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using System;
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using System.Threading;
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using System.Diagnostics;
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namespace Implab.Parallels {
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/// <summary>
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/// Implements a lightweight mechanism to aquire a shared or an exclusive lock.
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/// </summary>
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public class SharedLock {
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readonly object m_lock = new object();
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// the count of locks currently acquired by clients
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int m_locks;
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// the count of pending requests for upgrade
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int m_upgrades;
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bool m_exclusive;
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public bool LockExclusive(int timeout) {
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lock (m_lock) {
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var dt = timeout;
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if (m_locks > m_upgrades) {
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var t1 = Environment.TickCount;
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do {
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if (!Monitor.Wait(m_lock, timeout))
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return false;
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if (m_locks == m_upgrades)
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break;
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if (timeout > 0) {
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dt = timeout - Environment.TickCount + t1;
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if (dt < 0)
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return false;
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}
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} while(true);
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}
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m_exclusive = true;
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m_locks ++;
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return true;
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}
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}
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public void LockExclusive() {
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lock (m_lock) {
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while (m_locks > m_upgrades)
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Monitor.Wait(m_lock);
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m_exclusive = true;
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m_locks ++;
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}
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}
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/// <summary>
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/// Acquires a shared lock.
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/// </summary>
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/// <returns><c>true</c>, if the shared lock was acquired, <c>false</c> if the specified timeout was expired.</returns>
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/// <param name="timeout">Timeout.</param>
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public bool LockShared(int timeout) {
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lock (m_lock) {
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if (!m_exclusive) {
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m_locks++;
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return true;
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}
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if (m_locks == m_upgrades) {
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m_exclusive = false;
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m_locks = 1;
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return true;
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}
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var t1 = Environment.TickCount;
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var dt = timeout;
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do {
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if (!Monitor.Wait(m_lock, dt))
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return false;
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if (m_locks == m_upgrades || !m_exclusive)
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break;
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if (timeout >= 0) {
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dt = timeout - Environment.TickCount + t1;
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if (dt < 0)
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return false;
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}
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} while(true);
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m_locks ++;
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m_exclusive = false;
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return true;
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}
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}
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/// <summary>
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/// Acquires the shared lock.
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/// </summary>
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public void LockShared() {
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lock (m_lock) {
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if (!m_exclusive) {
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m_locks++;
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} else if (m_locks == m_upgrades) {
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m_exclusive = false;
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m_locks++;
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} else {
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while (m_exclusive && m_locks > m_upgrades)
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Monitor.Wait(m_lock);
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m_locks++;
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m_exclusive = false;
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}
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}
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}
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/// <summary>
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/// Upgrades the current lock to exclusive level.
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/// </summary>
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/// <remarks>If the current lock is exclusive already the method does nothing.</remarks>
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public void Upgrade() {
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lock (m_lock) {
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if (!m_exclusive) {
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if (m_locks <= m_upgrades)
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throw new InvalidOperationException();
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if (m_locks - m_upgrades == 1) {
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m_exclusive = true;
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} else {
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m_upgrades++;
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while (m_locks > m_upgrades)
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Monitor.Wait(m_lock);
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m_upgrades--;
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m_exclusive = true;
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}
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}
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}
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}
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/// <summary>
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/// Upgrades the current lock to exclusive level.
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/// </summary>
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/// <param name="timeout">Timeout.</param>
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/// <returns><c>true</c> if the current lock was updated, <c>false</c> the specified timeout was expired.</returns>
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/// <remarks>If the current lock is exclusive already the method does nothing.</remarks>
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public bool Upgrade(int timeout) {
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lock (m_lock) {
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if (m_exclusive)
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return true;
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if (m_locks <= m_upgrades)
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throw new InvalidOperationException();
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if (m_locks - m_upgrades == 1) {
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m_exclusive = true;
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} else {
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var t1 = Environment.TickCount;
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var dt = timeout;
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m_upgrades++;
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do {
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if (!Monitor.Wait(m_lock, dt)) {
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m_upgrades--;
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return false;
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}
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// we may get there but the shared lock already aquired
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if (m_locks == m_upgrades)
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break;
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if (timeout >= 0) {
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dt = timeout - Environment.TickCount + t1;
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if (dt < 0) {
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m_upgrades--;
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return false;
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}
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}
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} while(true);
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m_upgrades--;
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m_exclusive = true;
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}
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return true;
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}
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}
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/// <summary>
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/// Downgrades this lock to shared level.
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/// </summary>
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public void Downgrade() {
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lock (m_lock)
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m_exclusive = false;
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}
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/// <summary>
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/// Releases the current lock.
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/// </summary>
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public void Release() {
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lock (m_lock)
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// if no more running threads left
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if (--m_locks == m_upgrades)
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Monitor.PulseAll(m_lock);
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}
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}
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}
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