bpo-36813: Fix QueueListener to call task_done() upon termination. (GH-13113)

Fixed QueueListener in order to avoid random deadlocks.
Unable to add regression tests atm due to time constraints, will add it in a bit.
Regarding implementation, although it's nested, it does not cause performance issues whatsoever, and does not call task_done() in case of an exception (which is the right thing to do IMHO).

https://bugs.python.org/issue36813

diff --git a/Lib/logging/handlers.py b/Lib/logging/handlers.py
index 3727bf0677cdcef7afcb9c0e448a5e083e07ec77..a913d27389ab592e8f0b9acfc1758e8d13931f20 100644 (file)
--- a/Lib/logging/handlers.py
+++ b/Lib/logging/handlers.py
@@ -1477,6 +1477,8 @@ class QueueListener(object):
             try:
                 record = self.dequeue(True)
                 if record is self._sentinel:
+                    if has_task_done:
+                        q.task_done()
                     break
                 self.handle(record)
                 if has_task_done:

diff --git a/Lib/test/test_logging.py b/Lib/test/test_logging.py
index b884753ad397859000cdb4be5eeba1333634f6fb..50148dc2f252df1473a8aed8b25fe18be0be91d9 100644 (file)
--- a/Lib/test/test_logging.py
+++ b/Lib/test/test_logging.py
@@ -3633,6 +3633,16 @@ if hasattr(logging.handlers, 'QueueListener'):
                                     [m.msg if isinstance(m, logging.LogRecord)
                                      else m for m in items]))
 
+        def test_calls_task_done_after_stop(self):
+            # Issue 36813: Make sure queue.join does not deadlock.
+            log_queue = queue.Queue()
+            listener = logging.handlers.QueueListener(log_queue)
+            listener.start()
+            listener.stop()
+            with self.assertRaises(ValueError):
+                # Make sure all tasks are done and .join won't block.
+                log_queue.task_done()
+
 
 ZERO = datetime.timedelta(0)
 

diff --git a/Misc/NEWS.d/next/Library/2019-05-06-18-28-38.bpo-36813.NXD0KZ.rst b/Misc/NEWS.d/next/Library/2019-05-06-18-28-38.bpo-36813.NXD0KZ.rst
new file mode 100644 (file)
index 0000000..e89358a
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-05-06-18-28-38.bpo-36813.NXD0KZ.rst
@@ -0,0 +1,2 @@
+Fix :class:`~logging.handlers.QueueListener` to call ``queue.task_done()``
+upon stopping. Patch by Bar Harel.