//
// Executes the prologue of the BlockingQueue<T>.TryTake method.
//
internal override WaitNode TryTakePrologue(StParker pk, int key, out T di, ref WaitNode ignored)
{
//
// Acquire the queue's lock and check if the queue's is empty.
//
qlock.Enter();
if (count != 0)
{
//
// The queue isn't empty; so, ...
//
if (!pk.TryLock())
{
qlock.Exit();
di = default(T);
return(null);
}
pk.UnparkSelf(key);
//
// Retrieve a data item from the queue.
//
di = items[head];
if (++head == length)
{
head = 0;
}
count--;
//
// If the wait queue isn't empty, try to use the freed
// slot to release one of the waiter threads.
//
if (!waitQueue.IsEmpty)
{
do
{
WaitNode w = waitQueue.Dequeue();
StParker pk2 = w.parker;
if (pk2.TryLock())
{
items[tail] = w.channel;
if (++tail == length)
{
tail = 0;
}
count++;
qlock.Exit();
pk2.Unpark(w.waitKey);
return(null);
}
} while (!waitQueue.IsEmpty);
}
//
// Release the queue's lock and return null to signal that the
// take operation was accomplished.
//
qlock.Exit();
return(null);
}
//
// The queue's buffer is empty; so, create ...
//
WaitNode wn = new WaitNode(pk, key);
if (lifoQueue)
{
waitQueue.EnqueueHead(wn);
}
else
{
waitQueue.Enqueue(wn);
}
//
// Release the queue's lock and return the wait node
// inserted in the wait queue.
//
qlock.Exit();
di = default(T);
return(wn);
}