/**********************************************************************
 * Copyright (c) 2003, 2004 IBM Corporation and others. All rights reserved.   This
 * program and the accompanying materials are made available under the terms of
 * the Common Public License v1.0 which accompanies this distribution, and is
 * available at http://www.eclipse.org/legal/cpl-v10.html
 * 
 * Contributors: 
 * IBM - Initial API and implementation
 **********************************************************************/
package org.eclipse.core.internal.jobs;

import org.eclipse.core.internal.runtime.Assert;
import org.eclipse.core.runtime.jobs.ILock;
import org.eclipse.core.runtime.jobs.ISchedulingRule;

/**
 * A lock used to control write access to an exclusive resource.
 * 
 * The lock avoids circular waiting deadlocks by detecting the deadlocks
 * and resolving them through the suspension of all locks owned by one 
 * of the threads involved in the deadlock. This makes it impossible for n such 
 * locks to deadlock while waiting for each other.  The downside is that this means
 * that during an interval when a process owns a lock, it can be forced
 * to give the lock up and wait until all locks it requires become
 * available.  This removes the feature of exclusive access to the
 * resource in contention for the duration between acquire() and
 * release() calls.
 * 
 * The lock implementation prevents starvation by granting the
 * lock in the same order in which acquire() requests arrive. In
 * this scheme, starvation is only possible if a thread retains
 * a lock indefinitely.
 */
public class OrderedLock implements ILock, ISchedulingRule {

	private static final boolean DEBUG = false;
	/**
	 * Locks are sequentially ordered for debugging purposes.
	 */
	private static int nextLockNumber = 0;
	/**
	 * The thread of the operation that currently owns the lock.
	 */
	private volatile Thread currentOperationThread;
	/**
	 * Records the number of successive acquires in the same
	 * thread. The lock is released only when the depth
	 * reaches zero.
	 */
	private int depth;
	/**
	 * The manager that implements the deadlock detection and resolution protocol.
	 */
	private final LockManager manager;
	private final int number;
	/**
	 * Queue of semaphores for threads currently waiting
	 * on the lock.
	 */
	private final Queue operations = new Queue();

	/**
	 * Creates a new workspace lock.
	 */
	OrderedLock(LockManager manager) {
		this.manager = manager;
		this.number = nextLockNumber++;
	}

	/* (non-Javadoc)
	 * @see Locks.ILock#acquire()
	 */
	public void acquire() {
		//spin until the lock is successfully acquired
		//NOTE: spinning here allows the UI thread to service pending syncExecs
		//if the UI thread is waiting to acquire a lock.
		while (true) {
			try {
				if (acquire(Long.MAX_VALUE))
					return;
			} catch (InterruptedException e) {
				//ignore and loop
			}
		}
	}

	/* (non-Javadoc)
	 * @see Locks.ILock#acquire(long)
	 */
	public boolean acquire(long delay) throws InterruptedException {
		if (Thread.interrupted())
			throw new InterruptedException();

		boolean success = false;
		if (delay <= 0)
			return attempt();
		Semaphore semaphore = createSemaphore();
		if (semaphore == null)
			return true;
		if (DEBUG)
			System.out.println("[" + Thread.currentThread() + "] Operation waiting to be executed... " + this); //$NON-NLS-1$ //$NON-NLS-2$
		success = doAcquire(semaphore, delay);
		manager.resumeSuspendedLocks(Thread.currentThread());
		if (DEBUG && success)
			System.out.println("[" + Thread.currentThread() + "] Operation started... " + this); //$NON-NLS-1$ //$NON-NLS-2$
		else if (DEBUG)
			System.out.println("[" + Thread.currentThread() + "] Operation timed out... " + this); //$NON-NLS-1$ //$NON-NLS-2$	
		return success;
	}

	/**
	 * Attempts to acquire the lock.  Returns false if the lock is not available and
	 * true if the lock has been successfully acquired.
	 */
	private synchronized boolean attempt() {
		//return true if we already own the lock
		//also, if nobody is waiting, grant the lock immediately
		if ((currentOperationThread == Thread.currentThread()) || (currentOperationThread == null && operations.isEmpty())) {
			depth++;
			setCurrentOperationThread(Thread.currentThread());
			return true;
		}
		return false;
	}

	/* (non-Javadoc)
	 * @see org.eclipse.core.runtime.jobs.ISchedulingRule#contains(org.eclipse.core.runtime.jobs.ISchedulingRule)
	 */
	public boolean contains(ISchedulingRule rule) {
		return false;
	}

	/**
	 * Returns null if acquired and a Semaphore object otherwise. If a
	 * waiting semaphore already exists for this thread, it will be returned, 
	 * otherwise a new semaphore will be created, enqueued, and returned.
	 */
	private synchronized Semaphore createSemaphore() {
		return attempt() ? null : enqueue(new Semaphore(Thread.currentThread()));
	}

