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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"drain event is unreliable when using cork/uncork with ServerResponse","articleBody":"### Version\n\nv24.10.0\n\n### Platform\n\n```text\nDarwin blah.local 24.6.0 Darwin Kernel Version 24.6.0: Mon Aug 11 21:15:09 PDT 2025; root:xnu-11417.140.69.701.11~1/RELEASE_ARM64_T6041 arm64\n```\n\n### Subsystem\n\n_No response_\n\n### What steps will reproduce the bug?\n\nThis code snippet will occasionally hang while waiting for the second `drain` event on MacOS, and will reliably hang while waiting for the first `drain` event on Linux:\n\n```js\nconst { createServer } = require('node:http');\n\nfunction drainUncorked(target) {\n\treturn new Promise((resolve) =\u003e {\n\t\tconsole.log('drain required:', target.writableNeedDrain);\n\t\ttarget.once('drain', () =\u003e {\n\t\t\tconsole.log('drain complete');\n\t\t\ttarget.cork();\n\t\t\tresolve();\n\t\t});\n\t\ttarget.uncork();\n\t});\n}\n\nconst s = createServer(async (req, res) =\u003e {\n\tconsole.log('request');\n\tres.cork();\n\tconsole.log('1');\n\tif (!res.write('1'.repeat(100))) await drainUncorked(res);\n\tconsole.log('2');\n\tif (!res.write('2'.repeat(1000000))) await drainUncorked(res);\n\tconsole.log('3');\n\tif (!res.write('3'.repeat(100))) await drainUncorked(res);\n\tconsole.log('4');\n\tif (!res.write('4'.repeat(1000000))) await drainUncorked(res);\n\tconsole.log('5');\n\tres.uncork();\n\tres.end();\n\tconsole.log('done');\n});\n\ns.listen(8080, 'localhost', async () =\u003e {\n\tconsole.log('listening');\n\n\tconst res = await fetch('http://localhost:8080');\n\tawait res.text();\n\ts.close();\n});\n```\n\n### How often does it reproduce? Is there a required condition?\n\nThis reproduces intermittently on v24.10.0 on MacOS (tested: 3 failures in 30 attempts; ~10% failure rate), and reliably on v22.20.0 on Linux (specifically a Raspberry Pi running `Linux pi5 6.12.47+rpt-rpi-v8 #1 SMP PREEMPT Debian 1:6.12.47-1+rpt1~bookworm (2025-09-16) aarch64 GNU/Linux`)\n\n### What is the expected behavior? Why is that the expected behavior?\n\nWhen `writableNeedDrain` is `true` (and equivalently if `.write` returns `false`), there should always be a `drain` event emitted once the stream has been consumed. This should apply regardless of whether the `cork` feature is being used (as long as the stream is uncorked once it needs to drain), and any listener registered while `writableNeedDrain` is `true` should be guaranteed to receive this drain event.\n\n### What do you see instead?\n\non macOS, the code above frequently hangs at this point:\n\n\u003e ```\n\u003e listening\n\u003e request\n\u003e 1\n\u003e 2\n\u003e drain required: true\n\u003e drain complete\n\u003e 3\n\u003e 4\n\u003e drain required: true\n\u003e ```\n\nBy using `curl -vvv localhost:8080` instead of the `fetch` example code, I can see that the response content _is_ being drained successfully (i.e. the correct number of '4's are downloaded), so the `drain` event ought to be fired.\n\nTesting by adding additional delays surprisingly makes this _more_ likely to fail. For example, this adapted version of `drainUncorked` which waits for an event loop between each command fails every time on the second drain on macOS:\n\n```js\nfunction drainUncorked(target) {\n\treturn new Promise((resolve) =\u003e {\n\t\tconsole.log('drain required:', target.writableNeedDrain);\n\t\ttarget.once('drain', async () =\u003e {\n\t\t\tconsole.log('drain complete');\n\t\t\tawait new Promise((resolve) =\u003e setTimeout(resolve, 0));\n\t\t\ttarget.cork();\n\t\t\tawait new Promise((resolve) =\u003e setTimeout(resolve, 0));\n\t\t\tresolve();\n\t\t});\n\t\ttarget.uncork();\n\t});\n}\n```\n\n### Additional information\n\nWithout `cork`/`uncork`, the code always succeeds. I have also tested this on a raw socket and confirmed the `drain` event fires correctly there; this code succeeds every time:\n\n```sh\n# run netcat in the background as a destination for the socket\nnc -l 8080\n```\n\n```js\nconst { Socket } = require('node:net');\n\nfunction drainUncorked(target) {\n\treturn new Promise((resolve) =\u003e {\n\t\tconsole.log('drain required:', target.writableNeedDrain);\n\t\ttarget.once('drain', () =\u003e {\n\t\t\tconsole.log('drain complete');\n\t\t\ttarget.cork();\n\t\t\tresolve();\n\t\t});\n\t\ttarget.uncork();\n\t});\n}\n\n(async () =\u003e {\n\tconst s = new Socket();\n\ts.connect(8080, 'localhost');\n\ts.cork();\n\tconsole.log('1');\n\tif (!s.write('1'.repeat(100))) await drainUncorked(s);\n\tconsole.log('2');\n\tif (!s.write('2'.repeat(1000000))) await drainUncorked(s);\n\tconsole.log('3');\n\tif (!s.write('3'.repeat(100))) await drainUncorked(s);\n\tconsole.log('4');\n\tif (!s.write('4'.repeat(1000000))) await drainUncorked(s);\n\tconsole.log('5');\n\ts.uncork();\n\ts.end();\n})();\n```\n\nSince this is unique to ServerResponse, I _suspect_ it is related to the chunk merging behaviour from https://github.com/nodejs/node/pull/50167 (and chunk merging is why I want to use `cork` in the first place)","author":{"url":"https://github.com/davidje13","@type":"Person","name":"davidje13"},"datePublished":"2025-10-27T13:11:18.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/60432/node/issues/60432"}