presto-main/src/main/java/io/prestosql/operator/ExchangeClient.java/ExchangeClient/requestComplete
class ExchangeClient:    private synchronized void requestComplete(HttpPageBufferClient client)
    {
        if (!queuedClients.contains(client)) {
            queuedClients.add(client);
        }
        scheduleRequestIfNecessary();
    }