asyncio: Improve canceled timer handles cleanup. Closes issue #22448.

Patch by Joshua Moore-Oliva.

Lib/asyncio/base_events.py

@@ -40,6 +40,13 @@ __all__ = ['BaseEventLoop', 'Server']
 # Argument for default thread pool executor creation.
 _MAX_WORKERS = 5
 
+# Minimum number of _scheduled timer handles before cleanup of
+# cancelled handles is performed.
+_MIN_SCHEDULED_TIMER_HANDLES = 100
+
+# Minimum fraction of _scheduled timer handles that are cancelled
+# before cleanup of cancelled handles is performed.
+_MIN_CANCELLED_TIMER_HANDLES_FRACTION = 0.5
 
 def _format_handle(handle):
     cb = handle._callback
@@ -145,6 +152,7 @@ class Server(events.AbstractServer):
 class BaseEventLoop(events.AbstractEventLoop):
 
     def __init__(self):
+        self._timer_cancelled_count = 0
         self._closed = False
         self._ready = collections.deque()
         self._scheduled = []
@@ -349,6 +357,7 @@ class BaseEventLoop(events.AbstractEventLoop):
         if timer._source_traceback:
             del timer._source_traceback[-1]
         heapq.heappush(self._scheduled, timer)
+        timer._scheduled = True
         return timer
 
     def call_soon(self, callback, *args):
@@ -964,16 +973,19 @@ class BaseEventLoop(events.AbstractEventLoop):
         assert isinstance(handle, events.Handle), 'A Handle is required here'
         if handle._cancelled:
             return
-        if isinstance(handle, events.TimerHandle):
+        assert not isinstance(handle, events.TimerHandle)
-            heapq.heappush(self._scheduled, handle)
+        self._ready.append(handle)
-        else:
-            self._ready.append(handle)
 
     def _add_callback_signalsafe(self, handle):
         """Like _add_callback() but called from a signal handler."""
         self._add_callback(handle)
         self._write_to_self()
 
+    def _timer_handle_cancelled(self, handle):
+        """Notification that a TimerHandle has been cancelled."""
+        if handle._scheduled:
+            self._timer_cancelled_count += 1
+
     def _run_once(self):
         """Run one full iteration of the event loop.
 
@@ -981,9 +993,26 @@ class BaseEventLoop(events.AbstractEventLoop):
         schedules the resulting callbacks, and finally schedules
         'call_later' callbacks.
         """
-        # Remove delayed calls that were cancelled from head of queue.
+
-        while self._scheduled and self._scheduled[0]._cancelled:
+        # Remove delayed calls that were cancelled if their number is too high
-            heapq.heappop(self._scheduled)
+        sched_count = len(self._scheduled)
+        if (sched_count > _MIN_SCHEDULED_TIMER_HANDLES and
+            self._timer_cancelled_count / sched_count >
+                _MIN_CANCELLED_TIMER_HANDLES_FRACTION):
+            for handle in self._scheduled:
+                if handle._cancelled:
+                    handle._scheduled = False
+
+            self._scheduled = [x for x in self._scheduled if not x._cancelled]
+            self._timer_cancelled_count = 0
+
+            heapq.heapify(self._scheduled)
+        else:
+            # Remove delayed calls that were cancelled from head of queue.
+            while self._scheduled and self._scheduled[0]._cancelled:
+                self._timer_cancelled_count -= 1
+                handle = heapq.heappop(self._scheduled)
+                handle._scheduled = False
 
         timeout = None
         if self._ready:
@@ -1024,6 +1053,7 @@ class BaseEventLoop(events.AbstractEventLoop):
             if handle._when >= end_time:
                 break
             handle = heapq.heappop(self._scheduled)
+            handle._scheduled = False
             self._ready.append(handle)
 
         # This is the only place where callbacks are actually *called*.

Lib/asyncio/events.py

@@ -105,14 +105,15 @@ class Handle:
         return '<%s>' % ' '.join(info)
 
     def cancel(self):
-        self._cancelled = True
+        if not self._cancelled:
-        if self._loop.get_debug():
+            self._cancelled = True
-            # Keep a representation in debug mode to keep callback and
+            if self._loop.get_debug():
-            # parameters. For example, to log the warning "Executing <Handle
+                # Keep a representation in debug mode to keep callback and
-            # ...> took 2.5 second"
+                # parameters. For example, to log the warning
-            self._repr = repr(self)
+                # "Executing <Handle...> took 2.5 second"
-        self._callback = None
+                self._repr = repr(self)
-        self._args = None
+            self._callback = None
+            self._args = None
 
     def _run(self):
         try:
@@ -134,7 +135,7 @@ class Handle:
 class TimerHandle(Handle):
     """Object returned by timed callback registration methods."""
 
-    __slots__ = ['_when']
+    __slots__ = ['_scheduled', '_when']
 
     def __init__(self, when, callback, args, loop):
         assert when is not None
@@ -142,6 +143,7 @@ class TimerHandle(Handle):
         if self._source_traceback:
             del self._source_traceback[-1]
         self._when = when
+        self._scheduled = False
 
     def _repr_info(self):
         info = super()._repr_info()
@@ -180,6 +182,11 @@ class TimerHandle(Handle):
         equal = self.__eq__(other)
         return NotImplemented if equal is NotImplemented else not equal
 
+    def cancel(self):
+        if not self._cancelled:
+            self._loop._timer_handle_cancelled(self)
+        super().cancel()
+
 
 class AbstractServer:
     """Abstract server returned by create_server()."""
@@ -238,6 +245,10 @@ class AbstractEventLoop:
 
