http: emit 'drain' after flushing coalesced chunks by junyup0503 · Pull Request #66782 · nodejs/node

junyup0503 · 2026-10-16T06:40:16Z

Since #64987, chunked writes made while the response is auto-corked are coalesced, and write() returns false based on the byte length of the buffered chunks. When uncork() flushes them, net.Socket counts string chunks in UTF-16 code units, so with multi-byte text the socket can stay below its own highWaterMark and never emits 'drain'. If the flushed writes complete asynchronously, the check at the end of uncork() also runs while bytes are still pending, so the response stalls with writableNeedDrain === true and writableLength === 0.

This re-checks the drain condition in the write callback of the flushed data, unless the message or its socket has been destroyed. It does not change the accounting itself (e.g. encoding the coalesced strings to Buffers), to avoid adding cost to the write path that #64987 optimized.

The new test uses a Duplex that counts strings like net.Socket and completes writes asynchronously, so it does not depend on OS socket buffer sizes. It hangs on main without the change and passes with it. It also checks that 'drain' is not emitted after the response is destroyed from a write callback. parallel/test-http*, parallel/test-stream*, parallel/test-net*, sequential/test-http* and make lint-js pass locally.

Fixes: #66781
Refs: #64987

I used an AI coding agent to help investigate and write this change. I reproduced the issue and verified the fix and the tests myself.

Since chunked writes are coalesced while the message is corked, write() decides backpressure from the byte length of the buffered chunks and sets kNeedDrain. When uncork() flushes them, net.Socket accounts string chunks in UTF-16 code units, so with multi-byte text it can stay below its own highWaterMark. It then never needs to drain and never emits 'drain'. If the flushed writes complete asynchronously, the check at the end of uncork() also runs while bytes are still pending, so the response stalls with writableNeedDrain === true and writableLength === 0. Re-check the drain condition once the flushed data has been written, unless the message or its socket has been destroyed in the meantime. Fixes: nodejs#66781 Refs: nodejs#64987 Assisted-by: AI coding agent Signed-off-by: junyup0503 <97156898+junyup0503@users.noreply.github.com>

nodejs-github-bot · 2026-10-16T06:40:22Z

Review requested:

@nodejs/http
@nodejs/net

github-actions · 2026-10-16T06:40:39Z

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par333k · 2026-10-16T07:07:13Z

const currentSocket = this[kSocket]

The socket reference is captured when uncork() runs, so it is null if the message had no socket yet and the write was buffered in outputData. Re-reading this[kSocket] in the callback checks the socket the data was actually flushed to. In practice _flush() usually clears kNeedDrain before this callback runs, so this is a defensive guard rather than a fix for an observed crash.

par333k · 2026-10-16T07:09:47Z

+      // Re-check once the flushed data has been written.
+      if (!err && this.writableNeedDrain && !socket.destroyed &&
+          this.writableLength === 0) {


Suggested change

// Re-check once the flushed data has been written.

if (!err && this.writableNeedDrain && !socket.destroyed &&

this.writableLength === 0) {

const currentSocket = this[kSocket];

if (!err && this.writableNeedDrain && currentSocket &&

!currentSocket.destroyed && this.writableLength === 0) {

+1, I think without this there is likely a reachable crash here, good catch @par333k

Thanks for the suggestion! @par333k
Applied in 96a0377. The regression test and five related HTTP tests all pass.

codecov · 2026-10-26T10:54:36Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.69%. Comparing base (1c90519) to head (3f263a8).
⚠️ Report is 258 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66782      +/-   ##
==========================================
+ Coverage   90.49%   90.69%   +0.19%     
==========================================
  Files         797      797              
  Lines      277598   278906    +1308     
  Branches    53336    53631     +295     
==========================================
+ Hits       251220   252950    +1730     
+ Misses      16790    16311     -479     
- Partials     9588     9645      +57

Files with missing lines	Coverage Δ
lib/internal/http/outgoing.js	`97.99% <100.00%> (+0.01%)`	⬆️

... and 128 files with indirect coverage changes

🚀 New features to boost your workflow:

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pimterry

The comment here needs resolving, but otherwise I think this is good to go and fixes a real regression. Thanks for the fix @junyup0503!

pimterry · 2026-10-28T11:30:47Z

+      // Re-check once the flushed data has been written.
+      if (!err && this.writableNeedDrain && !socket.destroyed &&
+          this.writableLength === 0) {


+1, I think without this there is likely a reachable crash here, good catch @par333k

Read the current socket after the coalesced write callbacks run, since uncork() may initially buffer data before a socket is assigned. Check that the current socket exists and is not destroyed before emitting drain. Refs: nodejs#66782 (comment) Assisted-by: AI coding agent Signed-off-by: junyup0503 <97156898+junyup0503@users.noreply.github.com>

nodejs-github-bot · 2026-10-30T17:46:19Z

CI: https://ci.nodejs.org/job/node-test-pull-request/78747/

junyup0503 · 2026-10-30T18:06:20Z

I applied @par333k’s suggestion locally but missed the co-author attribution. Could we include them as a co-author in the final commit when landing this PR?

nodejs-github-bot · 2026-10-31T14:45:19Z

CI: https://ci.nodejs.org/job/node-test-pull-request/78767/

github-actions · 2026-11-01T13:51:33Z

Failed to resume CI

✖ Failed to load PR CI job 78767: Unexpected token '<', ..."

