TransformStream: unawaited writer.write() chunks dropped when writer.close() runs before the queued transform jobs

[proggeramlug]

Summary

Unawaited writer.write() chunks on a TransformStream are silently dropped when writer.close() is called in the same synchronous run: the close path closes the readable side before the queued transform jobs deliver their chunks, so the chunks hit a Closed readable and are discarded. Node delivers them.

Reproduces identically on a pristine main build (found while validating #6602 — unrelated to id recycling).

Repro

async function main() {
  const ts = new TransformStream();
  const writer = ts.writable.getWriter();
  writer.write("hello");
  writer.write(" world");
  writer.close();
  let out = "";
  for await (const chunk of ts.readable) out += chunk;
  console.log("transform:", out);
}
main();

• Node: transform: hello world
• Perry: transform: (empty)

Mechanism

transform_write (streams/transform.rs) queues the transformer invocation as a two-hop microtask job (transform_write_job → transform_write_job2) but bypasses the writable's write_queue. writer.close() → js_writer_close → transform_close runs synchronously, and with no user flush() promise to defer on, finish_transform_close calls js_readable_stream_controller_close(readable) immediately — before the queued write jobs run. When transform_write_job2 finally executes, controller_enqueue sees state == Closed and drops the chunk (the write promise still resolves, so the loss is silent).

Per WHATWG, TransformStreamDefaultSinkCloseAlgorithm only runs after queued writes complete — the sink close should chain behind the pending write jobs (or the transform write path should flow through the writable queue that finish_writable_close already waits on).

Awaiting each writer.write() avoids the drop, which is why pipeTo-driven paths (Next.js SSR) don't hit this.