Error swap between completing and failing query in build step (with max connections = 1)

• Tested node versions: v16.19.0, v20.9.0
• Tested postgres.js versions: 3.0.6, 3.4.3
• Tested postgres version: 14

Hi,
with the following conditions:

1. the max number of connections is set to 1
2. the connection is already open
3. two queries are run at the same time

If the second query fails the build step, the build error is thrown on the first query, and the second query never resolves nor throws.

The bug occurs also for any n >= 2, as in the squared parenthesis in the comments.

const postgres = require("postgres");

(async () => {
  const sql = postgres({
    // important: max should be 1 [ n - 1 ]
    max: 1
  });
  try {
    // connection must exists. If this query is not run the bug does not occurs
    await sql`SELECT 2`;

    // 2 [n] queries. The first [first to (n-1)th] one is ok, but the second [nth] should fail in the build step
    await Promise.all([
      // this is a valid query and should resolve. It throws the error in the second query instead
      sql`SELECT 1`.catch((e) => {
        console.log(
          "Catched error from other query",
          e,
          e.query /* SELECT 1*/
        );
      }),
      // this query should fail *in the build step*. It never resolves/throws
      sql`SELECT ${undefined}`.catch((e) => {
        console.log("Never catched: this line won't be logged", e, e.query);
      }),
    ]);
    console.log("This line will never be logged");
  } catch (error) {
    console.log("This line will never be logged. Error in one promise: ", error);
    throw error;
  }
  await sql.end();
})().catch((e) => {
  console.error("Error", e);
});

I'm sorry, I tried but cannot suggest the fix.

Start by running the supplied Node.js reproduction with max connections set to 1 and inspect the connection scheduling and query build-step handling it exercises. Done means the valid concurrent query resolves, the invalid query rejects with its own error, and both promises settle without swapping errors or hanging.