gh-120012: clarify the behaviour of `multiprocessing.Queue.empty` on closed queues. (GH-120102)

* improve doc for `multiprocessing.Queue.empty`
* add tests for checking emptiness of queues

Co-authored-by: Gregory P. Smith <greg@krypto.org>

1 files changed, 26 insertions, 0 deletions

diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index 301541a666e..4b3a0645cfc 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -1332,6 +1332,23 @@ class _TestQueue(BaseTestCase):
         self.assertTrue(not_serializable_obj.reduce_was_called)
         self.assertTrue(not_serializable_obj.on_queue_feeder_error_was_called)
 
+    def test_closed_queue_empty_exceptions(self):
+        # Assert that checking the emptiness of an unused closed queue
+        # does not raise an OSError. The rationale is that q.close() is
+        # a no-op upon construction and becomes effective once the queue
+        # has been used (e.g., by calling q.put()).
+        for q in multiprocessing.Queue(), multiprocessing.JoinableQueue():
+            q.close()  # this is a no-op since the feeder thread is None
+            q.join_thread()  # this is also a no-op
+            self.assertTrue(q.empty())
+
+        for q in multiprocessing.Queue(), multiprocessing.JoinableQueue():
+            q.put('foo')  # make sure that the queue is 'used'
+            q.close()  # close the feeder thread
+            q.join_thread()  # make sure to join the feeder thread
+            with self.assertRaisesRegex(OSError, 'is closed'):
+                q.empty()
+
     def test_closed_queue_put_get_exceptions(self):
         for q in multiprocessing.Queue(), multiprocessing.JoinableQueue():
             q.close()
@@ -5815,6 +5832,15 @@ class TestSimpleQueue(unittest.TestCase):
         finally:
             parent_can_continue.set()
 
+    def test_empty_exceptions(self):
+        # Assert that checking emptiness of a closed queue raises
+        # an OSError, independently of whether the queue was used
+        # or not. This differs from Queue and JoinableQueue.
+        q = multiprocessing.SimpleQueue()
+        q.close()  # close the pipe
+        with self.assertRaisesRegex(OSError, 'is closed'):
+            q.empty()
+
     def test_empty(self):
         queue = multiprocessing.SimpleQueue()
         child_can_start = multiprocessing.Event()