Add documentation for handler cancellation behaviour

[Dreamsorcerer]

Documentation for #7056.

[codecov]

Codecov Report

Merging #7128 (151bc04) into master (6798629) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7128   +/-   ##
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  Coverage   97.37%   97.37%           
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  Files         106      106           
  Lines       31093    31093           
  Branches     3875     3875           
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  Hits        30278    30278           
  Misses        613      613           
  Partials      202      202           

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[Dreamsorcerer]

@webknjaz Any concerns with this documentation?
I've also suggested to link to it at the end of the demo tutorial as well: aio-libs/aiohttp-demos#334

[patchback]

Backport to 3.9: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 743c21f on top of patchback/backports/3.9/743c21f4f683e11ecef588a0955b433a3b3102e9/pr-7128

Backporting merged PR #7128 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. 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

3. 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/3.9/743c21f4f683e11ecef588a0955b433a3b3102e9/pr-7128 upstream/3.9

4. Now, cherry-pick PR Add documentation for handler cancellation behaviour #7128 contents into that branch:

   $ git cherry-pick -x 743c21f4f683e11ecef588a0955b433a3b3102e9

   If it'll yell at you with something like fatal: Commit 743c21f4f683e11ecef588a0955b433a3b3102e9 is a merge but no -m option was given., add -m 1 as follows intead:

   $ git cherry-pick -m1 -x 743c21f4f683e11ecef588a0955b433a3b3102e9

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Add documentation for handler cancellation behaviour #7128 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.9/743c21f4f683e11ecef588a0955b433a3b3102e9/pr-7128

7. 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.

[mosquito]

@Dreamsorcerer is is possible to release this? I have to use ==3.8.1 for handler cancellation behaviour for all my projects.

[Dreamsorcerer]

It's in 3.9, will be looking at pushing a beta release soon.