TPL data block never terminates in PropagateCompletion

Question

Since the last change to my common-end pipeline, one of my buffer blocks has never completed. Let me summarize what works and what doesn't:

Previously worked:

A.LinkTo(B, PropagateCompletion);
B.LinkTo(C, PropagateCompletion);
C.LinkTo(D, PropagateCompletion);
D.Receive();

// everything completes

Does not work any more:

A.LinkTo(B, PropagateCompletion);
C.LinkTo(D, PropagateCompletion);

await A.Completion;
someWriteOnceBlock.Post(B.Count);
// B.Complete(); commented on purpose
B.LinkTo(C, PropagateCompletion);

D.Receive();

// Only A reaches completion
// B remains in 'waiting for activation'
// C executes but obviously never completes since B doesn't either

If I uncomment the commented line, everything works, but obviously the line is not needed.

Somehow, my BufferBlock B does not reach completion, although the block associated with it ends and its completion is distributed, and the block associated with it receives all buffered elements.

Answer 1

A, A. , , - , , . , B A , A , . .

Answer 2

- TransformBlock, , .

, , , , block.Complete, , .

, ActionBlock, . , :

Dim maxDop = Environment.ProcessorCount
Dim executionOptions = New ExecutionDataflowBlockOptions With {.MaxDegreeOfParallelism = maxDop}
Dim linkOptions = New DataflowLinkOptions With {.PropagateCompletion = True}

Dim inputBlock = New BufferBlock(Of String)
Dim transformBlock = New TransformBlock(Of String, IEnumerable(of JsonObject))(Function(fileName) DeserialiseFromFileAsync(fileName)
Dim outputBlock As New BufferBlock(Of IEnumerable(of JsonObject))
Dim combineBlock = New ActionBlock(Of IEnumerable(of JsonObject))(Sub(col) ' Do something to add all the collections together)

inputBlock.LinkTo(transformBlock, linkOptions)
transformBlock.LinkTo(outputBlock, linkOptions)
outputBlock.LinkTo(combineBlock, linkOptions)

For fileNo = 1 To 10
    inputBlock.Post(String.Concat("JsonFile", fileNo, ".txt"))
Next

inputBlock.Complete() 'Complete the first block, propogation will handle the rest
Await combineBlock.Completion 'Await the last block completing