René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"stream/iter: BroadcastWriter.end() strands backpressured writes and truncates the broadcast","articleBody":"### Version\n\nmain\n\n### Platform\n\n```text\nN/A\n```\n\n### Subsystem\n\nstream\n\n### What steps will reproduce the bug?\n\n```js\nimport { setTimeout } from 'node:timers/promises';\nimport { broadcast } from 'node:stream/iter';\n\nconst { writer, broadcast: bc } = broadcast({\n  budget: 16384,\n  backpressure: 'unbounded',\n});\nconst reader = bc.push()[Symbol.asyncIterator]();\n\nawait writer.write(new Uint8Array(16384));\n\nconst pendingWrite = writer.write(Uint8Array.of(1));\nconst endPromise = writer.end();\n\nconsole.log('first read done:', (await reader.next()).done);\n\nconst writeState = await Promise.race([\n  pendingWrite.then(\n    () => 'resolved',\n    () => 'rejected',\n  ),\n  setTimeout(100, 'still pending'),\n]);\nconsole.log('blocked write:', writeState);\n\nconsole.log('second read done:', (await reader.next()).done);\nconsole.log('final read done:', (await reader.next()).done);\nconsole.log('end():', await endPromise);\n```\n\n### How often does it reproduce? Is there a required condition?\n\nAlways\n\n### What is the expected behavior? Why is that the expected behavior?\n\n```console\nfirst read done: false\nblocked write: resolved\nsecond read done: false\nfinal read done: true\nend(): 16385\n```\n\nWrites queued before `end()` should remain ahead of end-of-stream. The pending write should be promoted when the first read frees capacity, and `end()` should resolve only after the consumer reaches the final end sentinel.\n\nAs per [§7.2.2, `Writer.write()`](https://iter-streams.proposal.wintertc.org/#writer-write):\n> “return a promise that resolves when the batch is transferred to the slots buffer.”\n\nAnd [§7.2.5, `Writer.end()` / `Writer.endSync()`](https://iter-streams.proposal.wintertc.org/#writer-end):\n> “signals end-of-stream and waits for buffered data to drain.”\n\n### What do you see instead?\n\n```console\nfirst read done: false\nblocked write: still pending\nsecond read done: true\nfinal read done: true\nend(): 16384\n```\n\nAfter `end()` marks the broadcast ended, freeing buffer capacity cannot promote the queued write. Its promise remains pending, the byte is never delivered, and `end()` reports only the first write.\n\n\n### Additional information\n\n_No response_","author":{"url":"https://github.com/trivikr","@type":"Person","name":"trivikr"},"datePublished":"2026-08-16T19:08:40Z","url":"https://github.com/nodejs/node/issues/65333"}