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, 6 insertions, 4 deletions

diff --git a/Lib/compileall.py b/Lib/compileall.py
index fe7f450..672cb43 100644
--- a/Lib/compileall.py
+++ b/Lib/compileall.py
@@ -84,12 +84,14 @@ def compile_dir(dir, maxlevels=None, ddir=None, force=False,
     if workers < 0:
         raise ValueError('workers must be greater or equal to 0')
     if workers != 1:
+        # Check if this is a system where ProcessPoolExecutor can function.
+        from concurrent.futures.process import _check_system_limits
         try:
-            # Only import when needed, as low resource platforms may
-            # fail to import it
-            from concurrent.futures import ProcessPoolExecutor
-        except ImportError:
+            _check_system_limits()
+        except NotImplementedError:
             workers = 1
+        else:
+            from concurrent.futures import ProcessPoolExecutor
     if maxlevels is None:
         maxlevels = sys.getrecursionlimit()
     files = _walk_dir(dir, quiet=quiet, maxlevels=maxlevels)