fix(fetch): dump the body of a 3xx response that will be followed

[zeexzeex]

This relates to...

Fixes #5728

Rationale

When fetch() follows a redirect, the body of the 3xx response is pushed into a Readable that nothing ever reads. Once the buffered body exceeds the stream highWaterMark, push() returns false and the socket is paused.

Nothing resumes it. The call that resumes the socket lives in that stream's read(), and read() only fires when a consumer pulls from the stream. For a followed redirect the response is never exposed to the caller, so no consumer is ever attached: resuming the socket requires reading the body, and the reader only appears once the redirect chain has been followed. The connection stays in a running state and is never returned to the pool.

With a pooled dispatcher this is worse than a single stalled request, since every redirect pins a connection until the connections limit is exhausted. In the reported case a 301 carrying a 128 KiB body with Agent({ connections: 1 }) makes fetch() never settle, and the follow-up request never reaches the server at all.

RedirectHandler already discards 3xx bodies on the dispatcher path, citing RFC 7231 section 6.4. This applies the same behaviour to the fetch path.

Changes

The fix reuses the willFollow value that onResponseStart already computes for content-encoding handling, and discards the chunks in onResponseData instead of pushing them into the stream.

The flag is stored on the dispatch handler rather than on fetchParams.controller. The controller is created once per fetch() call and shared across the whole redirect chain, so setting dump there would also discard the final response body. The handler object is created per dispatch, which scopes the change to the redirect response alone.

Features

N/A

Bug Fixes

• The body of a 3xx response that will be followed is now discarded as it arrives, so the connection is released instead of being pinned for the lifetime of the redirect chain.

Breaking Changes and Deprecations

N/A

Status

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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.46%. Comparing base (fc3450d) to head (b869351).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5729      +/-   ##
==========================================
- Coverage   93.47%   93.46%   -0.01%     
==========================================
  Files         110      110              
  Lines       38908    38915       +7     
==========================================
+ Hits        36368    36372       +4     
- Misses       2540     2543       +3     

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