gh-155912: Catch OSError from os.sysconf in _check_system_limits

Lib/concurrent/futures/process.py

@@ -707,8 +707,9 @@ def _check_system_limits():
         raise NotImplementedError(_system_limited)
     try:
         nsems_max = os.sysconf("SC_SEM_NSEMS_MAX")
-    except (AttributeError, ValueError):
-        # sysconf not available or setting not available
+    except (AttributeError, ValueError, OSError):
+        # sysconf not available, setting not available, or the read was
+        # denied (e.g. a sandbox profile that denies sysctl reads on macOS)
         return
     if nsems_max == -1:
         # indetermined limit, assume that limit is determined

Lib/test/test_concurrent_futures/test_process_pool.py

@@ -8,6 +8,7 @@
 import unittest.mock
 import weakref
 from concurrent import futures
+from concurrent.futures import process as futures_process
 from concurrent.futures.process import BrokenProcessPool
 
 from test import support
@@ -543,6 +544,48 @@ def test_force_shutdown_workers_stops_pool(self, function_name):
                     break
 
 
+@unittest.skipUnless(hasattr(os, "sysconf"), "requires os.sysconf()")
+class CheckSystemLimitsTest(unittest.TestCase):
+    # gh-155912: a denied sysconf read is the same condition as an
+    # unavailable one and must not be fatal.
+
+    def setUp(self):
+        support.skip_if_broken_multiprocessing_synchronize()
+        saved = (futures_process._system_limits_checked,
+                 futures_process._system_limited)
+
+        def restore():
+            (futures_process._system_limits_checked,
+             futures_process._system_limited) = saved
+
+        self.addCleanup(restore)
+
+    def check(self, **kwargs):
+        futures_process._system_limits_checked = False
+        futures_process._system_limited = None
+        with unittest.mock.patch.object(os, "sysconf", **kwargs):
+            futures_process._check_system_limits()
+
+    def test_denied_read_is_not_fatal(self):
+        self.check(side_effect=PermissionError(1, "Operation not permitted"))
+
+    def test_other_oserror_is_not_fatal(self):
+        self.check(side_effect=OSError(5, "Input/output error"))
+
+    def test_unsupported_name_is_not_fatal(self):
+        self.check(side_effect=ValueError)
+
+    def test_indeterminate_limit_is_not_fatal(self):
+        self.check(return_value=-1)
+
+    def test_sufficient_semaphores(self):
+        self.check(return_value=87381)
+
+    def test_too_few_semaphores_still_raises(self):
+        with self.assertRaises(NotImplementedError):
+            self.check(return_value=10)
+
+
 create_executor_tests(globals(), ProcessPoolExecutorTest,
                       executor_mixins=(ProcessPoolForkMixin,
                                        ProcessPoolForkserverMixin,

Misc/NEWS.d/next/Library/2026-08-23-14-54-39.gh-issue-155912.Hk7Qa2.rst

@@ -0,0 +1,3 @@
+Fix :class:`concurrent.futures.ProcessPoolExecutor` construction when
+:func:`os.sysconf` raises :exc:`OSError`, for instance when a sandbox denies
+the read.