fix(responseInterceptor): prevent trailer/content-length conflict

[chimurai]

Causing this error to be thrown:

node:_http_outgoing:522
    throw new ERR_HTTP_TRAILER_INVALID();
    ^

Error [ERR_HTTP_TRAILER_INVALID]: Trailers are invalid with this transfer encoding

closes #830

Summary by CodeRabbit

• Bug Fixes
  • Fixed proxied response header handling to strip incompatible framing headers while preserving content-length, content-type, set-cookie, and cache-control so responses are correctly framed.
• Tests
  • Added end-to-end tests covering responses with chunked transfer encoding and trailer headers to verify correct header filtering and body rewriting.
• Chores
  • Updated changelog with the fix entry.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 213327a4-bb3a-4c82-aced-614c6b451093

📥 Commits

Reviewing files that changed from the base of the PR and between da3bad3 and 93cf883.

📒 Files selected for processing (3)

• CHANGELOG.md
• src/handlers/response-interceptor.ts
• test/e2e/response-interceptor.spec.ts

---

📝 Walkthrough

Walkthrough

responseInterceptor removes trailer and transfer-encoding before setting content-length; copyHeaders now ignores trailer when copying upstream headers. An end-to-end test verifies proxied responses with chunked transfer and trailers are rewritten and that trailer/transfer-encoding headers are omitted while content-length and other headers are preserved or set.

Changes

Trailer Header Handling

Layer / File(s)	Summary
Header stripping and filtering src/handlers/response-interceptor.ts, CHANGELOG.md	Before setting content-length, the interceptor removes trailer and transfer-encoding from the outgoing ServerResponse. copyHeaders now filters out trailer alongside content-encoding and transfer-encoding. A changelog entry documents the fix.
Trailer header handling test test/e2e/response-interceptor.spec.ts	New e2e test configures an upstream endpoint with chunked transfer encoding and a trailer header, applies responseInterceptor to rewrite the JSON body, and asserts the proxied response omits trailer and transfer-encoding while ensuring content-length, content-type, set-cookie, and cache-control are present or adjusted.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 I nibbled headers by the stream,
Found trailers tangled in the seam,
I lop the bits that break the length,
So buffered bytes regain their strength,
Hooray — the proxy hops along, all clean! 🥕

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: preventing a trailer/content-length conflict in the responseInterceptor, which directly aligns with the primary objective of fixing the ERR_HTTP_TRAILER_INVALID error.
Linked Issues check	✅ Passed	All coding requirements from issue #830 are met: trailer/content-length conflict is resolved, disallowed headers are removed when trailers present, and end-to-end test coverage is provided.
Out of Scope Changes check	✅ Passed	All changes are directly related to addressing the trailer/content-length conflict issue. The responseInterceptor modifications, copyHeaders adjustments, and test additions are all within scope of issue #830.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ERR_HTTP_TRAILER_INVALID

---

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[pkg-pr-new]

npm i https://pkg.pr.new/http-proxy-middleware@1249

commit: da3bad3

[coveralls]

coverage: 94.697% (+0.04%) from 94.656% — ERR_HTTP_TRAILER_INVALID into master