refactor: replace filterFlags with mime.ParseMediaType

[wanghaolong613]

Pull Request Checklist

Please ensure your pull request meets the following requirements:

• Open your pull request against the master branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.
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Changes

Replace handmade filterFlags simple string truncation logic with standard library mime.ParseMediaType to parse Content-Type header.

Purpose

• Fix multiple parsing defects of filterFlags: leading whitespace, case sensitivity, incomplete whitespace handling
• Follow RFC standard for MIME type parsing
• Unify media type to lowercase automatically
• Gain better compatibility with malformed HTTP request headers from clients

Testing

Added test cases to verify parsing behavior for:

• Leading / trailing whitespaces
• Case-insensitive media types
• Various irregular whitespace layouts
• Malformed and incomplete Content-Type headers

[rashmi-tondare]

Good direction since mime.ParseMediaType fixes real defects in the old scan (leading whitespace, case sensitivity). One thing to check though: ParseMediaType can return a non-empty mediatype and a non-nil error (ErrInvalidMediaParameter) for headers with a valid base type but a malformed param. The current err != nil fallback doesn't handle that correctly.

Also, could use some table-driven tests in utils_test.go for the cases this is meant to fix (whitespace, case, missing space before ;), plus a couple of malformed-header cases to pin down the fallback behavior.

[wanghaolong613]

Good direction since mime.ParseMediaType fixes real defects in the old scan (leading whitespace, case sensitivity). One thing to check though: ParseMediaType can return a non-empty mediatype and a non-nil error (ErrInvalidMediaParameter) for headers with a valid base type but a malformed param. The current err != nil fallback doesn't handle that correctly.

Also, could use some table-driven tests in utils_test.go for the cases this is meant to fix (whitespace, case, missing space before ;), plus a couple of malformed-header cases to pin down the fallback behavior.

OK, I'll fix it

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (3dc1cd6) to head (8060370).
⚠️ Report is 289 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4758      +/-   ##
==========================================
- Coverage   99.21%   98.33%   -0.89%     
==========================================
  Files          42       48       +6     
  Lines        3182     3174       -8     
==========================================
- Hits         3157     3121      -36     
- Misses         17       43      +26     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.31% <100.00%> (?)
-tags go_json	98.25% <100.00%> (?)
-tags nomsgpack	98.30% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.33% <100.00%> (?)
go-1.26	98.33% <100.00%> (?)
macos-latest	98.33% <100.00%> (-0.89%)	⬇️
ubuntu-latest	98.33% <100.00%> (-0.89%)	⬇️

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[rashmi-tondare]

Changes look good to me! Just one small thing, would be nice to add the pull request template in the PR description and tick off appropriate items from the checklist.

[wanghaolong613]

Changes look good to me! Just one small thing, would be nice to add the pull request template in the PR description and tick off appropriate items from the checklist.

Thanks for the feedback! I'll add the PR template to the description and tick off the relevant checklist items right away.

[rashmi-tondare]

LGTM!
@appleboy this fix looks sound to me. It contains tests for the fixed issue as well and I believe would be a good addition to the lib.