BufferBlock deadlock with OutputAvailableAsync after TryReceiveAll

Answer 1

Alas, it's the end of September 2015, and although i3arnon fixed the error it is not solved in the version that was released two days after the error was fixed: Microsoft TPL Dataflow version 4.5.24.

However IReceivableSourceBlock.TryReceive(...) works correctly. An extension method will solve the problem. After a new release of TPL Dataflow it will be easy to change the extension method.

/// <summary>
/// This extension method returns all available items in the IReceivableSourceBlock
/// or an empty sequence if nothing is available. The functin does not wait.
/// </summary>
/// <typeparam name="T">The type of items stored in the IReceivableSourceBlock</typeparam>
/// <param name="buffer">the source where the items should be extracted from </param>
/// <returns>The IList with the received items. Empty if no items were available</returns>
public static IList<T> TryReceiveAllEx<T>(this IReceivableSourceBlock<T> buffer)
{
    /* Microsoft TPL Dataflow version 4.5.24 contains a bug in TryReceiveAll
     * Hence this function uses TryReceive until nothing is available anymore
     * */
    IList<T> receivedItems = new List<T>();
    T receivedItem = default(T);
    while (buffer.TryReceive<T>(out receivedItem))
    {
        receivedItems.Add(receivedItem);
    }
    return receivedItems;
}

usage:

while (await this.bufferBlock.OutputAvailableAsync())
{
    // some data available
    var receivedItems = this.bufferBlock.TryReceiveAllEx();
    if (receivedItems.Any())
    {
        ProcessReceivedItems(bufferBlock);
    }
}

Answer 2

This is a bug in SourceCore being used internally by BufferBlock. Its TryReceiveAll method doesn't turn on the _enableOffering boolean data member while TryReceive does. That results in the task returned from OutputAvailableAsync never completing.

Here's a minimal reproduce:

var buffer = new BufferBlock<object>();
buffer.Post(null);

IList<object> items;
buffer.TryReceiveAll(out items);

var outputAvailableAsync = buffer.OutputAvailableAsync();
buffer.Post(null);

await outputAvailableAsync; // Never completes

I've just fixed it in the .Net core repository with this pull request. Hopefully the fix finds itself in the nuget package soon.