fix(h2): deliver an early final response to an Expect: 100-continue request

[jeswr]

Fixes #5465.

Problem

Over HTTP/2, when a server answers a request carrying Expect: 100-continue with a final response (e.g. 401) instead of 100 (Continue), undici withholds the request body correctly but never delivers the response — the request hangs.

The body write is bound to the stream's 'continue' event; on an early final response that never fires, so the client never sends END_STREAM. The h2 stream stays half-open, its 'close' never fires, and onRequestStreamClose (which finalizes the response) never runs. RFC 9110 §10.1.1 permits a server to answer with a final status determinable from the header fields in place of 100 (Continue), so this is a client conformance bug. HTTP/2 only — HTTP/1.x already handles it.

Fix

In onResponse (lib/dispatcher/client-h2.js): when a final response arrives and the body was never sent, remove the continue listener and stream.end() the writable half, so the stream completes and the response is delivered without sending the body. "Body not sent" is tracked with a new state.bodySent, set when writeBodyH2 runs.

Tests

New case in test/http2-continue.js: an early 401 to an Expect: 100-continue POST is delivered, and the server receives 0 body bytes. The existing h2 suite passes.

Minimal reproduction: https://github.com/jeswr/undici-h2-expect-continue-bug

[mcollina]

lgtm

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.46%. Comparing base (907bc33) to head (1c82ecf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5470   +/-   ##
=======================================
  Coverage   93.46%   93.46%           
=======================================
  Files         110      110           
  Lines       37115    37124    +9     
=======================================
+ Hits        34690    34699    +9     
  Misses       2425     2425           

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