test_pthread_kill is flaky / racey

[sbc100]

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FAIL: test_pthread_kill (test_other.other)
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Traceback (most recent call last):
  File "/Users/chrome-bot/Library/Caches/vpython-root.502/store/cpython+pvkc65g756tssn1rh3h5u9e3lg/contents/lib/python3.8/unittest/case.py", line 60, in testPartExecutor
    yield
  File "/Users/chrome-bot/Library/Caches/vpython-root.502/store/cpython+pvkc65g756tssn1rh3h5u9e3lg/contents/lib/python3.8/unittest/case.py", line 676, in run
    self._callTestMethod(testMethod)
  File "/Users/chrome-bot/Library/Caches/vpython-root.502/store/cpython+pvkc65g756tssn1rh3h5u9e3lg/contents/lib/python3.8/unittest/case.py", line 633, in _callTestMethod
    method()
  File "/Volumes/Work/s/w/ir/x/w/install/emscripten/test/common.py", line 366, in decorated
    f(self, *args, **kwargs)
  File "/Volumes/Work/s/w/ir/x/w/install/emscripten/test/test_other.py", line 14279, in test_pthread_kill
    self.do_run_in_out_file_test('pthread/test_pthread_kill.c')
  File "/Volumes/Work/s/w/ir/x/w/install/emscripten/test/common.py", line 1957, in do_run_in_out_file_test
    output = self._build_and_run(srcfile, expected, **kwargs)
  File "/Volumes/Work/s/w/ir/x/w/install/emscripten/test/common.py", line 2003, in _build_and_run
    self.assertContained(o, js_output, regex=regex)
  File "/Volumes/Work/s/w/ir/x/w/install/emscripten/test/common.py", line 1672, in assertContained
    self.fail("Expected to find '%s' in '%s', diff:\n\n%s\n%s" % (
  File "/Users/chrome-bot/Library/Caches/vpython-root.502/store/cpython+pvkc65g756tssn1rh3h5u9e3lg/contents/lib/python3.8/unittest/case.py", line 753, in fail
    raise self.failureException(msg)
AssertionError: Expected to find 'thread has started, sending SIGTERM
SIGTERM sent
signal: 15 onthread=1
got term signal, shutting down thread
' in 'thread has started, sending SIGTERM
signal: 15 onthread=1
got term signal, shutting down thread
SIGTERM sent
', diff:

--- expected
+++ actual
@@ -1,5 +1,5 @@
 thread has started, sending SIGTERM
-SIGTERM sent
 signal: 15 onthread=1
 got term signal, shutting down thread
+SIGTERM sent