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, 8 insertions, 0 deletions

diff --git a/Lib/concurrent/futures/process.py b/Lib/concurrent/futures/process.py
index 90bc98b..7647198 100644
--- a/Lib/concurrent/futures/process.py
+++ b/Lib/concurrent/futures/process.py
@@ -533,6 +533,14 @@ def _check_system_limits():
             raise NotImplementedError(_system_limited)
     _system_limits_checked = True
     try:
+        import multiprocessing.synchronize
+    except ImportError:
+        _system_limited = (
+            "This Python build lacks multiprocessing.synchronize, usually due "
+            "to named semaphores being unavailable on this platform."
+        )
+        raise NotImplementedError(_system_limited)
+    try:
         nsems_max = os.sysconf("SC_SEM_NSEMS_MAX")
     except (AttributeError, ValueError):
         # sysconf not available or setting not available