Fix cancelled payload send leading to hung connection

[Dreamsorcerer]

I've thought it through here, and I don't think we can safely reuse the connection at this point when we haven't finished sending the payload.
If it's not chunked, and therefore has a Content-Length, reusing the connection will result in the server thinking it's still reading the last message.
If it is chunked, then we can technically send a closing chunk to complete the message and then reuse it. However, that may lead the server to think that it has received the full message when it hasn't.

Fixes #7599.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.29%. Comparing base (5c3d50f) to head (ba5e421).
Report is 5 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8992   +/-   ##
=======================================
  Coverage   98.28%   98.29%           
=======================================
  Files         107      107           
  Lines       34317    34320    +3     
  Branches     4064     4063    -1     
=======================================
+ Hits        33729    33734    +5     
+ Misses        415      413    -2     
  Partials      173      173           

Flag	Coverage Δ
CI-GHA	98.18% <100.00%> (+<0.01%)	⬆️
OS-Linux	97.84% <100.00%> (+<0.01%)	⬆️
OS-Windows	96.25% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.52% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.62% <100.00%> (+<0.01%)	⬆️
Py-3.10.14	97.55% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	97.78% <100.00%> (+<0.01%)	⬆️
Py-3.12.5	97.90% <100.00%> (+<0.01%)	⬆️
Py-3.9.13	97.51% <100.00%> (+<0.01%)	⬆️
Py-3.9.19	97.45% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.16	97.06% <100.00%> (+<0.01%)	⬆️
VM-macos	97.52% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	97.84% <100.00%> (+<0.01%)	⬆️
VM-windows	96.25% <100.00%> (+<0.01%)	⬆️

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[patchback]

Backport to 3.10: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 5c0b8e4 on top of patchback/backports/3.10/5c0b8e4a7897f48063f684ac16e7dd18f6218274/pr-8992

Backporting merged PR #8992 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/3.10/5c0b8e4a7897f48063f684ac16e7dd18f6218274/pr-8992 upstream/3.10

4. Now, cherry-pick PR Fix cancelled payload send leading to hung connection #8992 contents into that branch:

   $ git cherry-pick -x 5c0b8e4a7897f48063f684ac16e7dd18f6218274

   If it'll yell at you with something like fatal: Commit 5c0b8e4a7897f48063f684ac16e7dd18f6218274 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 5c0b8e4a7897f48063f684ac16e7dd18f6218274

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix cancelled payload send leading to hung connection #8992 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.10/5c0b8e4a7897f48063f684ac16e7dd18f6218274/pr-8992

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.

[patchback]

Backport to 3.11: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 5c0b8e4 on top of patchback/backports/3.11/5c0b8e4a7897f48063f684ac16e7dd18f6218274/pr-8992

Backporting merged PR #8992 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/3.11/5c0b8e4a7897f48063f684ac16e7dd18f6218274/pr-8992 upstream/3.11

4. Now, cherry-pick PR Fix cancelled payload send leading to hung connection #8992 contents into that branch:

   $ git cherry-pick -x 5c0b8e4a7897f48063f684ac16e7dd18f6218274

   If it'll yell at you with something like fatal: Commit 5c0b8e4a7897f48063f684ac16e7dd18f6218274 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 5c0b8e4a7897f48063f684ac16e7dd18f6218274

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix cancelled payload send leading to hung connection #8992 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.11/5c0b8e4a7897f48063f684ac16e7dd18f6218274/pr-8992

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.