public override bool TryWrite(T item)
{
UnboundedChannel <T> parent = _parent;
while (true)
{
AsyncOperation <T> blockedReader = null;
AsyncOperation <bool> waitingReadersTail = null;
lock (parent.SyncObj)
{
// If writing has already been marked as done, fail the write.
parent.AssertInvariants();
if (parent._doneWriting != null)
{
return(false);
}
// If there aren't any blocked readers, just add the data to the queue,
// and let any waiting readers know that they should try to read it.
// We can only complete such waiters here under the lock if they run
// continuations asynchronously (otherwise the synchronous continuations
// could be invoked under the lock). If we don't complete them here, we
// need to do so outside of the lock.
if (parent._blockedReaders.IsEmpty)
{
parent._items.Enqueue(item);
waitingReadersTail = parent._waitingReadersTail;
if (waitingReadersTail == null)
{
return(true);
}
parent._waitingReadersTail = null;
}
else
{
// There were blocked readers. Grab one, and then complete it outside of the lock.
blockedReader = parent._blockedReaders.DequeueHead();
}
}
if (blockedReader != null)
{
// Complete the reader. It's possible the reader was canceled, in which
// case we loop around to try everything again.
if (blockedReader.TrySetResult(item))
{
return(true);
}
}
else
{
// Wake up all of the waiters. Since we've released the lock, it's possible
// we could cause some spurious wake-ups here, if we tell a waiter there's
// something available but all data has already been removed. It's a benign
// race condition, though, as consumers already need to account for such things.
ChannelUtilities.WakeUpWaiters(ref waitingReadersTail, result: true);
return(true);
}
}
}
public override bool TryComplete(Exception error)
{
UnboundedChannel <T> parent = _parent;
bool completeTask;
lock (parent.SyncObj)
{
parent.AssertInvariants();
// If we've already marked the channel as completed, bail.
if (parent._doneWriting != null)
{
return(false);
}
// Mark that we're done writing.
parent._doneWriting = error ?? ChannelUtilities.s_doneWritingSentinel;
completeTask = parent._items.IsEmpty;
}
// If there are no items in the queue, complete the channel's task,
// as no more data can possibly arrive at this point. We do this outside
// of the lock in case we'll be running synchronous completions, and we
// do it before completing blocked/waiting readers, so that when they
// wake up they'll see the task as being completed.
if (completeTask)
{
ChannelUtilities.Complete(parent._completion, error);
}
// At this point, _blockedReaders and _waitingReaders will not be mutated:
// they're only mutated by readers while holding the lock, and only if _doneWriting is null.
// freely manipulate _blockedReaders and _waitingReaders without any concurrency concerns.
ChannelUtilities.FailOperations <AsyncOperation <T>, T>(parent._blockedReaders, ChannelUtilities.CreateInvalidCompletionException(error));
ChannelUtilities.WakeUpWaiters(ref parent._waitingReadersTail, result: false, error: error);
// Successfully transitioned to completed.
return(true);
}
/// <summary>Dequeues an item, and then fixes up our state around writers and completion.</summary>
/// <returns>The dequeued item.</returns>
private T DequeueItemAndPostProcess()
{
BoundedChannel <T> parent = _parent;
Debug.Assert(Monitor.IsEntered(parent.SyncObj));
// Dequeue an item.
T item = parent._items.DequeueHead();
// If we're now empty and we're done writing, complete the channel.
if (parent._doneWriting != null && parent._items.IsEmpty)
{
ChannelUtilities.Complete(parent._completion, parent._doneWriting);
}
// If there are any writers blocked, there's now room for at least one
// to be promoted to have its item moved into the items queue. We need
// to loop while trying to complete the writer in order to find one that
// hasn't yet been canceled (canceled writers transition to canceled but
// remain in the physical queue).
while (!parent._blockedWriters.IsEmpty)
{
WriterInteractor <T> w = parent._blockedWriters.DequeueHead();
if (w.Success(default(VoidResult)))
{
parent._items.EnqueueTail(w.Item);
return(item);
}
}
// There was no blocked writer, so see if there's a WaitToWriteAsync
// we should wake up.
ChannelUtilities.WakeUpWaiters(ref parent._waitingWriters, result: true);
// Return the item
return(item);
}
