test_signal.test_interprocess_signal() has a race condition:

with self.subprocess_send_signal(pid, "SIGUSR1") as child: # here
    self.wait_signal(child, 'SIGUSR1', SIGUSR1Exception)

The test only except SIGUSR1Exception inside wait_signal(), but the signal can be sent during subprocess_send_signal() call. assertRaises(SIGUSR1Exception) should surround these two lines instead. wait_signal() shouldn't handle the signal.

Or wait_signal() should call subprocess_send_signal().


It seems like Python 2.7 has the proper design. It might be a regression introduced by myself in:

commit 32eb840a42ec0e131daac48d43aa35290e72571e
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Tue Mar 15 11:12:35 2016 +0100

    Issue #26566: Rewrite test_signal.InterProcessSignalTests
    
    * Add Lib/test/signalinterproctester.py
    * Don't disable the garbage collector anymore
    * Don't use os.fork() with a subprocess to not inherit existing signal handlers
      or threads: start from a fresh process
    * Don't use UNIX kill command to send a signal but Python os.kill()
    * Use a timeout of 10 seconds to wait for the signal instead of 1 second
    * Always use signal.pause(), instead of time.wait(1), to wait for a signal
    * Use context manager on subprocess.Popen
    * remove code to retry on EINTR: it's no more needed since the PEP 475
    * remove unused function exit_subprocess()
    * Cleanup the code


FAIL: test_interprocess_signal (test.test_signal.PosixTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/Lib/test/test_signal.py", line 62, in test_interprocess_signal
    assert_python_ok(script)
  File "/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/Lib/test/support/script_helper.py", line 157, in assert_python_ok
    return _assert_python(True, *args, **env_vars)
  File "/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/Lib/test/support/script_helper.py", line 143, in _assert_python
    res.fail(cmd_line)
  File "/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/Lib/test/support/script_helper.py", line 84, in fail
    err))
AssertionError: Process return code is 1
command line: ['/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/python', '-X', 'faulthandler', '-I', '/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/Lib/test/signalinterproctester.py']

stdout:
---

---

stderr:
---
E/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/Lib/subprocess.py:858: ResourceWarning: subprocess 64567 is still running
  ResourceWarning, source=self)
ResourceWarning: Enable tracemalloc to get the object allocation traceback

======================================================================
ERROR: test_interprocess_signal (__main__.InterProcessSignalTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/Lib/test/signalinterproctester.py", line 68, in test_interprocess_signal
    with self.subprocess_send_signal(pid, "SIGUSR1") as child:
  File "/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/Lib/test/signalinterproctester.py", line 50, in subprocess_send_signal
    return subprocess.Popen(args)
  File "/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/Lib/subprocess.py", line 775, in __init__
    restore_signals, start_new_session)
  File "/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/Lib/subprocess.py", line 1476, in _execute_child
    part = os.read(errpipe_read, 50000)
  File "/usr/home/buildbot/python/3.7.koobs-freebsd10.nondebug/build/Lib/test/signalinterproctester.py", line 22, in sigusr1_handler
    raise SIGUSR1Exception
SIGUSR1Exception

----------------------------------------------------------------------
Ran 1 test in 0.223s

FAILED (errors=1)
---

----------------------------------------------------------------------

Ran 43 tests in 31.872s

FAILED (failures=1, skipped=2)
test test_signal failed

I forgot the URL to the build:
https://buildbot.python.org/all/#/builders/170/builds/175

New changeset 2ab2afd387084ba38a37f5944fcb0675113b64dc by Pablo Galindo in branch 'master':
bpo-35426: Eliminate race condition in test_interprocess_signal (GH-11087)
https://github.com/python/cpython/commit/2ab2afd387084ba38a37f5944fcb0675113b64dc

New changeset f7404a5a08b70ec171279a277c1817e82430fa83 by Miss Islington (bot) in branch '3.7':
bpo-35426: Eliminate race condition in test_interprocess_signal (GH-11087)
https://github.com/python/cpython/commit/f7404a5a08b70ec171279a277c1817e82430fa83

New changeset 869e23e0af806ed3a10d0484827cb1b5f5cd5e5f by Miss Islington (bot) in branch '3.6':
bpo-35426: Eliminate race condition in test_interprocess_signal (GH-11087)
https://github.com/python/cpython/commit/869e23e0af806ed3a10d0484827cb1b5f5cd5e5f

Thanks Pablo Galindo for the quick fix!