gh-74028: `concurrent.futures.Executor.map`: avoid reference cycles when an exception is raised#131701

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ebonnal wants to merge 49 commits into python:main from ebonnal:test/executor-map-ref-cycles

+42 −4

Lib/concurrent/futures/interpreter.py

-Original file line number
+Diff line change
@@ Expand Up / @@ -205,8 +205,11 @@ def run(self, task): @@
  assert res is None, res
  assert pickled
  assert exc_wrapper is not None
- exc = pickle.loads(excdata)
- raise exc from exc_wrapper
+ try:
+ raise pickle.loads(excdata) from exc_wrapper
+ finally:
+ # avoid a ref cycle where exc_wrapper is captured in its traceback
+ exc_wrapper = None
  return pickle.loads(res) if pickled else res
@@ Expand Down @@

Lib/test/test_concurrent_futures/executor.py

-Original file line number
+Diff line change
@@ -1,6 +1,8 @@
+import gc
 import itertools
 import threading
 import time
+import traceback
 import weakref
 from concurrent import futures
 from operator import add
@@ Expand DownExpand Up / @@ -55,8 +57,41 @@ def test_map_exception(self): @@
  i = self.executor.map(divmod, [1, 1, 1, 1], [2, 3, 0, 5])
  self.assertEqual(i.__next__(), (0, 1))
  self.assertEqual(i.__next__(), (0, 1))
- with self.assertRaises(ZeroDivisionError):
+ exception = None
+ try:
  i.__next__()
+ except Exception as e:
+ exception = e
+ self.assertTrue(
+ isinstance(exception, ZeroDivisionError),
+ msg="should raise a ZeroDivisionError",
+ )
+ # pause needed for free-threading builds on Ubuntu (ARM) and Windows
+ time.sleep(1)
+ self.assertFalse(
+ gc.get_referrers(exception),
+ msg="the exception should not have any referrers",
+ )
+ self.assertFalse(
+ [
+ (var, val)
+ # go through the frames of the exception's traceback
+ for frame, _ in traceback.walk_tb(exception.__traceback__)
+ # skipping the current frame
+ if frame is not exception.__traceback__.tb_frame
+ # go through the locals captured in that frame
+ for var, val in frame.f_locals.items()
+ # check if one of them is an exception
+ if isinstance(val, Exception)
+ # check if it is captured in its own traceback
+ and frame is val.__traceback__.tb_frame
+ ],
+ msg=f"the exception's traceback should not contain an exception captured in its own traceback",
+ )
  @support.requires_resource('walltime')
  def test_map_timeout(self):
@@ Expand DownExpand Up / @@ -140,7 +175,7 @@ def test_map_buffersize_when_buffer_is_full(self): @@
  self.assertEqual(
  next(ints),
  buffersize,
- msg="should have fetched only `buffersize` elements from `ints`.",
+ msg="should have fetched only `buffersize` elements from `ints`",
  )
  def test_shutdown_race_issue12456(self):
@@ Expand Down @@

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gh-74028: `concurrent.futures.Executor.map`: avoid reference cycles when an exception is raised#131701

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kumaraditya303Aug 13, 2025

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gh-74028: concurrent.futures.Executor.map: avoid reference cycles when an exception is raised#131701

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gh-74028: `concurrent.futures.Executor.map`: avoid reference cycles when an exception is raised#131701

gh-74028: `concurrent.futures.Executor.map`: avoid reference cycles when an exception is raised #131701