http: fix drain event with cork/uncork

[davidje13]

Fixes #60432

This is a copy of PR #60437, so credit for the fix goes to @ThierryMT. Sadly that PR was abandoned and I wasn't able to get a response from the author. I've just applied their changes to the latest main, patched up the lint errors, and applied the suggested change to the tests. I've signed the certificate of origin on the basis that the test is based on the reproduction I provided in the original bug report #60432 (hence, created in part by me), but let me know if this is too tenuous and I can write a new test from scratch instead. The fix itself is so trivial that I'm not sure there's a way to write it which isn't effectively the same code.

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Original PR description:

When using cork() and uncork() with ServerResponse, the drain event was not reliably emitted after uncorking. This occurred because the uncork() method did not check if a drain was pending (kNeedDrain flag) after flushing the chunked buffer.

The Problem:

• When corked, write() buffers data in kChunkedBuffer instead of sending immediately
• If write() returns false, kNeedDrain is set to true
• When uncork() is called, it flushes the buffer by calling _send()
• After flushing, uncork() never checked if drain was needed or emitted the event
• This left the response waiting for a drain event that would never fire

The Fix:
This commit ensures that when uncork() successfully flushes buffered data and a drain was needed, the drain event is emitted immediately.

Changes:

• Modified OutgoingMessage.prototype.uncork() in lib/_http_outgoing.js
• Capture return value from final _send() call
• Check if drain was pending after successful flush
• Emit drain event when appropriate

Testing:
The issue could be reproduced by using cork() with multiple large writes and checking for drain events. With this fix, the drain event now fires correctly after uncork().

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[Copilot]

Pull request overview

This pull request fixes a reliability issue where ServerResponse could get stuck with writableNeedDrain === true when using cork()/uncork(), because uncork() flushed the internal chunked buffer without emitting 'drain' when no socket-level backpressure occurred.

Changes:

• Update OutgoingMessage.prototype.uncork() to emit 'drain' after flushing the chunked buffer when a drain was pending.
• Add a regression test covering ServerResponse 'drain' behavior when corked writes hit backpressure.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
lib/_http_outgoing.js	Emits 'drain' from uncork() when the buffered chunked payload is flushed successfully and kNeedDrain was set.
test/parallel/test-http-response-drain-cork.js	Adds a regression test for the cork/uncork drain behavior on ServerResponse.

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[ronag]

Please fix copilot feedback.

[davidje13]

Applied the requested changes, and simplified the test a little since setting an explicit high water mark means we don't need to be so extreme with the volume of data.

I also removed the '3' part of the test, which I don't think was testing anything useful here, and is likely just a remnant of how I originally diagnosed the issue.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74331/

[mcollina]

lgtm

[davidje13]

Amended first commit message to fix commit lint error (also rebased on latest main)