Make FileResponse.prepare re-entrant

[AG0708]

What do these changes do?

Makes FileResponse.prepare() return the existing response writer when the response has already been prepared, matching StreamResponse.prepare() instead of reopening and sending the file a second time. A regression test covers two calls on the same response.

Are there changes in behavior for the user?

Yes. Applications may explicitly await FileResponse.prepare() before returning the response—for example, so a temporary file can then be removed—without aiohttp preparing and sending it again. First-call behavior is unchanged.

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

No. The guard delegates the already-prepared case to the base implementation and adds no new state or dependency.

Related issue number

Fixes #4647.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• I added myself to CONTRIBUTORS.txt in alphabetical order
• I added CHANGES/4647.feature.rst

Local validation

• Relevant unit tests: 134 passed
• Mypy on both changed Python files: passed
• Changed-file pre-commit hooks: passed
• Targeted Sphinx build with warnings as errors: passed

Drafted with OpenAI Codex (GPT-5); reviewed by @AG0708.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.99%. Comparing base (d041d4d) to head (af230c0).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #13399   +/-   ##
=======================================
  Coverage   98.99%   98.99%           
=======================================
  Files         132      132           
  Lines       49454    49468   +14     
  Branches     2572     2573    +1     
=======================================
+ Hits        48959    48973   +14     
  Misses        371      371           
  Partials      124      124           

Flag	Coverage Δ
Autobahn	22.08% <7.14%> (-0.01%)	⬇️
CI-GHA	98.91% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.68% <100.00%> (-0.01%)	⬇️
OS-Windows	97.04% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.93% <100.00%> (+<0.01%)	⬆️
Py-3.10	98.13% <100.00%> (+<0.01%)	⬆️
Py-3.11	98.39% <100.00%> (-0.01%)	⬇️
Py-3.12	98.47% <100.00%> (-0.01%)	⬇️
Py-3.13	98.46% <100.00%> (+<0.01%)	⬆️
Py-3.14	98.47% <100.00%> (-0.01%)	⬇️
Py-3.14t	97.56% <100.00%> (-0.01%)	⬇️
Py-pypy-3.11	97.43% <100.00%> (+<0.01%)	⬆️
VM-macos	97.93% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.68% <100.00%> (-0.01%)	⬇️
VM-windows	97.04% <100.00%> (+<0.01%)	⬆️
cython-coverage	38.14% <0.00%> (-0.02%)	⬇️

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[codspeed]

Merging this PR will improve performance by 9.64%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 83 untouched benchmarks
⏩ 83 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_simple_web_file_response[ssl-large]	307.7 ms	280.6 ms	+9.64%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing AG0708:codex/4647-reentrant-file-response (af230c0) with master (d041d4d)²}

Footnotes

1. 83 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on master (77c5108) during the generation of this report, so d041d4d was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

_{Reviews (2): Last reviewed commit: "Clarify FileResponse.prepare return valu..." | Re-trigger Greptile}