Full Auto Start CI output

�[36m⠋�[39m Validating Jenkins credentials
�[36m⠋�[39m Validating Jenkins credentials
✔  Jenkins credentials valid
�[36m⠙�[39m Looking for CI runs for pull request 66782
�[36m⠙�[39m Looking for CI runs for pull request 66782
�[36m⠙�[39m Getting PR from nodejs/node/pull/66782
�[36m⠙�[39m Getting reviews from nodejs/node/pull/66782
�[36m⠙�[39m Getting comments from nodejs/node/pull/66782
✔  Found PR CI job 78767
�[36m⠹�[39m Querying data for job/node-test-pull-request/78767/
�[36m⠹�[39m Querying data for job/node-test-pull-request/78767/
�[36m⠹�[39m Querying API for job/node-test-pull-request/78767/

[DEBUG] [SyntaxError: Unexpected token '<', ..."    
  
  <!DOCTYPE "... is not valid JSON]
✖  Failed to load PR CI job 78767: Unexpected token '<', ..."    
  
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ronag · 2026-11-01T13:57:45Z

+      if (!err && this.writableNeedDrain && currentSocket &&
+          !currentSocket.destroyed && this.writableLength === 0) {


Suggested change

if (!err && this.writableNeedDrain && currentSocket &&

!currentSocket.destroyed && this.writableLength === 0) {

if (!err && this.writableNeedDrain && !this.destroyed && this.writableLength === 0) {

Maybe?

Applied in 3f263a8, thanks. CI needs a fresh run — the auto-resume failed with an infra error.

Rely on the message's own destroyed state instead of the socket's when re-checking the drain condition after the coalesced chunks are flushed. writableNeedDrain already excludes destroyed and finished messages. Refs: nodejs#66782 (comment) Co-authored-by: Robert Nagy <ronagy@icloud.com> Co-authored-by: Park Jin Woong <35551417+par333k@users.noreply.github.com> Assisted-by: AI coding agent Signed-off-by: junyup0503 <97156898+junyup0503@users.noreply.github.com>

nodejs-github-bot · 2026-11-03T07:59:36Z

CI: https://ci.nodejs.org/job/node-test-pull-request/78804/

sxa · 2026-11-03T10:32:41Z

The AIX test here is an issue while will not occur on a re-run, so once the existing PR run is complete a re-run should allow it to pass

aduh95 · 2026-11-03T22:23:10Z

Related Windows failure:


---
duration_ms: 195.014
exitcode: 1
severity: fail
stack: |-
  node:assert:173
    throw err;
    ^

  AssertionError [ERR_ASSERTION]: function should not have been called at c:\workspace\node-test-binary-windows-js-suites\node\test\parallel\test-http-outgoing-chunked-drain-multibyte.js:63
      at ServerResponse.mustNotCall (c:\workspace\node-test-binary-windows-js-suites\node\test\common\index.js:570:12)
      at ServerResponse.emit (node:events:514:20)
      at socketOnDrain (node:internal/http/server:925:9)
      at Duplex.emit (node:events:514:20)
      at afterWrite (node:internal/streams/writable:744:12)
      at onwrite (node:internal/streams/writable:727:7)
      at WritableState.onwrite (node:internal/streams/writable:351:26)
      at Immediate.flush [as _onImmediate] (c:\workspace\node-test-binary-windows-js-suites\node\test\parallel\test-http-outgoing-chunked-drain-multibyte.js:28:22)
      at process.processImmediate (node:internal/timers:567:21) {
    generatedMessage: false,
    code: 'ERR_ASSERTION',
    actual: undefined,
    expected: undefined,
    operator: 'fail',
    diff: 'simple'
  }

  Node.js v28.0.0-pre
...

The default stream highWaterMark is 16 KiB on Windows and 64 KiB elsewhere. With the smaller value the fake socket exceeds its own highWaterMark in UTF-16 code units and emits 'drain' by itself before the write callback destroys the response, which trips mustNotCall(). Pin the highWaterMark so the payload sizes stay on the intended side of it on every platform, and assert that the socket never drains on its own in the destroy scenario. Refs: nodejs#66782 (comment) Assisted-by: AI coding agent Signed-off-by: junyup0503 <97156898+junyup0503@users.noreply.github.com>

junyup0503 · 2026-11-04T03:59:22Z

Thanks. That was a test assumption: the default highWaterMark is 16 KiB on Windows, so the fake socket emitted 'drain' on its own before the write callback destroyed the response. Fixed in 0ecaa63 by pinning the socket's highWaterMark; reproduced and verified locally with the 16 KiB default. A fresh CI run is needed.

nodejs-github-bot added http Issues and PRs related to the http subsystem. needs-ci PRs that need a full CI run. labels Oct 16, 2026

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robobun mentioned this pull request Oct 21, 2026

node:http: gate 'drain' on the bytes of the handle, give write callbacks to the stream oven-sh/bun#46014

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hjdhjd mentioned this pull request Oct 24, 2026

fix(hb-service): stop the colored log download stalling on node 24.22+ and 26.9+ homebridge/homebridge-config-ui-x#3083

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H4ad mentioned this pull request Oct 30, 2026

On Node.js 24.22.0+, every streamed response hangs (the fake socket has no writableLength / writableHighWaterMark) H4ad/serverless-adapter#423

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		if (!err && this.writableNeedDrain && currentSocket &&
		!currentSocket.destroyed && this.writableLength === 0) {

	if (!err && this.writableNeedDrain && currentSocket &&
	!currentSocket.destroyed && this.writableLength === 0) {
	if (!err && this.writableNeedDrain && !this.destroyed && this.writableLength === 0) {

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