presto-main/src/main/java/io/prestosql/operator/ExchangeClient.java/ExchangeClient/close
class ExchangeClient:    @Override
    public synchronized void close()
    {
        if (!closed.compareAndSet(false, true)) {
            return;
        }

        for (HttpPageBufferClient client : allClients.values()) {
            closeQuietly(client);
        }
        pageBuffer.clear();
        systemMemoryContext.setBytes(0);
        bufferRetainedSizeInBytes = 0;
        if (pageBuffer.peekLast() != NO_MORE_PAGES) {
            checkState(pageBuffer.add(NO_MORE_PAGES), "Could not add no more pages marker");
        }
        notifyBlockedCallers();
    }