Switch to ThreadedChildWatcher and test #5877

sweatybridge · 2021-07-14T04:20:14Z

What do these changes do?

Switch back to using ThreadedChildWatcher (originally introduced in #5862). Adds a unit test to ensure loop can be setup from non-main thread by pytest-xdist.

Are there changes in behavior for the user?

NA

Related issue number

#5852

Checklist

I think the code is well written
Unit tests for the changes exist
Documentation reflects the changes
If you provide code modification, please add yourself to CONTRIBUTORS.txt
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Add a new news fragment into the CHANGES folder
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- Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

codecov · 2021-07-14T04:32:54Z

Codecov Report

Merging #5877 (b99f35c) into master (8c5bb41) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5877   +/-   ##
=======================================
  Coverage   96.75%   96.75%           
=======================================
  Files          44       44           
  Lines        9851     9852    +1     
  Branches     1591     1591           
=======================================
+ Hits         9531     9532    +1     
  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%> (ø)`
aiohttp/client.py	`94.00% <0.00%> (+0.01%)`	⬆️

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AustinScola · 2021-07-15T13:59:04Z

Hey @sweatybridge, can you help me understand this change please? My understanding is that pytest-xdist uses sub-processes and each of the sub-processes run their own event loop. And asyncio.ThreadedChildWatcher handles this scenario better than asyncio.MultiLoopChildWatcher does? Is that an accurate description?

sweatybridge · 2021-07-15T14:22:19Z

Hi @AustinScola, pytest-xdist uses execnet to run workers in subprocesses. According to this issue pytest-dev/pytest-xdist#469, modern versions of execnet no longer guarantees that every worker is run on the main thread of its own subprocess. As a result, when these tests are scheduled occasionally on non-main thread by pytest-xdist, the test will crash due to MultiLoopChildWatcher trying to register signal handlers, which requires main thread. Using ThreadedChildWatcher avoids this issue because it doesn't register signal handlers.

AustinScola · 2021-07-15T14:50:22Z

@sweatybridge, that makes sense. Thank you for the wonderful explanation!

sweatybridge · 2021-07-16T03:59:26Z

Happy to help! I'm looking forward to xdist support.

AustinScola · 2021-07-18T19:22:11Z

Is this ready to be merged?

sweatybridge · 2021-07-19T03:44:24Z

@webknjaz could you take a look at this and merge it if possible?

CHANGES/5877.bugfix

webknjaz · 2021-07-22T07:34:26Z

tests/test_loop.py

+def test_setup_loop_non_main_thread() -> None:
+    def target() -> None:
+        with loop_context():
+            pass


Could we run something in this loop and maybe process some signal? This is what may go wrong potentially.

Setting up the loop context in python 3.7 still uses SafeChildWatcher, which in turn registers signals. I've updated the test to expect failure on python 3.7 and success in 3.8+. This way we have both code paths covered.

Subsequently after merging pytest-xdist, we might see occasional failures on python 3.7 tests in CI for the same reason. This could happen with any tests that use the asyncio event loop, not specific to this test alone. This will be ok once we deprecate 3.7.

I'd rather vendor a backport and use it under Python 3.7. Also, it's not yet time to drop it anyway. aiohttp 3.8 still supports Python 3.6 even. It'll be EOL in 5 months but for CPython 3.7 there are 2 years to go.
Being a framework/library forces us to support a wider range of versions, unlike projects that are just apps bound to a single env.

Yup, agree that backporting is better.

here's the backport with manually resolved merge conflicts #5919

tests/test_loop.py

webknjaz · 2021-07-27T19:07:55Z

tests/test_loop.py

+        try:
+            with loop_context() as loop:
+                assert asyncio.get_event_loop() is loop
+                loop.run_until_complete(test_subprocess_co(loop))


I see this calls subprocesses but what about registering signal handlers? This is what actually fails on old watchers.

Registering signal handlers will always fail in non-main thread, regardless of whether we are using old or new watcher. The new watcher avoids this issue by not registering signal handlers at all (using waitpid as alternative).

What we want to ensure in this test is that child processes can be watched, not whether signals can be successfully registered. In other words, since the library no longer registers signal handler on python 3.8, it's not relevant to cover signal registering in tests.

Am I misunderstanding something?

Yes. The original problem with xdist was exactly the fact that there's a problem with the signal handlers that happens in an inconsistent manner. This is what blocks #5431 and all the previous attempts. I was sure that the new watchers were supposed to fix this based on what @asvetlov mentioned to me privately. Are you sure it's still problematic?

What I'm saying is signal handling is an independent problem from watching child process. The old watcher uses signal handling and will crash when mixed with xdist. The new watcher doesn't use signal handling and therefore is not problematic. Our test only need to verify that child process is watched, it doesn't matter whether it's implemented signals or not.

For reference, here's cpython's implementation for ThreadedChildWatcher https://github.com/python/cpython/blob/3.8/Lib/asyncio/unix_events.py#L1296. It starts a new thread and calls waitpid.

