[3.7] closes bpo-34650: Check if sched_getscheduler returns ENOSYS before declaring it supported. (GH-9228)

musl doesn't support the scheduler API, but declares stubs that alway return ENOSYS..
(cherry picked from commit c704222)

Co-authored-by: Benjamin Peterson <benjamin@python.org>

Author: benjaminp

Lib/test/test_posix.py

@@ -23,6 +23,18 @@
 requires_32b = unittest.skipUnless(sys.maxsize < 2**32,
         'test is only meaningful on 32-bit builds')
 
+def _supports_sched():
+    if not hasattr(posix, 'sched_getscheduler'):
+        return False
+    try:
+        posix.sched_getscheduler(0)
+    except OSError as e:
+        if e.errno == errno.ENOSYS:
+            return False
+    return True
+
+requires_sched = unittest.skipUnless(_supports_sched(), 'requires POSIX scheduler API')
+
 class PosixTester(unittest.TestCase):
 
     def setUp(self):
@@ -1276,7 +1288,7 @@ def test_sched_priority(self):
             self.assertRaises(OSError, posix.sched_get_priority_min, -23)
             self.assertRaises(OSError, posix.sched_get_priority_max, -23)
 
-    @unittest.skipUnless(hasattr(posix, 'sched_setscheduler'), "can't change scheduler")
+    @requires_sched
     def test_get_and_set_scheduler_and_param(self):
         possible_schedulers = [sched for name, sched in posix.__dict__.items()
                                if name.startswith("SCHED_")]