Fix test_data_file race condition on Python 3.14 free-threaded

[rodrigobnogueira]

What do these changes do?

This PR fixes a flaky test (test_data_file) that fails on Python 3.14 free-threaded (3.14t) builds.

The test asserts asyncio.isfuture(req._writer) immediately after await req._send(conn). However, on Python 3.12+, asyncio.Task accepts eager_start=True. Because the 2-byte file payload in this test is exceptionally small, the eagerly-started task completes synchronously during _send().

When task.done() handles this, req._writer is left as None, causing the assertion to fail (e.g. assert False + where False = asyncio.isfuture(None)).

This PR applies the exact same mock strategy used by the sibling test test_data_stream. We mock the _write_bytes method to execute await asyncio.sleep(0), guaranteeing the task yields back to the event loop. This ensures req._writer holds a pending Future when the assertion fires.

The fixed test has failed in PRs #12089 and #12153, for example.

Are there changes in behavior for the user?

No user-facing changes. This strictly addresses test flakiness in the CI environment.

Is it a substantial burden for the maintainers to support this?

No. This follows the exact same pattern already used in the same test file.

Related issue number

N/A.

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
• Add a new news fragment into the CHANGES/ folder

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (c1981ef) to head (eace48d).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #12170   +/-   ##
=======================================
  Coverage   98.77%   98.78%           
=======================================
  Files         128      128           
  Lines       45321    45326    +5     
  Branches     2405     2405           
=======================================
+ Hits        44768    44776    +8     
+ Misses        393      390    -3     
  Partials      160      160           

Flag	Coverage Δ
CI-GHA	98.64% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.38% <100.00%> (+<0.01%)	⬆️
OS-Windows	96.75% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.63% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.18% <100.00%> (+<0.01%)	⬆️
Py-3.10.19	97.67% <100.00%> (+<0.01%)	⬆️
Py-3.11.14	97.86% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	97.39% <100.00%> (+<0.01%)	⬆️
Py-3.12.10	97.48% <100.00%> (-0.01%)	⬇️
Py-3.12.12	97.95% <100.00%> (+<0.01%)	⬆️
Py-3.13.12	98.20% <100.00%> (+<0.01%)	⬆️
Py-3.14.3	98.16% <100.00%> (+<0.01%)	⬆️
Py-3.14.3t	97.26% <100.00%> (+<0.01%)	⬆️
Py-pypy3.11.13-7.3.20	97.29% <100.00%> (+<0.01%)	⬆️
VM-macos	97.63% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.38% <100.00%> (+<0.01%)	⬆️
VM-windows	96.75% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 59 untouched benchmarks

---

_{Comparing rodrigobnogueira:fix-test-data-file-3.14t (eace48d) with master (c1981ef)}

[patchback]

Backport to 3.13: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 2d03bc4 on top of patchback/backports/3.13/2d03bc45722f4153c2a993f084ecac04023d54c4/pr-12170

Backporting merged PR #12170 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/3.13/2d03bc45722f4153c2a993f084ecac04023d54c4/pr-12170 upstream/3.13

4. Now, cherry-pick PR Fix test_data_file race condition on Python 3.14 free-threaded #12170 contents into that branch:

   $ git cherry-pick -x 2d03bc45722f4153c2a993f084ecac04023d54c4

   If it'll yell at you with something like fatal: Commit 2d03bc45722f4153c2a993f084ecac04023d54c4 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 2d03bc45722f4153c2a993f084ecac04023d54c4

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix test_data_file race condition on Python 3.14 free-threaded #12170 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.13/2d03bc45722f4153c2a993f084ecac04023d54c4/pr-12170

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 3.14: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 2d03bc4 on top of patchback/backports/3.14/2d03bc45722f4153c2a993f084ecac04023d54c4/pr-12170

Backporting merged PR #12170 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/3.14/2d03bc45722f4153c2a993f084ecac04023d54c4/pr-12170 upstream/3.14

4. Now, cherry-pick PR Fix test_data_file race condition on Python 3.14 free-threaded #12170 contents into that branch:

   $ git cherry-pick -x 2d03bc45722f4153c2a993f084ecac04023d54c4

   If it'll yell at you with something like fatal: Commit 2d03bc45722f4153c2a993f084ecac04023d54c4 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 2d03bc45722f4153c2a993f084ecac04023d54c4

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix test_data_file race condition on Python 3.14 free-threaded #12170 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.14/2d03bc45722f4153c2a993f084ecac04023d54c4/pr-12170

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.