bpo-40692: Run more test_concurrent_futures tests (GH-20239)

In the case of multiprocessing.synchronize() being missing, the
test_concurrent_futures test suite now skips only the tests that
require multiprocessing.synchronize().

Validate that multiprocessing.synchronize exists as part of
_check_system_limits(), allowing ProcessPoolExecutor to raise
NotImplementedError during __init__, rather than crashing with
ImportError during __init__ when creating a lock imported from
multiprocessing.synchronize.

Use _check_system_limits() to disable tests of
ProcessPoolExecutor on systems without multiprocessing.synchronize.

Running the test suite without multiprocessing.synchronize reveals
that Lib/compileall.py crashes when it uses a ProcessPoolExecutor.
Therefore, change Lib/compileall.py to call _check_system_limits()
before creating the ProcessPoolExecutor.

Note that both Lib/compileall.py and Lib/test/test_compileall.py
were attempting to sanity-check ProcessPoolExecutor by expecting
ImportError. In multiprocessing.resource_tracker, sem_unlink() is also absent
on platforms where POSIX semaphores aren't available. Avoid using
sem_unlink() if it, too, does not exist.

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/Lib/multiprocessing/resource_tracker.py b/Lib/multiprocessing/resource_tracker.py
index c9bfa9b82b6..cc42dbdda05 100644
--- a/Lib/multiprocessing/resource_tracker.py
+++ b/Lib/multiprocessing/resource_tracker.py
@@ -37,8 +37,16 @@ if os.name == 'posix':
     import _multiprocessing
     import _posixshmem
 
+    # Use sem_unlink() to clean up named semaphores.
+    #
+    # sem_unlink() may be missing if the Python build process detected the
+    # absence of POSIX named semaphores. In that case, no named semaphores were
+    # ever opened, so no cleanup would be necessary.
+    if hasattr(_multiprocessing, 'sem_unlink'):
+        _CLEANUP_FUNCS.update({
+            'semaphore': _multiprocessing.sem_unlink,
+        })
     _CLEANUP_FUNCS.update({
-        'semaphore': _multiprocessing.sem_unlink,
         'shared_memory': _posixshmem.shm_unlink,
     })