[PR #13010/9e00085c backport][3.14] 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 (e1cad97) to head (becc4eb).
⚠️ Report is 2 commits behind head on 3.14.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             3.14   #13096   +/-   ##
=======================================
  Coverage   98.24%   98.24%           
=======================================
  Files         135      135           
  Lines       48824    48824           
  Branches     2619     2619           
=======================================
+ Hits        47966    47967    +1     
  Misses        678      678           
+ Partials      180      179    -1     

Flag	Coverage Δ
CI-GHA	98.29% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.06% <100.00%> (+<0.01%)	⬆️
OS-Windows	95.73% <100.00%> (-0.01%)	⬇️
OS-macOS	97.26% <100.00%> (ø)
Py-3.10	97.45% <100.00%> (ø)
Py-3.11	97.72% <100.00%> (ø)
Py-3.12	97.80% <100.00%> (ø)
Py-3.13	97.79% <100.00%> (+<0.01%)	⬆️
Py-3.14	97.89% <100.00%> (-0.01%)	⬇️
Py-3.14t	96.89% <100.00%> (+<0.01%)	⬆️
Py-pypy-3.11	96.74% <100.00%> (-0.01%)	⬇️
VM-macos	97.26% <100.00%> (ø)
VM-ubuntu	98.06% <100.00%> (+<0.01%)	⬆️
VM-windows	95.73% <100.00%> (-0.01%)	⬇️
cython-coverage	38.43% <0.00%> (-0.01%)	⬇️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 72 untouched benchmarks
⏩ 7 skipped benchmarks¹

---

_{Comparing patchback/backports/3.14/9e00085c958fbcd40de37867e773bf7cd3bbee0c/pr-13010 (becc4eb) with 3.14 (e1cad97)}

Footnotes

1. 7 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. ↩