net: burst of 40k writes then end() delivers 0 bytes to a local server (Node: all 2.56 MB) #11106

A net client that makes one large burst of writes and then calls end() delivers 0 bytes to a local net server. Node delivers all 2,560,000 bytes.

Found while A/B-testing tokio lane A (#11105). Base (bd9cd48da, #11083's head) and branch behave identically, so this is not caused by that PR, and it is not specific to either socket backend.

Shape

import net from "node:net";
const N = 40000, CHUNK = Buffer.alloc(64, 0x61);
const srv = net.createServer((s) => {
  let got = 0;
  s.on("data", (d) => { got += d.length; });
  s.on("end", () => { console.log("received", got); srv.close(); });
});
srv.listen(0, () => {
  const c = net.connect((srv.address() as any).port, () => {
    for (let i = 0; i < N; i++) c.write(CHUNK);
    c.end();
  });
});

Node 26.5.1 prints received 2560000. Perry prints received 0.

Likely suspects:

• the write queue is dropped when end() arrives before it drains
• the half-close (FIN) is sent ahead of the queued data

The fixture above is a reconstruction of the agent's probe; confirm the exact counts before minimizing further.