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test_multiprocessing fails on platforms lacking a functioning sem_open (Android for example) with:
======================================================================
ERROR: test_multiprocessing (test.test_venv.BasicTest)
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Traceback (most recent call last):
 File "/data/local/tmp/python/lib/python3.8/test/test_venv.py", line 317, in test_multiprocessing
 out, err = check_output([envpy, '-c',
 File "/data/local/tmp/python/lib/python3.8/test/test_venv.py", line 37, in check_output
 raise subprocess.CalledProcessError(
subprocess.CalledProcessError: Command '['/data/local/tmp/python/tmp/tmpli0_hkdl/bin/python', '-c', 'from multiprocessing import Pool; print(Pool(1).apply_async("Python".lower).get(3))']' returned non-zero exit status 1.

Are you sure that multiprocessing is the issue? On bpo-35978, the test fails because of venv, not because of multiprocessing. Can it be the same issue?

You are right, this is a duplicate of bpo-35978.
Closing as duplicate.

Too hasty again, remembering now why I linked this failure with sem_open :-(
Re-opening this issue as the test does fail because Android lacks a functioning sem_open implementation. And tagging this issue as a dependency of bpo-35978 as we should wait for the solution on bpo-35978 before fixing this one, if it is still needed.
The link between the failure and sem_open shows up when running the python statements of test_multiprocessing() in the interpreter on Android:
generic_x86_64:/data/local/tmp/python $ python
Python 3.8.0a2+ (heads/bpo-36341-dirty:41f0b78cbf, Mar 18 2019, 10:43:05)
[Clang 3.8.275480 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from multiprocessing import Pool
>>> print(Pool(1).apply_async("Python".lower).get(3))
Traceback (most recent call last):
 File "/data/local/tmp/python/lib/python3.8/multiprocessing/synchronize.py", line 28, in <module>
 from _multiprocessing import SemLock, sem_unlink
ImportError: cannot import name 'SemLock' from '_multiprocessing' (/data/local/tmp/python/lib/python3.8/lib-dynload/_multiprocessing.cpython-38dm.so)
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/data/local/tmp/python/lib/python3.8/multiprocessing/context.py", line 118, in Pool
 return Pool(processes, initializer, initargs, maxtasksperchild,
 File "/data/local/tmp/python/lib/python3.8/multiprocessing/pool.py", line 166, in __init__
 self._setup_queues()
 File "/data/local/tmp/python/lib/python3.8/multiprocessing/pool.py", line 300, in _setup_queues
 self._inqueue = self._ctx.SimpleQueue()
 File "/data/local/tmp/python/lib/python3.8/multiprocessing/context.py", line 112, in SimpleQueue
 return SimpleQueue(ctx=self.get_context())
 File "/data/local/tmp/python/lib/python3.8/multiprocessing/queues.py", line 336, in __init__
 self._rlock = ctx.Lock()
 File "/data/local/tmp/python/lib/python3.8/multiprocessing/context.py", line 66, in Lock
 from .synchronize import Lock
 File "/data/local/tmp/python/lib/python3.8/multiprocessing/synchronize.py", line 30, in <module>
 raise ImportError("This platform lacks a functioning sem_open" +
ImportError: This platform lacks a functioning sem_open implementation, therefore, the required synchronization primitives needed will not function, see issue 3770.
>>>

So this is a test_multiprocessing issue? If it is then the title is misleading.

It is a test_venv issue related to bpo-32126 and also related to the other issues listed in bpo-32126.
Changing the title as it is ambiguous indeed.
check_output() in test_venv.py does not print the full stack trace as it is done by test_asyncio in bpo-32126, is this another issue ?

I guess my confusion comes from the fact that test_venv in isolation is a totally fine test suite, it just fails when test_multiprocessing runs it due to something multiprocessing-related.
I've tried tweaking the title to reflect the fact that it's a test_venv failure triggered by test_multiprocessing.

Removing the bpo-35978 dependency as changeset 8bba81fd55873148c65b7d0e6a6effbd63048c76 that fixes bpo-35978 only skips test_multiprocessing when test_venv is run from a venv.

New changeset 5437ccca1424e415a938c583df43d8cc74047d16 by Miss Islington (bot) (xdegaye) in branch 'master':
bpo-36342: Fix test_multiprocessing in test_venv (GH-12513)
https://github.com/python/cpython/commit/5437ccca1424e415a938c583df43d8cc74047d16

It seems like this issue has been fixed in the master branch. Reopen the issue if you consider that the fix should be backported.