PostOffice.java

package nachos.network;

import nachos.machine.*;
import nachos.threads.*;

/**
 * A collection of message queues, one for each local port. A
 * <tt>PostOffice</tt> interacts directly with the network hardware. Because
 * of the network hardware, we are guaranteed that messages will never be
 * corrupted, but they might get lost.
 *
 * <p>
 * The post office uses a "postal worker" thread to wait for messages to arrive
 * from the network and to place them in the appropriate queues. This cannot
 * be done in the receive interrupt handler because each queue (implemented
 * with a <tt>SynchList</tt>) is protected by a lock.
 */
public class PostOffice {
    /**
     * Allocate a new post office, using an array of <tt>SynchList</tt>s.
     * Register the interrupt handlers with the network hardware and start the
     * "postal worker" thread.
     */
    public PostOffice() {
        messageReceived = new Semaphore(0);
        messageSent = new Semaphore(0);
        sendLock = new Lock();

        queues = new SynchList[MailMessage.portLimit];
        for (int i=0; i<queues.length; i++)
            queues[i] = new SynchList();

        Runnable receiveHandler = new Runnable() {
            public void run() {
                receiveInterrupt();
            }
        };
        Runnable sendHandler = new Runnable() {
            public void run() {
                sendInterrupt();
            }
        };
        Machine.networkLink().setInterruptHandlers(receiveHandler,
                sendHandler);

        KThread t = new KThread(new Runnable() {
            public void run() {
                postalDelivery();
            }
        });

        t.fork();
    }

    /**
     * Retrieve a message on the specified port, waiting if necessary.
     *
     * @param	port	the port on which to wait for a message.
     *
     * @return	the message received.
     */
    public MailMessage receive(int port) {
        Lib.assertTrue(port >= 0 && port < queues.length);

        Lib.debug(dbgNet, "waiting for mail on port " + port);

        MailMessage mail = (MailMessage) queues[port].removeFirst();

        if (Lib.test(dbgNet))
            System.out.println("got mail on port " + port + ": " + mail);

        return mail;
    }

    /**
     * Wait for incoming messages, and then put them in the correct mailbox.
     */
    private void postalDelivery() {
        while (true) {
            messageReceived.P();

            Packet p = Machine.networkLink().receive();

            MailMessage mail;

            try {
                mail = new MailMessage(p);
            } catch (MalformedPacketException e) {
                continue;
            }

            if (Lib.test(dbgNet))
                System.out.println("delivering mail to port " + mail.dstPort
                                   + ": " + mail);

            // atomically add message to the mailbox and wake a waiting thread
            queues[mail.dstPort].add(mail);
        }
    }

    /**
     * Called when a packet has arrived and can be dequeued from the network
     * link.
     */
    private void receiveInterrupt() {
        messageReceived.V();
    }

    /**
     * Send a message to a mailbox on a remote machine.
     */
    public void send(MailMessage mail) {
        if (Lib.test(dbgNet))
            System.out.println("sending mail: " + mail);

        sendLock.acquire();

        Machine.networkLink().send(mail.packet);
        messageSent.P();

        sendLock.release();
    }

    /**
     * Called when a packet has been sent and another can be queued to the
     * network link. Note that this is called even if the previous packet was
     * dropped.
     */
    private void sendInterrupt() {
        messageSent.V();
    }

    private SynchList[] queues;
    private Semaphore messageReceived;	// V'd when a message can be dequeued
    private Semaphore messageSent;	// V'd when a message can be queued
    private Lock sendLock;

    private static final char dbgNet = 'n';
}