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, 16 insertions, 3 deletions

diff --git a/Lib/test/test_concurrent_futures.py b/Lib/test/test_concurrent_futures.py
index a182b14fb9b..99651f5f4ed 100644
--- a/Lib/test/test_concurrent_futures.py
+++ b/Lib/test/test_concurrent_futures.py
@@ -4,8 +4,6 @@ from test.support import threading_helper
 
 # Skip tests if _multiprocessing wasn't built.
 import_helper.import_module('_multiprocessing')
-# Skip tests if sem_open implementation is broken.
-support.skip_if_broken_multiprocessing_synchronize()
 
 from test.support import hashlib_helper
 from test.support.script_helper import assert_python_ok
@@ -27,7 +25,7 @@ from concurrent import futures
 from concurrent.futures._base import (
     PENDING, RUNNING, CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED, Future,
     BrokenExecutor)
-from concurrent.futures.process import BrokenProcessPool
+from concurrent.futures.process import BrokenProcessPool, _check_system_limits
 from multiprocessing import get_context
 
 import multiprocessing.process
@@ -161,6 +159,10 @@ class ProcessPoolForkMixin(ExecutorMixin):
     ctx = "fork"
 
     def get_context(self):
+        try:
+            _check_system_limits()
+        except NotImplementedError:
+            self.skipTest("ProcessPoolExecutor unavailable on this system")
         if sys.platform == "win32":
             self.skipTest("require unix system")
         return super().get_context()
@@ -170,12 +172,23 @@ class ProcessPoolSpawnMixin(ExecutorMixin):
     executor_type = futures.ProcessPoolExecutor
     ctx = "spawn"
 
+    def get_context(self):
+        try:
+            _check_system_limits()
+        except NotImplementedError:
+            self.skipTest("ProcessPoolExecutor unavailable on this system")
+        return super().get_context()
+
 
 class ProcessPoolForkserverMixin(ExecutorMixin):
     executor_type = futures.ProcessPoolExecutor
     ctx = "forkserver"
 
     def get_context(self):
+        try:
+            _check_system_limits()
+        except NotImplementedError:
+            self.skipTest("ProcessPoolExecutor unavailable on this system")
         if sys.platform == "win32":
             self.skipTest("require unix system")
         return super().get_context()