Connection pool sometimes does not end properly

Hello. I discovered race condition with sql.end().

It seems that if there are more queries queued than max pool size, something hangs.

I'm running on deno 1.43.1 with postgres.js v3.4.4

When CLOSE_POOL_TIMEOUT is around 33, there is 50/50 chance of both conditions below.
If CLOSE_POOL_TIMEOUT is much greater, process will hang.
If CLOSE_POOL_TIMEOUT is lesser, process will exit normally.

import postgres from "https://deno.land/x/postgresjs/mod.js";

const CLOSE_POOL_TIMEOUT = 33;

const sql = postgres({
  debug: true,
  max: 2,
  connect_timeout: 1,
});

const log = (str: string, ...rest: unknown[]) =>
  console.log(
    `[${(performance.now() / 1000).toFixed(3).padStart(6)}s] ${str}`,
    ...rest,
  );

const delay = (ms: number) => new Promise((r) => setTimeout(r, ms));

const longQuery = () =>
  sql`SELECT CURRENT_TIMESTAMP as ts, pg_sleep(0.05) as sleep`.execute();

let ended = false;
const parallelQueries = async () => {
  for (let i = 1; i <= 100; i++) {
    await delay(10);

    if (ended) {
      return;
    }

    const queryId = i;
    log(`${queryId} run`);
    longQuery()
      .then((r) => {
        log(`${queryId} OK:  ts: ${r?.[0]?.ts.toISOString()}`);
      })
      .catch((e) => {
        log(`${queryId} ERR:`, e.message);
      });
  }
};

parallelQueries()
  .then(() => {
    log("parallelQueries done");
  })
  .catch(() => {
    log("parallelQueries error");
  });

setTimeout(async () => {
  log("closing pool...");
  await sql.end({
    timeout: 1,
  });
  ended = true;
  log("pool closed");
}, CLOSE_POOL_TIMEOUT);

globalThis.addEventListener("unload", () => {
  log("goodbye!");
});

The output is as follows:
First run, OK, finished successfully 👍

% deno run -A postgres/client.ts
[ 0.106s] 1 run
[ 0.121s] 2 run
[ 0.130s] closing pool...
[ 0.132s] 3 run
[ 0.132s] 3 ERR: write CONNECTION_ENDED /var/run/postgresql/.s.PGSQL.5444
[ 0.143s] 4 run
[ 0.143s] 4 ERR: write CONNECTION_ENDED /var/run/postgresql/.s.PGSQL.5444
[ 0.156s] 5 run
[ 0.156s] 5 ERR: write CONNECTION_ENDED /var/run/postgresql/.s.PGSQL.5444
[ 0.167s] 6 run
[ 0.167s] 6 ERR: write CONNECTION_ENDED /var/run/postgresql/.s.PGSQL.5444
[ 0.172s] 1 OK:  ts: 2024-05-04T19:50:46.984Z
[ 0.179s] 7 run
[ 0.179s] 7 ERR: write CONNECTION_ENDED /var/run/postgresql/.s.PGSQL.5444
[ 0.181s] 2 OK:  ts: 2024-05-04T19:50:46.994Z
[ 0.181s] pool closed
[ 0.190s] parallelQueries done
[ 0.192s] goodbye!

Second run does not ternimate. 🤕

% deno run -A postgres/client.ts
[ 0.102s] 1 run
[ 0.114s] 2 run
[ 0.126s] 3 run  <-- 3rd client queued when pool max == 2
[ 0.126s] closing pool...
[ 0.138s] 4 run
[ 0.138s] 4 ERR: write CONNECTION_ENDED /var/run/postgresql/.s.PGSQL.5444
[ 0.151s] 5 run
[ 0.151s] 5 ERR: write CONNECTION_ENDED /var/run/postgresql/.s.PGSQL.5444
[ 0.162s] 6 run
[ 0.162s] 6 ERR: write CONNECTION_ENDED /var/run/postgresql/.s.PGSQL.5444
[ 0.165s] 1 OK:  ts: 2024-05-04T19:50:48.303Z
[ 0.173s] 7 run
[ 0.173s] 7 ERR: write CONNECTION_ENDED /var/run/postgresql/.s.PGSQL.5444
[ 0.175s] 2 OK:  ts: 2024-05-04T19:50:48.314Z
[ 0.176s] pool closed
[ 0.183s] parallelQueries done
[ 0.234s] 3 OK:  ts: 2024-05-04T19:50:48.372Z  <-- response after pool closed?
^C

Start by running the issue's Deno reproduction with max: 2 and queued queries, then trace sql.end through the connection-pool queue in mod.js. Done means queued queries consistently settle or reject and the process exits without hanging, including when closing races with a queued query.