private ValueTask <T> ReadAsync(CancellationToken cancellationToken = default(CancellationToken))
{
// Fast-path cancellation check
if (cancellationToken.IsCancellationRequested)
{
return(new ValueTask <T>(Task.FromCanceled <T>(cancellationToken)));
}
lock (SyncObj)
{
AssertInvariants();
// If there are any items, hand one back.
if (!_items.IsEmpty)
{
return(new ValueTask <T>(DequeueItemAndPostProcess()));
}
// There weren't any items. If we're done writing so that there
// will never be more items, fail.
if (_doneWriting != null)
{
return(ChannelUtilities.GetErrorValueTask <T>(_doneWriting));
}
// Otherwise, queue the reader.
var reader = ReaderInteractor <T> .Create(_runContinuationsAsynchronously, cancellationToken);
_blockedReaders.EnqueueTail(reader);
return(new ValueTask <T>(reader.Task));
}
}
/// <summary>Dequeues an item, and then fixes up our state around writers and completion.</summary>
/// <returns>The dequeued item.</returns>
private T DequeueItemAndPostProcess()
{
Debug.Assert(Monitor.IsEntered(SyncObj));
// Dequeue an item.
T item = _items.DequeueHead();
// If we're now empty and we're done writing, complete the channel.
if (_doneWriting != null && _items.IsEmpty)
{
ChannelUtilities.Complete(_completion, _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 (!_blockedWriters.IsEmpty)
{
WriterInteractor <T> w = _blockedWriters.DequeueHead();
if (w.Success(default(VoidResult)))
{
_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 _waitingWriters, result: true);
// Return the item
return(item);
}
private ValueTask <T> ReadAsyncCore(CancellationToken cancellationToken)
{
if (cancellationToken.IsCancellationRequested)
{
return(new ValueTask <T>(Task.FromCanceled <T>(cancellationToken)));
}
lock (SyncObj)
{
AssertInvariants();
// If there are any items, return one.
T item;
if (_items.TryDequeue(out item))
{
// Dequeue an item
if (_doneWriting != null && _items.IsEmpty)
{
// If we've now emptied the items queue and we're not getting any more, complete.
ChannelUtilities.Complete(_completion, _doneWriting);
}
return(new ValueTask <T>(item));
}
// There are no items, so if we're done writing, fail.
if (_doneWriting != null)
{
return(ChannelUtilities.GetInvalidCompletionValueTask <T>(_doneWriting));
}
// Otherwise, queue the reader.
ReaderInteractor <T> reader = ReaderInteractor <T> .Create(_runContinuationsAsynchronously, cancellationToken);
_blockedReaders.EnqueueTail(reader);
return(new ValueTask <T>(reader.Task));
}
}
private Task WriteAsync(T item, CancellationToken cancellationToken = default(CancellationToken))
{
if (cancellationToken.IsCancellationRequested)
{
return(Task.FromCanceled(cancellationToken));
}
ReaderInteractor <T> blockedReader = null;
ReaderInteractor <bool> waitingReaders = null;
lock (SyncObj)
{
AssertInvariants();
// If we're done writing, trying to write is an error.
if (_doneWriting != null)
{
return(Task.FromException(ChannelUtilities.CreateInvalidCompletionException()));
}
// Get the number of items in the channel currently.
int count = _items.Count;
if (count == 0)
{
// There are no items in the channel, which means we may have blocked/waiting readers.
// If there are any blocked readers, find one that's not canceled
// and store it to complete outside of the lock, in case it has
// continuations that'll run synchronously
while (!_blockedReaders.IsEmpty)
{
ReaderInteractor <T> r = _blockedReaders.DequeueHead();
r.UnregisterCancellation(); // ensure that once we grab it, we own its completion
if (!r.Task.IsCompleted)
{
blockedReader = r;
break;
}
}
if (blockedReader == null)
{
// If there wasn't a blocked reader, then store the item. If no one's waiting
// to be notified about a 0-to-1 transition, we're done.
_items.EnqueueTail(item);
waitingReaders = _waitingReaders;
if (waitingReaders == null)
{
return(ChannelUtilities.TrueTask);
}
_waitingReaders = null;
}
}
else if (count < _bufferedCapacity)
{
// There's room in the channel. Since we're not transitioning from 0-to-1 and
// since there's room, we can simply store the item and exit without having to
// worry about blocked/waiting readers.
_items.EnqueueTail(item);
return(ChannelUtilities.TrueTask);
}
else if (_mode == BoundedChannelFullMode.Wait)
{
// The channel is full and we're in a wait mode.
// Queue the writer.
var writer = WriterInteractor <T> .Create(true, cancellationToken, item);
_blockedWriters.EnqueueTail(writer);
return(writer.Task);
}
else
{
// The channel is full, and we're in a dropping mode.
// Drop either the oldest or the newest and write the new item.
T droppedItem = _mode == BoundedChannelFullMode.DropNewest ?
_items.DequeueTail() :
_items.DequeueHead();
_items.EnqueueTail(item);
return(ChannelUtilities.TrueTask);
}
}
// We either wrote the item already, or we're transfering it to the blocked reader we grabbed.
if (blockedReader != null)
{
// Transfer the written item to the blocked reader.
bool success = blockedReader.Success(item);
Debug.Assert(success, "We should always be able to complete the reader.");
}
else
{
// We stored an item bringing the count up from 0 to 1. Alert
// any waiting readers that there may be something for them to consume.
// Since we're no longer holding the lock, it's possible we'll end up
// waking readers that have since come in.
waitingReaders.Success(item: true);
}
return(ChannelUtilities.TrueTask);
}