Concurrent calls to notify in transaction results in deadlock, but pg_notify works

PoC

  const sql = postgres({ ssl: "require", max: 10 });

  const promises = [];
  for (let i = 0; i < 10; i++) {
    promises.push((async () => {
      console.log(`beginning thread ${i}`)
      await sql.begin(async (sql) => {
        console.log(`notifying ${i}`)

        await sql.notify('test_channel', i.toString());

        console.log(`done ${i}`)
      })
      console.log(`exiting thread ${i}`)
    })());
  }

  await Promise.all(promises);

  console.log('done promises');

  await sql.end();

Output:

beginning thread 0
beginning thread 1
beginning thread 2
beginning thread 3
beginning thread 4
beginning thread 5
beginning thread 6
beginning thread 7
beginning thread 8
beginning thread 9
notifying 6
notifying 7
notifying 4
notifying 3
notifying 1
notifying 9
notifying 8
notifying 0
notifying 5
notifying 2
^C

Changing max=11 results in this

beginning thread 0
beginning thread 1
beginning thread 2
beginning thread 3
beginning thread 4
beginning thread 5
beginning thread 6
beginning thread 7
beginning thread 8
beginning thread 9
notifying 4
notifying 3
notifying 9
notifying 1
notifying 2
notifying 8
notifying 0
notifying 5
notifying 7
notifying 6
done 4
exiting thread 4
done 3
done 9
done 1
done 2
done 8
exiting thread 1
exiting thread 3
exiting thread 8
done 5
done 7
done 6
exiting thread 2
exiting thread 9
done 0
exiting thread 5
exiting thread 0
exiting thread 6
exiting thread 7
done promises

Interestingly, replacing with await sql`select pg_notify('test_channel', ${i})`; results in no deadlock

Start by running the provided concurrent sql.begin/sql.notify PoC with max 10, then compare it with the direct pg_notify query and max 11. Trace the transaction-aware notify path involved in sql.notify; done means concurrent transactional notifications complete without deadlocking at max 10.