return len(self._tests)
+
 
 def _RunTestsFromQueue(runner, test_collection, out_results, watcher,
                        num_retries, tag_results_with_device=False):
@@ -236,14 +240,14 @@ def _RunAllTests(runners, test_collection_factory, num_retries, timeout=None,
   results = []
   exit_code = 0
   watcher = watchdog_timer.WatchdogTimer(timeout)
+  test_collections = [test_collection_factory() for _ in runners]
 
   workers = reraiser_thread.ReraiserThreadGroup(
       [reraiser_thread.ReraiserThread(
           _RunTestsFromQueue,
-          [r, test_collection_factory(), results, watcher, num_retries,
-           tag_results_with_device],
+          [r, tc, results, watcher, num_retries, tag_results_with_device],
           name=r.device[-4:])
-       for r in runners])
+       for r, tc in zip(runners, test_collections)])
   run_results = base_test_result.TestRunResults()
   workers.StartAll()
 
@@ -254,6 +258,10 @@ def _RunAllTests(runners, test_collection_factory, num_retries, timeout=None,
     logging.error(e)
     exit_code = constants.WARNING_EXIT_CODE
 
+  assert all([len(tc) == 0 for tc in test_collections]), (
+      'Some tests were not run, all devices are likely offline (ran %d tests)' %
+      len(run_results.GetAll()))
+
   for r in results:
     run_results.AddTestRunResults(r)
   if not run_results.DidRunPass():