[issue34819] Executor.map and as_completed timeouts are able to deviate

[orlnub123]

New submission from orlnub123 <orlnub123 at gmail.com>:

The map and as_completed functions can randomly timeout earlier or later due to NTP stepping the clock or something changing the time and/or date.

Here's some code to reproduce the issue for map:

import concurrent.futures
import time

with concurrent.futures.ThreadPoolExecutor() as executor:
    list(executor.map(time.sleep, [29, 30], timeout=3000))

And similar code to reproduce it for as_completed:

import concurrent.futures
import time

with concurrent.futures.ThreadPoolExecutor() as executor:
    future1 = executor.submit(time.sleep, 29)
    future2 = executor.submit(time.sleep, 30)
    list(concurrent.futures.as_completed([future1, future2], timeout=3000))

It doesn't error normally, as it shouldn't, but if you move your clock an hour ahead within 30 seconds of running it you get this:

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "C:\Python\Python37\lib\concurrent\futures\_base.py", line 588, in result_iterator
    yield fs.pop().result(end_time - time.time())
  File "C:\Python\Python37\lib\concurrent\futures\_base.py", line 434, in result
    raise TimeoutError()
concurrent.futures._base.TimeoutError

Or this if you used the as_completed example:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
  File "C:\Python\Python37\lib\concurrent\futures\_base.py", line 238, in as_completed
    len(pending), total_futures))
concurrent.futures._base.TimeoutError: 1 (of 2) futures unfinished

The error stems from map and as_completed using time.time to calculate the duration for the timeout. The problem with time.time is that it's adjustable (i.e. it can skip around) which makes it unideal for comparing relative times. A solution would be to replace the time.time calls with either time.monotonic (preferable?) or time.perf_counter.

----------
components: Library (Lib)
messages: 326535
nosy: orlnub123
priority: normal
severity: normal
status: open
title: Executor.map and as_completed timeouts are able to deviate
type: behavior
versions: Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34819>
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