[PR #10101/678993a4 backport][3.11] Fix race in FileResponse if file is replaced during prepare

[patchback]

This is a backport of PR #10101 as merged into master (678993a).

There was a race in FileResponse where the stat would be incorrect if the file was changed out between the stat and open syscalls. This would lead to various unexpected behaviors such as trying to read beyond the length of the file or sending a partial file. This problem is likely to occour when files are being renamed/linked into place.

An example of how this can happen with a system that provides weather data every 60s:

An external process writes .weather.txt at the top of each minute, and than renames it into place as weather.txt. In this case FileResponse.prepare may stat the old weather.txt, and than open the new weather.txt, and use the os.stat_result from the original file.

To fix this we now fstat the open file on operating systems where fstat is available.

This change looks much larger than it actually is because most of the function now needs to be wrapped in the try/finally to ensure that we close the file cleanly as we open it a lot sooner now. Its best to compare without white space as well.

fixes #8013

[codspeed-hq]

CodSpeed Performance Report

Merging #10105 will improve performances by 15.19%

_{Comparing patchback/backports/3.11/678993a4bd071200fbcc14fa121c6673221aa540/pr-10101 (fe03729) with 3.11 (23a4b31)}

Summary

⚡ 2 improvements
✅ 44 untouched benchmarks

Benchmarks breakdown

	Benchmark	3.11	patchback/backports/3.11/678993a4bd071200fbcc14fa121c6673221aa540/pr-10101	Change
⚡	test_simple_web_file_response[pyloop]	95.6 ms	83 ms	+15.19%
⚡	test_simple_web_file_sendfile_fallback_response[pyloop]	101 ms	88.9 ms	+13.59%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.06%. Comparing base (23a4b31) to head (fe03729).
Report is 1 commits behind head on 3.11.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             3.11   #10105      +/-   ##
==========================================
- Coverage   98.06%   98.06%   -0.01%     
==========================================
  Files         123      123              
  Lines       37098    37113      +15     
  Branches     4459     4466       +7     
==========================================
+ Hits        36381    36395      +14     
  Misses        540      540              
- Partials      177      178       +1     

Flag	Coverage Δ
CI-GHA	97.95% <100.00%> (+<0.01%)	⬆️
OS-Linux	97.65% <100.00%> (+<0.01%)	⬆️
OS-Windows	94.57% <100.00%> (-0.01%)	⬇️
OS-macOS	96.77% <100.00%> (-0.01%)	⬇️
Py-3.10.11	96.62% <100.00%> (+<0.01%)	⬆️
Py-3.10.15	97.16% <100.00%> (-0.01%)	⬇️
Py-3.11.10	97.22% <100.00%> (-0.02%)	⬇️
Py-3.11.9	96.69% <100.00%> (-0.03%)	⬇️
Py-3.12.7	97.72% <100.00%> (-0.01%)	⬇️
Py-3.13.0	96.55% <100.00%> (-0.01%)	⬇️
Py-3.13.1	97.69% <100.00%> (-0.01%)	⬇️
Py-3.9.13	96.55% <93.93%> (-0.01%)	⬇️
Py-3.9.20	97.12% <93.93%> (-0.01%)	⬇️
Py-pypy7.3.16	96.69% <93.93%> (+<0.01%)	⬆️
VM-macos	96.77% <100.00%> (-0.01%)	⬇️
VM-ubuntu	97.65% <100.00%> (+<0.01%)	⬆️
VM-windows	94.57% <100.00%> (-0.01%)	⬇️

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