gh-145703: Fix asyncio.BaseEventLoop low clock resolution

[CaptainFlint]

With this fix we make sure that even if the clock resolution becomes too small, the "next tick" will still be different from the current time, and the comparison remains valid.

Example: clock resolution = 1e-9, current uptime = 200 days (17280000 seconds). Floating point addition does not change the value, because the precision is too small. In this case the patch will use math.ulp(), which is the smallest possible increment that will modify the current value.

• Issue: asyncio.BaseEventLoop clock resolution becomes too low over time #145703

[picnixz]

/home/runner/work/cpython/cpython/Doc/build/NEWS:40: WARNING: py:class reference target not found: asyncio.BaseEventLoop [ref.class]

Oh so BaseEventLoop doesn't have a documented entry. Maybe you can directly refer to call_soon (https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.call_soon) ?

[CaptainFlint]

Oh so BaseEventLoop doesn't have a documented entry. Maybe you can directly refer to call_soon (https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.call_soon) ?

I found an existing reference to call_soon in other NEWS entries and copied it. Looks like it worked.

[picnixz]

If there are other methods that are affected by that, you can also mention them (I actually don't know if it's used by call_soon, I just assumed so because it's about callbacks)

[CaptainFlint]

Well, the code that triggered the issue for me was using call_at. That implies that call_later is also affected, since they are basically the one.

I can't tell whether call_soon is affected, actually, it's using a different approach, and I'm not familiar with the asyncio code structure to quickly see how the scheduler would process it.

I think, then, it's better to mention only call_at and call_later as they are definitely affected.

[picnixz]

I'm not an asyncio expert so I'll leave it for the maintainers but this looks like the best alternative.

[CaptainFlint]

Thanks a lot for your input and help! I really appreciate it!

[kumaraditya303]

This looks good, we aren't backporting this to I am fine with getting this in and see if something breaks.

[gvanrossum]

Wonderful. Go ahead and merge.

(Are there other loops doing a similar thing elsewhere? In asyncio or outside it?)

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot s390x RHEL9 Refleaks 3.x (tier-3) has failed when building commit 77c06f3.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1589/builds/3089) and take a look at the build logs.
4. Check if the failure is related to this commit (77c06f3) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1589/builds/3089

Failed tests:

• test_profiling

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/var/lib/buildbot/worker/cstratak-rhel9-s390x/3.x.cstratak-rhel9-s390x.refleak/build/Lib/test/test_profiling/test_sampling_profiler/test_advanced.py", line 169, in test_native_frames_enabled
    self.assertFalse(any(stack.endswith(";<native>") for stack in stacks))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: True is not false

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 FreeBSD Refleaks 3.x (tier-3) has failed when building commit 77c06f3.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1613/builds/2977) and take a look at the build logs.
4. Check if the failure is related to this commit (77c06f3) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1613/builds/2977

Test leaking resources:

• test_events: memory blocks

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-freebsd.refleak/build/Lib/test/support/__init__.py", line 947, in gc_collect
    gc.collect()
    ~~~~~~~~~~^^
ResourceWarning: unclosed <socket.socket fd=10, family=2, type=1, proto=6, laddr=('127.0.0.1', 62446), raddr=('127.0.0.1', 62447)>
Task was destroyed but it is pending!
task: <Task pending name='Task-1498' coro=<BaseSelectorEventLoop._accept_connection2() done, defined at /buildbot/buildarea/3.x.ware-freebsd.refleak/build/Lib/asyncio/selector_events.py:217> wait_for=<Future pending cb=[Task.task_wakeup()]>>
Warning -- Unraisable exception
Exception ignored while calling deallocator <function _SelectorTransport.__del__ at 0x844149610>:
Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-freebsd.refleak/build/Lib/asyncio/selector_events.py", line 873, in __del__
    _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ResourceWarning: unclosed transport <_SelectorSocketTransport closing fd=10>
k


Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-freebsd.refleak/build/Lib/test/support/__init__.py", line 947, in gc_collect
    gc.collect()
    ~~~~~~~~~~^^
ResourceWarning: unclosed <socket.socket fd=8, family=2, type=1, proto=6, laddr=('127.0.0.1', 59198), raddr=('127.0.0.1', 59199)>
Task was destroyed but it is pending!
task: <Task pending name='Task-1730' coro=<BaseSelectorEventLoop._accept_connection2() done, defined at /buildbot/buildarea/3.x.ware-freebsd.refleak/build/Lib/asyncio/selector_events.py:217> wait_for=<Future pending cb=[Task.task_wakeup()]>>
Warning -- Unraisable exception
Exception ignored while calling deallocator <function _SelectorTransport.__del__ at 0x8459deb10>:
Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-freebsd.refleak/build/Lib/asyncio/selector_events.py", line 873, in __del__
    _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ResourceWarning: unclosed transport <_SelectorSocketTransport closing fd=8>
k


Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-freebsd.refleak/build/Lib/test/support/__init__.py", line 947, in gc_collect
    gc.collect()
    ~~~~~~~~~~^^
ResourceWarning: unclosed <socket.socket fd=10, family=2, type=1, proto=6, laddr=('127.0.0.1', 62837), raddr=('127.0.0.1', 62838)>
Task was destroyed but it is pending!
task: <Task pending name='Task-3623' coro=<BaseSelectorEventLoop._accept_connection2() done, defined at /buildbot/buildarea/3.x.ware-freebsd.refleak/build/Lib/asyncio/selector_events.py:217> wait_for=<Future pending cb=[Task.task_wakeup()]>>
Warning -- Unraisable exception
Exception ignored while calling deallocator <function _SelectorTransport.__del__ at 0x844149610>:
Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-freebsd.refleak/build/Lib/asyncio/selector_events.py", line 873, in __del__
    _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ResourceWarning: unclosed transport <_SelectorSocketTransport closing fd=10>
k