Compare that with SafeChildWatcher which inherits from BaseChildWatcher https://github.com/python/cpython/blob/3.8/Lib/asyncio/unix_events.py#L927. It tries to register signal handler and hence will raise an exception in non-main thread.

tests/test_loop.py

Co-authored-by: Sviatoslav Sydorenko <wk.cvs.github@sydorenko.org.ua>

webknjaz

I'll merge this "as is" for now. It's an improvement over what we had before. But it'd be really great to tackle that problem with the signals in order to make xdist enabled by default.

patchback · 2021-07-28T12:09:10Z

Backport to 3.8: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 33a38b8 on top of patchback/backports/3.8/33a38b8c358011fc8bc9198cd62a2a50b69bbc14/pr-5877

Backporting merged PR #5877 into master

Ensure you have a local repo clone of your fork. Unless you cloned it
from the upstream, this would be your origin remote.
Make sure you have an upstream repo added as a remote too. In these
instructions you'll refer to it by the name upstream. If you don't
have it, here's how you can add it:
```
$ git remote add upstream https://github.com/aio-libs/aiohttp.git
```

Ensure you have the latest copy of upstream and prepare a branch
that will hold the backported code:

$ git fetch upstream
$ git checkout -b patchback/backports/3.8/33a38b8c358011fc8bc9198cd62a2a50b69bbc14/pr-5877 upstream/3.8

Now, cherry-pick PR Switch to ThreadedChildWatcher and test #5877 contents into that branch:
```
$ git cherry-pick -x 33a38b8c358011fc8bc9198cd62a2a50b69bbc14
```
If it'll yell at you with something like fatal: Commit 33a38b8c358011fc8bc9198cd62a2a50b69bbc14 is a merge but no -m option was given., add -m 1 as follows intead:
```
$ git cherry-pick -m1 -x 33a38b8c358011fc8bc9198cd62a2a50b69bbc14
```
At this point, you'll probably encounter some merge conflicts. You must
resolve them in to preserve the patch from PR Switch to ThreadedChildWatcher and test #5877 as close to the
original as possible.

Push this branch to your fork on GitHub:

$ git push origin patchback/backports/3.8/33a38b8c358011fc8bc9198cd62a2a50b69bbc14/pr-5877

Create a PR, ensure that the CI is green. If it's not — update it so that
the tests and any other checks pass. This is it!
Now relax and wait for the maintainers to process your pull request
when they have some cycles to do reviews. Don't worry — they'll tell you if
any improvements are necessary when the time comes!

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

webknjaz · 2021-07-28T12:09:37Z

@sweatybridge could you backport this manually?

Co-authored-by: Sviatoslav Sydorenko <wk.cvs.github@sydorenko.org.ua> (cherry picked from commit 33a38b8)

* Switch to ThreadedChildWatcher and test (#5877) Co-authored-by: Sviatoslav Sydorenko <wk.cvs.github@sydorenko.org.ua> (cherry picked from commit 33a38b8) * Fix event loop access for py310 Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

Switch to ThreadedChildWatcher and test

dc361c4

sweatybridge requested a review from asvetlov as a code owner July 14, 2021 04:20

Add to changes

95b0e5d

psf-chronographer bot added the bot:chronographer:provided There is a change note present in this PR label Jul 14, 2021

Fix mypy

6b2d5af

Han Qiao added 2 commits July 14, 2021 12:57

Tear down test loop in thread

3072ae0

Fix mypy

8af64ab

AustinScola mentioned this pull request Jul 18, 2021

Ascola/add parallel testing #5852

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webknjaz reviewed Jul 22, 2021

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Han Qiao added 3 commits July 27, 2021 10:29

Fix changelog

3b51df0

Create subprocess in test loop

0d624ca

Fail test on python 3.7

b56b139

webknjaz reviewed Jul 27, 2021

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webknjaz reviewed Jul 27, 2021

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Update tests/test_loop.py

b99f35c

Co-authored-by: Sviatoslav Sydorenko <wk.cvs.github@sydorenko.org.ua>

webknjaz approved these changes Jul 28, 2021

View reviewed changes

webknjaz added the backport-3.8 label Jul 28, 2021

webknjaz merged commit 33a38b8 into aio-libs:master Jul 28, 2021

sweatybridge added a commit to sweatybridge/aiohttp that referenced this pull request Jul 28, 2021

Switch to ThreadedChildWatcher and test (aio-libs#5877)

8385688

Co-authored-by: Sviatoslav Sydorenko <wk.cvs.github@sydorenko.org.ua> (cherry picked from commit 33a38b8)

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Switch to ThreadedChildWatcher and test #5877

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Switch to ThreadedChildWatcher and test #5877

Switch to ThreadedChildWatcher and test #5877

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sweatybridge commented Jul 14, 2021 • edited Loading

What do these changes do?

Are there changes in behavior for the user?

Related issue number

Checklist

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Backport to 3.8: 💔 cherry-picking failed — conflicts found

Backporting merged PR #5877 into master

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