Thread Watcher: don't fail when test already failed.

testing_support/thread_watcher.py

 # Copyright (c) 2016 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 logging
 import sys
 import threading
 import traceback
 import unittest
 
 
 class ThreadWatcherMixIn(object):
   def setUp(self):
     self._pre_test_threads = [t.ident for t in threading.enumerate()]
 
   def tearDown(self):
+    # If test failed before this check, don't raise another exception
+    # overwriting the original one, and making it harder to debug.
+    # Besides, that exception might be the reason cleanup didn't happen.
+    if self._test_definitely_failed():
+      return
+
     post_test_threads = threading.enumerate()
     new_threads = [t for t in post_test_threads
                    if t.ident not in self._pre_test_threads]
     if new_threads:
       details = []
       for th in new_threads:
         details.append('\nThread %s stacktrace:\n' % th)
         try:
           details.extend(traceback.format_stack(sys._current_frames()[t.ident]))
         except Exception:
           if t.ident in sys._current_frames():
             raise
           # This can happen due to threads starting or stopping concurrently.
           details.append('  Thread stopped while acquiring stacktrace.\n')
       details = ''.join(details)
 
       self.fail('Found %d running thread(s) after the test.\n%s' % (
-        len(new_threads), details))
+                len(new_threads), details))
 
+  def _test_definitely_failed(self):
+    """Returns True only if test has definitely failed already.
+
+    This is necessary to determine whether we should fail the test when finding
+    stray threads. If the test has failed in some other way, we should avoid
+    throwing an exception because it will mask the original error.
+    """
+    # When run using unittest runner,
+    # self._resultForDoCleanups is set to internal unittest object.
+    if hasattr(self, '_resultForDoCleanups') and self._resultForDoCleanups:
+      return not self._resultForDoCleanups.wasSuccessful()
+    # expect_tests runner does so differently (see
+    # https://codereview.chromium.org/2121343004).
+    if hasattr(self, '_test_failed_with_exception'):
+      return self._test_failed_with_exception
+    # Default case - be conservative.
+    return False
 
 
 class TestCase(unittest.TestCase, ThreadWatcherMixIn):
   """Base unittest class that fails on leaked threads after the test."""
   def setUp(self):
     super(TestCase, self).setUp()
     ThreadWatcherMixIn.setUp(self)
 
   def tearDown(self):
     ThreadWatcherMixIn.tearDown(self)
     super(TestCase, self).tearDown()