stream/iter: broadcast consumer can leave earlier next() pending after overlapping next() calls #63499

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1. trivikr

   self-assigned this

   on May 23, 2026
2. trivikr commented on May 25, 2026

   

   trivikr

   on May 25, 2026

   MemberAuthor

   More actions

   As per spec 13.1.3, the second next should be pending and not resolve.

   Broadcast.push() creates a consumer cursor that “advances the cursor on each step, waiting for new data if needed” in the spec. One written chunk should satisfy the earliest pending next() for that iterator. The second next() has no second chunk to read, and the broadcast has not been ended/cancelled, so it should wait for new data. It should only resolve { done: true, value: undefined } after clean completion, for example writer.end() or broadcast.cancel() with no reason.

   New repro

   import { broadcast } from 'node:stream/iter';
   import { setTimeout } from 'node:timers/promises';
   
   const { writer, broadcast: bc } = broadcast();
   const it = bc.push()[Symbol.asyncIterator]();
   
   const first = it.next();
   const second = it.next();
   
   await writer.write('x');
   
   const secondResult = await Promise.race([
     second.then((value) => ['settled', value]),
     setTimeout(50, ['pending']),
   ]);
   
   console.log('second next():', ...secondResult);
   
   const result = await Promise.race([
     first.then((value) => ['settled', value]),
     setTimeout(50, ['pending']),
   ]);
   
   console.log('first next():', ...result);

   Observed

   second next(): settled [Object: null prototype] {
     done: false,
     value: [ Uint8Array(1) [ 120 ] ]
   }
   first next(): pending

   Expected as previous ask

   second next(): settled [Object: null prototype] { done: true, value: undefined }
   first next(): settled [Object: null prototype] {
     done: false,
     value: [ Uint8Array(1) [ 120 ] ]
   }

   Expected (actual)

   second next(): pending
   first next(): settled [Object: null prototype] {
     done: false,
     value: [ Uint8Array(1) [ 120 ] ]
   }
3. 2 remaining items

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4. trivikr

   added a commit that references this issue on Jun 18, 2026

   stream: keep overlapping broadcast reads pending

   ...

   8a996e3
5. nodejs-github-bot

   closed this as completedin #63500on Jun 20, 2026
6. nodejs-github-bot

   added a commit that references this issue on Jun 20, 2026

   stream: keep overlapping broadcast reads pending

   ...

   d1eaaf6
7. aduh95

   added a commit that references this issue on Jun 20, 2026

   stream: keep overlapping broadcast reads pending

   ...

   a9f9a3d
8. aduh95

   added a commit that references this issue on Aug 6, 2026

   stream: keep overlapping broadcast reads pending

   ...

   97ef13c