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gh-131466: concurrent.futures.Executor.map: avoid temporarily exceeding buffersize while collecting the next result #131467

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gh-131466: concurrent.futures.Executor.map: avoid temporarily exceeding buffersize while collecting the next result #131467

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233ccc1
`Executor.map`: avoid temporarily exceeding the `buffersize` while co…
ebonnal Mar 19, 2025
b864ef9
Merge remote-tracking branch 'cpython/main' into feat/executor-map-bu…
ebonnal Mar 19, 2025
72d7028
avoid keeping a ref to result for test_thread_pool.ThreadPoolExecutor…
ebonnal Mar 19, 2025
2a30697
update comment about not keeping references to popped future/result
ebonnal Mar 19, 2025
ab4182b
introduce `current_timeout` variable
ebonnal Mar 19, 2025
7a1ae46
comment on the necessity of the result container
ebonnal Mar 19, 2025
268927d
avoid container
ebonnal Mar 20, 2025
de09aff
remove current_timeout usage
ebonnal Mar 20, 2025
1814bfe
fix comments format
ebonnal Mar 20, 2025
7206321
rephrase comments
ebonnal Mar 20, 2025
162add1
order imports
ebonnal Mar 23, 2025
f2c5fd0
format comments
ebonnal Mar 23, 2025
9474769
remove _result_or_cancel
ebonnal Mar 23, 2025
2a2119e
access awaited result via _result attribute
ebonnal Mar 23, 2025
3be6956
break a reference cycle with `fs[-1]._exception`
ebonnal Mar 24, 2025
0d70be9
break other potential reference cycles with fs, not only the one caus…
ebonnal Mar 24, 2025
f509097
lighter ref cycle break
ebonnal Mar 25, 2025
d50dabd
move the ref cycle break into the finally block
ebonnal Mar 25, 2025
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avoid container
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@ebonnal
ebonnal committed Mar 20, 2025
commit 268927d965e1a369e7f49d0d56253dd9ae89717b
9 changes: 5 additions & 4 deletions Lib/concurrent/futures/_base.py
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Original file line number Diff line number Diff line change
Expand Up @@ -621,8 +621,6 @@ def map(self, fn, *iterables, timeout=None, chunksize=1, buffersize=None):
# collected independently of the result_iterator closure.
executor_weakref = weakref.ref(self)

# used by the result_iterator to avoid keeping a reference to the result
result = collections.deque(maxlen=1)
# Yield must be hidden in closure so that the futures are submitted
# before the first iterator value is required.
def result_iterator():
Expand All @@ -635,16 +633,19 @@ def result_iterator():
if current_timeout is not None:
current_timeout = end_time - time.monotonic()

result.append(_result_or_cancel(fs.pop(), current_timeout))
# wait for the next result
_result_or_cancel(fs[-1], current_timeout)

# buffer next task
if (
buffersize
and (executor := executor_weakref())
and (args := next(zipped_iterables, None))
):
fs.appendleft(executor.submit(fn, *args))

yield result.pop()
# yield the awaited result
yield fs.pop().result()

@ebonnal ebonnal Mar 22, 2025 •
edited
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to be discussed: this could be replaced by a lighter yield fs.pop()._result because the prior call to _result_or_cancel guarantees that at this point the result is available.

finally:
for future in fs:
future.cancel()
Expand Down