[mrityunjay-pandey]

This relates to...

Fixes #6027

Rationale

The HTTP/1.1 and HTTP/2 clients deliver interim 1xx responses (e.g. 103 Early Hints) through onResponseStart before the final response. DecoratorHandler.onResponseStart asserts it is only called once, so any handler built on it (the dns interceptor's DNSDispatchHandler in the issue) fails the request with ERR_ASSERTION when the final response arrives after an interim one.

Changes

• DecoratorHandler.onResponseStart only applies the once-only check (and sets the flag) for final responses (statusCode >= 200). Interim responses are still forwarded to the wrapped handler, which already ignores them where needed (e.g. fetch).
• Tests: a DecoratorHandler unit test (a 103 then a 200 are both forwarded, and a second final response still asserts), and a dns interceptor test fetching from a server that sends 103 Early Hints. Both fail on main.

Features

N/A

Bug Fixes

• Interceptors built on DecoratorHandler (such as interceptors.dns()) no longer fail when a response is preceded by a 1xx interim response.

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

Tested locally on Linux with Node 24: npm run lint and npm run test:unit (1634 tests, 1629 pass, 5 skipped, 0 fail).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.59%. Comparing base (935aa75) to head (fcbf9e9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6028      +/-   ##
==========================================
+ Coverage   91.56%   91.59%   +0.02%     
==========================================
  Files         113      113              
  Lines       43938    43942       +4     
==========================================
+ Hits        40234    40248      +14     
+ Misses       3704     3694      -10     

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