     # Methods scheduling callbacks.  All these return Handles.
 
+    def _timer_handle_cancelled(self, handle):
+        """Notification that a TimerHandle has been cancelled."""
+        raise NotImplementedError
+
     def call_soon(self, callback, *args):
         return self.call_later(0, callback, *args)
 

Lib/test/test_asyncio/test_base_events.py

@@ -2,6 +2,7 @@
 
 import errno
 import logging
+import math
 import socket
 import sys
 import time
@@ -73,13 +74,6 @@ class BaseEventLoopTests(test_utils.TestCase):
         self.assertFalse(self.loop._scheduled)
         self.assertIn(h, self.loop._ready)
 
-    def test__add_callback_timer(self):
-        h = asyncio.TimerHandle(time.monotonic()+10, lambda: False, (),
-                                self.loop)
-
-        self.loop._add_callback(h)
-        self.assertIn(h, self.loop._scheduled)
-
     def test__add_callback_cancelled_handle(self):
         h = asyncio.Handle(lambda: False, (), self.loop)
         h.cancel()
@@ -283,6 +277,82 @@ class BaseEventLoopTests(test_utils.TestCase):
         self.assertTrue(processed)
         self.assertEqual([handle], list(self.loop._ready))
 
+    def test__run_once_cancelled_event_cleanup(self):
+        self.loop._process_events = mock.Mock()
+
+        self.assertTrue(
+            0 < base_events._MIN_CANCELLED_TIMER_HANDLES_FRACTION < 1.0)
+
+        def cb():
+            pass
+
+        # Set up one "blocking" event that will not be cancelled to
+        # ensure later cancelled events do not make it to the head
+        # of the queue and get cleaned.
+        not_cancelled_count = 1
+        self.loop.call_later(3000, cb)
+
+        # Add less than threshold (base_events._MIN_SCHEDULED_TIMER_HANDLES)
+        # cancelled handles, ensure they aren't removed
+
+        cancelled_count = 2
+        for x in range(2):
+            h = self.loop.call_later(3600, cb)
+            h.cancel()
+
+        # Add some cancelled events that will be at head and removed
+        cancelled_count += 2
+        for x in range(2):
+            h = self.loop.call_later(100, cb)
+            h.cancel()
+
+        # This test is invalid if _MIN_SCHEDULED_TIMER_HANDLES is too low
+        self.assertLessEqual(cancelled_count + not_cancelled_count,
+            base_events._MIN_SCHEDULED_TIMER_HANDLES)
+
+        self.assertEqual(self.loop._timer_cancelled_count, cancelled_count)
+
+        self.loop._run_once()
+
+        cancelled_count -= 2
+
+        self.assertEqual(self.loop._timer_cancelled_count, cancelled_count)
+
+        self.assertEqual(len(self.loop._scheduled),
+            cancelled_count + not_cancelled_count)
+
+        # Need enough events to pass _MIN_CANCELLED_TIMER_HANDLES_FRACTION
+        # so that deletion of cancelled events will occur on next _run_once
+        add_cancel_count = int(math.ceil(
+            base_events._MIN_SCHEDULED_TIMER_HANDLES *
+            base_events._MIN_CANCELLED_TIMER_HANDLES_FRACTION)) + 1
+
+        add_not_cancel_count = max(base_events._MIN_SCHEDULED_TIMER_HANDLES -
+            add_cancel_count, 0)
+
+        # Add some events that will not be cancelled
+        not_cancelled_count += add_not_cancel_count
+        for x in range(add_not_cancel_count):
+            self.loop.call_later(3600, cb)
+
+        # Add enough cancelled events
+        cancelled_count += add_cancel_count
+        for x in range(add_cancel_count):
+            h = self.loop.call_later(3600, cb)
+            h.cancel()
+
+        # Ensure all handles are still scheduled
+        self.assertEqual(len(self.loop._scheduled),
+            cancelled_count + not_cancelled_count)
+
+        self.loop._run_once()
+
+        # Ensure cancelled events were removed
+        self.assertEqual(len(self.loop._scheduled), not_cancelled_count)
+
+        # Ensure only uncancelled events remain scheduled
+        self.assertTrue(all([not x._cancelled for x in self.loop._scheduled]))
+
     def test_run_until_complete_type_error(self):
         self.assertRaises(TypeError,
             self.loop.run_until_complete, 'blah')

Lib/test/test_asyncio/test_events.py

@@ -1890,9 +1890,17 @@ class HandleTests(test_utils.TestCase):
 
         # cancelled handle
         h.cancel()
-        self.assertEqual(repr(h),
+        self.assertEqual(
-                        '<Handle cancelled noop(1, 2) at %s:%s created at %s:%s>'
+            repr(h),
-                        % (filename, lineno, create_filename, create_lineno))
+            '<Handle cancelled noop(1, 2) at %s:%s created at %s:%s>'
+            % (filename, lineno, create_filename, create_lineno))
+
+        # double cancellation won't overwrite _repr
+        h.cancel()
+        self.assertEqual(
+            repr(h),
+            '<Handle cancelled noop(1, 2) at %s:%s created at %s:%s>'
+            % (filename, lineno, create_filename, create_lineno))
 
     def test_handle_source_traceback(self):
         loop = asyncio.get_event_loop_policy().new_event_loop()

Misc/NEWS

@@ -137,6 +137,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #22448: Improve canceled timer handles cleanup to prevent
+  unbound memory usage. Patch by Joshua Moore-Oliva.
+
 - Issue #22427: TemporaryDirectory no longer attempts to clean up twice when
   used in the with statement in generator.