Use `MultiLoopChildWatcher` in tests where available #5862

sweatybridge · 2021-07-07T11:19:56Z

What do these changes do?

Python 3.8 adds a new child watcher class that doesn't require running loop in the main thread: https://docs.python.org/3/library/asyncio-policy.html#asyncio.MultiLoopChildWatcher. This PR uses the new watcher if it is available in asyncio module.

It should reduce test flakiness when parallelized with pytest-xdist.

Are there changes in behavior for the user?

No change for end users. Existing tests in test_loop.py covers this change given the current text matrix.

Related issue number

#3450
#2075

Checklist

I think the code is well written
Unit tests for the changes exist
Documentation reflects the changes
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webknjaz · 2021-07-07T11:23:53Z

This is already being introduced in #5431 but with a threaded watcher. It's also being discussed in #5852. Why do you think that multiloop would be a better fit? Do you want to join those discussions instead?

sweatybridge · 2021-07-07T11:26:13Z

ok, will join the other thread. Wasn't sure where to look since there are many related tickets.

webknjaz · 2021-07-07T11:30:35Z

Let's keep this open for now. I think we could accept it after some minor polishing. But I still need an answer to my question above, okay?

I encourage you to participate in other discussions, though. And it's indeed better to merge small PRs. I only mentioned other efforts so that people interested in achieving the common goal could collaborate more effectively and not repeat each other's work twice.

CHANGES/3450.bugfix

webknjaz · 2021-07-07T11:42:53Z

aiohttp/test_utils.py

+        if hasattr(asyncio, "MultiLoopChildWatcher"):
+            watcher = asyncio.MultiLoopChildWatcher()
+        else:
+            watcher = asyncio.SafeChildWatcher()


I have a few comments on this block: it is preferable to use the EAFP style in Python and also we need to have comments about the motivation of this.

To solve this, I suggest you cherry-pick my commit bea959a (#5431) (same patch: https://github.com/aio-libs/aiohttp/commit/bea959a4ce195405939c40f184c5189206064447.patch) and then change ThreadedChildWatcher with MultiLoopChildWatcher in a follow-up commit. If you know how to use interactive rebase in Git, it should be easy for you :)

Then, we could merge this change and backport it.

After that, I think it would be useful to vendor a backported watcher right in this repo and make use of it under older supported Pythons too. But this is a separate effort that should go into a separate PR.

Thanks for the review.

I'm reading this ticket https://bugs.python.org/issue38591 and there seems to be many instabilities with MultiLoopChildWatcher. I think ThreadedChildWatcher might actually be a better choice here...

Regarding the performance concerns of creating a new thread, how about additionally exposing skip_watcher: bool as a param? SafeChildWatcher was originally introduced to support asyncio.create_subprocess_*. In my use case, this support is not needed. If we expose the param, no one pays the cost unnecessarily.

Oh, maybe that's why I chose ThreadedChildWatcher. I think I talked to Andrew and he said to use it. It was still problematic under the heavy load on my laptop, IIRC.

As for skip_watcher, please file a proposal issue or a draft PR to demonstrate how you see it being exposed, API-wise.

Refs: * pytest-dev/pytest-xdist#620 * https://stackoverflow.com/a/58614689/595220 * https://bugs.python.org/issue35621 * python/cpython#14344

codecov · 2021-07-07T12:40:08Z

Codecov Report

Merging #5862 (c6594ea) into master (5b23426) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5862   +/-   ##
=======================================
  Coverage   96.75%   96.75%           
=======================================
  Files          44       44           
  Lines        9848     9851    +3     
  Branches     1591     1591           
=======================================
+ Hits         9528     9531    +3     
  Misses        182      182           
  Partials      138      138

Flag	Coverage Δ
unit	`96.65% <100.00%> (+<0.01%)`	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiohttp/test_utils.py	`99.68% <100.00%> (+<0.01%)`	⬆️

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patchback · 2021-07-07T12:42:35Z

Backport to 3.8: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.8/7080a8bba86db1ea38372183a90d82bfa7d6d8b1/pr-5862

Backported as #5863

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

PR #5862 Refs: * pytest-dev/pytest-xdist#620 * https://stackoverflow.com/a/58614689/595220 * https://bugs.python.org/issue35621 * python/cpython#14344 * https://docs.python.org/3/library/asyncio-policy.html#asyncio.MultiLoopChildWatcher Co-authored-by: Sviatoslav Sydorenko <[email protected]> (cherry picked from commit 7080a8b)

…s where available (#5863) Co-authored-by: Sviatoslav Sydorenko <[email protected]> Co-authored-by: Han Qiao <[email protected]>

sweatybridge requested review from asvetlov and webknjaz as code owners July 7, 2021 11:19

psf-chronographer bot added the bot:chronographer:provided There is a change note present in this PR label Jul 7, 2021

sweatybridge closed this Jul 7, 2021

webknjaz reopened this Jul 7, 2021

webknjaz reviewed Jul 7, 2021

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webknjaz reviewed Jul 7, 2021

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webknjaz and others added 2 commits July 7, 2021 19:52

Use ThreadedChildWatcher in tests where available

a0c69ca

Refs: * pytest-dev/pytest-xdist#620 * https://stackoverflow.com/a/58614689/595220 * https://bugs.python.org/issue35621 * python/cpython#14344

Use multi loop child watcher when possible

c6594ea

sweatybridge force-pushed the loop-watcher branch from a022285 to c6594ea Compare July 7, 2021 11:53

webknjaz added the backport-3.8 label Jul 7, 2021

webknjaz enabled auto-merge (squash) July 7, 2021 12:35

webknjaz disabled auto-merge July 7, 2021 12:35

webknjaz changed the title ~~Use MultiLoopChildWatcher when setting up test loop~~ Use MultiLoopChildWatcher in tests where available Jul 7, 2021

webknjaz enabled auto-merge (squash) July 7, 2021 12:41

webknjaz merged commit 7080a8b into aio-libs:master Jul 7, 2021

patchback bot mentioned this pull request Jul 7, 2021

[PR #5862/7080a8bb backport][3.8] Use MultiLoopChildWatcher in tests where available #5863

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webknjaz mentioned this pull request Jul 7, 2021

Ascola/add parallel testing #5852

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webknjaz added a commit that referenced this pull request Jul 7, 2021

[PR #5862/7080a8bb backport][3.8] Use MultiLoopChildWatcher in test…

68f0262

…s where available (#5863) Co-authored-by: Sviatoslav Sydorenko <[email protected]> Co-authored-by: Han Qiao <[email protected]>

sweatybridge mentioned this pull request Jul 14, 2021

Switch to ThreadedChildWatcher and test #5877

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sweatybridge commented Jul 7, 2021

What do these changes do?

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Related issue number

Checklist

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