[PR #13010/9e00085c backport][3.15] Fix parse_mimetype producing spurious empty-key parameter for whitespace-only segments after semicolons

[patchback]

This is a backport of PR #13010 as merged into master (9e00085).

Summary

parse_mimetype incorrectly adds a spurious {'': ''} parameter entry when a MIME type string has a trailing semicolon followed by whitespace (e.g. 'text/html; ').

Closes #13009

Root cause

aiohttp/helpers.py splits on ; and skips empty segments with:

if not item:
    continue

A bare trailing ; (like text/html;) produces an empty-string segment, which is correctly skipped. But text/html; produces a whitespace-only segment (' ') which is truthy — so it is NOT skipped — and ends up as params.add('', '').

Fix

-        if not item:
+        if not item.strip():

Reproduction / before

from aiohttp.helpers import parse_mimetype

dict(parse_mimetype('text/html; ').parameters)         # {'': ''}  ← wrong
dict(parse_mimetype('text/html;  ').parameters)        # {'': ''}  ← wrong
dict(parse_mimetype('text/html;\t').parameters)        # {'': ''}  ← wrong
dict(parse_mimetype('text/html; charset=utf-8; ').parameters)  # {'charset': 'utf-8', '': ''}  ← wrong

After fix

All four return the expected values with no empty-key parameter.

Testing

Four new parametrize cases added to tests/test_helpers.py::test_parse_mimetype. All 141 existing tests continue to pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.24%. Comparing base (4078e04) to head (f67e98f).
⚠️ Report is 1 commits behind head on 3.15.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             3.15   #13097   +/-   ##
=======================================
  Coverage   98.24%   98.24%           
=======================================
  Files         135      135           
  Lines       48920    48920           
  Branches     2632     2632           
=======================================
  Hits        48062    48062           
  Misses        678      678           
  Partials      180      180           

Flag	Coverage Δ
CI-GHA	98.27% <100.00%> (-0.01%)	⬇️
OS-Linux	98.04% <100.00%> (-0.01%)	⬇️
OS-Windows	95.72% <100.00%> (-0.01%)	⬇️
OS-macOS	97.25% <100.00%> (+<0.01%)	⬆️
Py-3.10	97.41% <100.00%> (-0.01%)	⬇️
Py-3.11	97.71% <100.00%> (+<0.01%)	⬆️
Py-3.12	97.78% <100.00%> (ø)
Py-3.13	97.76% <100.00%> (-0.01%)	⬇️
Py-3.14	97.87% <100.00%> (-0.01%)	⬇️
Py-3.14t	96.88% <100.00%> (+<0.01%)	⬆️
Py-pypy-3.11	96.72% <100.00%> (-0.01%)	⬇️
VM-macos	97.25% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.04% <100.00%> (-0.01%)	⬇️
VM-windows	95.72% <100.00%> (-0.01%)	⬇️
cython-coverage	38.41% <0.00%> (+<0.01%)	⬆️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 83 untouched benchmarks
⏩ 86 skipped benchmarks¹

---

_{Comparing patchback/backports/3.15/9e00085c958fbcd40de37867e773bf7cd3bbee0c/pr-13010 (f67e98f) with 3.15 (4078e04)}

Footnotes

1. 86 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. ↩