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I've been debugging this and I can repro on El Capitan on a different machine as well -- it's an infrequent hang, I've been running in a loop:
mattb@mattb-mbp:~/src/misc/cpython master$ for i in $(seq 1000); do echo "Run: $i -- $(date)"; time./python.exe -m test --timeout 180 test_multiprocessing_forkserver || break; sleep 10; done
And in this case after ~30 successful runs:
Run: 31 -- Tue Jun 20 05:12:19 PDT 2017
Run tests sequentially
0:00:00 load avg: 4.28 [1/1] test_multiprocessing_forkserver
Timeout (0:03:00)!
Thread 0x00007fffa1b3a3c0 (most recent call first):
 File "/Users/mattb/src/misc/cpython/Lib/selectors.py", line 415 in select
 File "/Users/mattb/src/misc/cpython/Lib/multiprocessing/connection.py", line 916 in wait
 File "/Users/mattb/src/misc/cpython/Lib/multiprocessing/popen_fork.py", line 45 in wait
 File "/Users/mattb/src/misc/cpython/Lib/multiprocessing/process.py", line 121 in join
 File "/Users/mattb/src/misc/cpython/Lib/test/_test_multiprocessing.py", line 431 in test_many_processes
 File "/Users/mattb/src/misc/cpython/Lib/unittest/case.py", line 605 in run
 File "/Users/mattb/src/misc/cpython/Lib/unittest/case.py", line 653 in __call__
 File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 122 in run
 File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 84 in __call__
 File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 122 in run
 File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 84 in __call__
 File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 122 in run
 File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 84 in __call__
 File "/Users/mattb/src/misc/cpython/Lib/test/support/__init__.py", line 1772 in run
 File "/Users/mattb/src/misc/cpython/Lib/test/support/__init__.py", line 1896 in _run_suite
 File "/Users/mattb/src/misc/cpython/Lib/test/support/__init__.py", line 1936 in run_unittest
 File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/runtest.py", line 168 in test_runner
 File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/runtest.py", line 169 in runtest_inner
 File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/runtest.py", line 137 in runtest
 File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/main.py", line 374 in run_tests_sequential
 File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/main.py", line 454 in run_tests
 File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/main.py", line 530 in _main
 File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/main.py", line 504 in main
 File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/main.py", line 573 in main
 File "/Users/mattb/src/misc/cpython/Lib/test/__main__.py", line 2 in <module>
 File "/Users/mattb/src/misc/cpython/Lib/runpy.py", line 85 in _run_code
 File "/Users/mattb/src/misc/cpython/Lib/runpy.py", line 193 in _run_module_as_main
real 3m0.115s
user 0m5.804s
sys 0m0.683s
I also ran ~600 passes overnight on this test on an Ubuntu 16.04 machine without a hang, so it seems to be OSX specific.