	/**
	 * Attempts to acquire this lock.  Callers will block  until this lock comes available to 
	 * them, or until the specified delay has elapsed.
	 */
	private boolean doAcquire(Semaphore semaphore, long delay) throws InterruptedException {
		boolean success = false;
		//notify hook to service pending syncExecs before falling asleep
		if (manager.aboutToWait(this.currentOperationThread)) {
			//hook granted immediate access
			//remove semaphore for the lock request from the queue
			//do not log in graph because this thread did not really get the lock
			operations.remove(semaphore);
			depth++;
			manager.addLockThread(currentOperationThread, this);
			return true;
		}
		//Make sure the semaphore is in the queue before we start waiting
		//It might have been removed from the queue while servicing syncExecs
		//This is will return our existing semaphore if it is still in the queue
		semaphore = createSemaphore();
		if (semaphore == null)
			return true;
		manager.addLockWaitThread(Thread.currentThread(), this);
		try {
			success = semaphore.acquire(delay);
		} catch (InterruptedException e) {
			if (DEBUG)
				System.out.println("[" + Thread.currentThread() + "] Operation interrupted while waiting... :-|"); //$NON-NLS-1$ //$NON-NLS-2$
			throw e;
		}
		if (success) {
			depth++;
			updateCurrentOperation();
		} else {
			//operation timed out
			//remove request semaphore from queue and update graph
			operations.remove(semaphore);
			manager.removeLockWaitThread(Thread.currentThread(), this);
		}
		return success;
	}

	/**
	 * Releases this lock from the thread that used to own it.
	 * Grants this lock to the next thread in the queue.  
	 */
	private synchronized void doRelease() {
		//notify hook
		manager.aboutToRelease();
		depth = 0;
		Semaphore next = (Semaphore) operations.peek();
		setCurrentOperationThread(null);
		if (next != null)
			next.release();
	}

	/**
	 * If there is another semaphore with the same runnable in the
	 * queue, the other is returned and the new one is not added.
	 */
	private synchronized Semaphore enqueue(Semaphore newSemaphore) {
		Semaphore semaphore = (Semaphore) operations.get(newSemaphore);
		if (semaphore == null) {
			operations.enqueue(newSemaphore);
			return newSemaphore;
		}
		return semaphore;
	}

	/**
	 * Suspend this lock by granting the lock to the next lock in the queue.
	 * Return the depth of the suspended lock. 
	 */
	protected int forceRelease() {
		int oldDepth = depth;
		doRelease();
		return oldDepth;
	}

	/* (non-Javadoc)
	 * @see Locks.ILock#getDepth()
	 */
	public int getDepth() {
		return depth;
	}

	/* (non-Javadoc)
	 * @see org.eclipse.core.runtime.jobs.ISchedulingRule#isConflicting(org.eclipse.core.runtime.jobs.ISchedulingRule)
	 */
	public boolean isConflicting(ISchedulingRule rule) {
		return rule == this;
	}

	/* (non-Javadoc)
	 * @see Locks.ILock#release()
	 */
	public void release() {
		if (depth == 0)
			return;
		//only release the lock when the depth reaches zero
		Assert.isTrue(depth >= 0, "Lock released too many times"); //$NON-NLS-1$
		if (--depth == 0)
			doRelease();
		else
			manager.removeLockThread(currentOperationThread, this);
	}

	/**
	 * If newThread is null, release this lock from its previous owner.
	 * If newThread is not null, grant this lock to newThread.
	 */
	private void setCurrentOperationThread(Thread newThread) {
		if ((currentOperationThread != null) && (newThread == null))
			manager.removeLockThread(currentOperationThread, this);
		this.currentOperationThread = newThread;
		if (currentOperationThread != null)
			manager.addLockThread(currentOperationThread, this);
	}

	/**
	 * Forces the lock to be at the given depth.
	 * Used when re-acquiring a suspended lock.
	 */
	protected void setDepth(int newDepth) {
		for (int i = depth; i < newDepth; i++) {
			manager.addLockThread(currentOperationThread, this);
		}
		this.depth = newDepth;
	}

	/**
	 * For debugging purposes only.
	 */
	public String toString() {
		return "OrderedLock (" + number + ")"; //$NON-NLS-1$ //$NON-NLS-2$
	}

	/**
	 * This lock has just been granted to a new thread (the thread waited for it).
	 * Remove the request from the queue and update both the graph and the lock.
	 */
	private synchronized void updateCurrentOperation() {
		operations.dequeue();
		setCurrentOperationThread(Thread.currentThread());
	}
}