// Unlock a resource: find the entry and call unregister lock
private void Unlock(TP.Transaction context, TP.Lockable resource, LockMode mode)
{
ResourceEntry lockTarget;
// Get exclusive access to the lock table
lock (this.ResourceTable)
{
this.ResourceTable.TryGetValue(resource, out lockTarget);
// Check if the resource wasn't locked, and if so, then return
if (lockTarget == null)
{
return;
}
}
// Get exclusive access to the resource
lock (lockTarget)
{
lockTarget.Unregister(context, mode);
}
}
// A shortcut to unlock a write lock
public void UnlockWrite(TP.Transaction context, TP.Lockable resource)
{
Unlock(context, resource, LockMode.Write);
}
// A shortcut to unlock a read lock
public void UnlockRead(TP.Transaction context, TP.Lockable resource)
{
Unlock(context, resource, LockMode.Read);
}
// Get a write lock for the resource
public void LockForWrite(TP.Transaction context, TP.Lockable resource)
{
Lock(context, resource, MyLM.LockMode.Write);
}
// Get a read lock for the resource
public void LockForRead(TP.Transaction context, TP.Lockable resource)
{
Lock(context, resource, MyLM.LockMode.Read);
}
/* Lock passed in resource _resource_ in mode _mode_
* This method needs additional code to implement lock conversion.
* It does deadlock detection by timeout */
private void Lock(TP.Transaction context, TP.Lockable resource, LockMode mode)
{
ResourceEntry lockTarget;
/* Get exclusive access to the lock table
* This avoids race conditions, such as two conflicting locks being granted to concurrent threads (i.e., transactions),
* or two physical resources created for one logical resource on behalf of two threads. */
lock (this.ResourceTable)
{
// Pick the needed resource from ResourceTable
this.ResourceTable.TryGetValue(resource, out lockTarget);
// Create a ResourceEntry for resource, if there is none
if (lockTarget == null)
{
lockTarget = new ResourceEntry();
this.ResourceTable[resource] = lockTarget;
}
}
for (int c = 0; ; c++)
{
/* If someone else holds a lock
* (the loop already executed once and failed to set the lock)
* wait for 5 seconds for the lock to be released and
* if it doesn't happen, timeout for deadlock,
* else try again to set the lock */
if (c > 0)
{
if (!lockTarget.UnlockEvent.WaitOne(System.TimeSpan.FromMilliseconds((double)deadlockTimeout), false))
{
throw new DeadLockDetected(string.Format("Resource {0} timed out", resource));
}
}
if (c > 0)
{
System.Console.WriteLine(string.Format("Attempt {0} in resource {1}", c, resource));
}
// Get exclusive access to the resource
lock (lockTarget)
{
// Set the lock, if you can
if (lockTarget.Compatible(mode))
{
lockTarget.Register(context, mode);
return;
}
// If the request is read, see if the transaction alreday has a write lock on the resource
else if (mode == LockMode.Read && lockTarget.DownGradedLockRequest(context, mode))
{
// ‘context’ has a write lock on lockTarget and requested a read lock so no action is required. //
return;
}
// If the request is write and the transaction already has a read lock on the resource, try
// to upgrade the read lock to a write lock if no other transactions has a lock on this resource
else if (mode == LockMode.Write && lockTarget.UpgradeLockRequest(context, mode))
{
return;
}
}
}
// Debug
throw new System.Exception("Internal Error");
}