ReceiveAsync中断/中断消息传递

Question

这个问题在试图实现建议的this problem解决方案时引起了人们的注意。

问题摘要

执行ReceiveAsync()从TransformBlock到WriteOnceBlock的调用会导致TransformBlock从流中从本质上删除自身。它停止传播任何类型的信息，无论是数据还是完成信号。

系统设计

该系统旨在通过一系列步骤解析大型CSV文件。

流程中有问题的部分可以(不熟练地)可视化如下：

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平行四边形是BufferBlock，菱形是BroadcastBlocks，三角形是WriteOnceBlocks，箭头是TransformBlocks。实线表示用LinkTo()创建的链接，虚线表示从ParsedHeaderAndRecordJoiner到ParsedHeaderContainer块的ReceiveAsync()调用。我知道这个流有点不太理想，但这不是问题的主要原因。

代码

应用根

下面是类的一部分，它使用PropagateCompletion创建必要的块并将它们链接到一起

using (var cancellationSource = new CancellationTokenSource())
{
    var cancellationToken = cancellationSource.Token;
    var temporaryEntityInstance = new Card(); // Just as an example

    var producerQueue = queueFactory.CreateQueue<string>(new DataflowBlockOptions{CancellationToken = cancellationToken});
    var recordDistributor = distributorFactory.CreateDistributor<string>(s => (string)s.Clone(), 
        new DataflowBlockOptions { CancellationToken = cancellationToken });
    var headerRowContainer = containerFactory.CreateContainer<string>(s => (string)s.Clone(), 
        new DataflowBlockOptions { CancellationToken = cancellationToken });
    var headerRowParser = new HeaderRowParserFactory().CreateHeaderRowParser(temporaryEntityInstance.GetType(), ';', 
        new ExecutionDataflowBlockOptions { CancellationToken = cancellationToken });
    var parsedHeaderContainer = containerFactory.CreateContainer<HeaderParsingResult>(HeaderParsingResult.Clone, 
        new DataflowBlockOptions { CancellationToken = cancellationToken});
    var parsedHeaderAndRecordJoiner = new ParsedHeaderAndRecordJoinerFactory().CreateParsedHeaderAndRecordJoiner(parsedHeaderContainer, 
        new ExecutionDataflowBlockOptions { CancellationToken = cancellationToken });
    var entityParser = new entityParserFactory().CreateEntityParser(temporaryEntityInstance.GetType(), ';',
        dataflowBlockOptions: new ExecutionDataflowBlockOptions { CancellationToken = cancellationToken });
    var entityDistributor = distributorFactory.CreateDistributor<EntityParsingResult>(EntityParsingResult.Clone, 
        new DataflowBlockOptions{CancellationToken = cancellationToken});

    var linkOptions = new DataflowLinkOptions {PropagateCompletion = true};

    // Producer subprocess
    producerQueue.LinkTo(recordDistributor, linkOptions);

    // Header subprocess
    recordDistributor.LinkTo(headerRowContainer, linkOptions);
    headerRowContainer.LinkTo(headerRowParser, linkOptions);
    headerRowParser.LinkTo(parsedHeaderContainer, linkOptions);
    parsedHeaderContainer.LinkTo(errorQueue, new DataflowLinkOptions{MaxMessages = 1, PropagateCompletion = true}, dataflowResult => !dataflowResult.WasSuccessful);

    // Parsing subprocess
    recordDistributor.LinkTo(parsedHeaderAndRecordJoiner, linkOptions);
    parsedHeaderAndRecordJoiner.LinkTo(entityParser, linkOptions, joiningResult => joiningResult.WasSuccessful);
    entityParser.LinkTo(entityDistributor, linkOptions);
    entityDistributor.LinkTo(errorQueue, linkOptions, dataflowResult => !dataflowResult.WasSuccessful);
}

HeaderRowParser

此块解析CSV文件中的标题行并进行一些验证。

public class HeaderRowParserFactory
{
    public TransformBlock<string, HeaderParsingResult> CreateHeaderRowParser(Type entityType,
        char delimiter,
        ExecutionDataflowBlockOptions dataflowBlockOptions = null)
    {
        return new TransformBlock<string, HeaderParsingResult>(headerRow =>
        {
            // Set up some containers
            var result = new HeaderParsingResult(identifier: "N/A", wasSuccessful: true);
            var fieldIndexesByPropertyName = new Dictionary<string, int>();

            // Get all serializable properties on the chosen entity type
            var serializableProperties = entityType.GetProperties()
                .Where(prop => prop.IsDefined(typeof(CsvFieldNameAttribute), false))
                .ToList();

            // Add their CSV fieldnames to the result
            var entityFieldNames = serializableProperties.Select(prop => prop.GetCustomAttribute<CsvFieldNameAttribute>().FieldName);
            result.SetEntityFieldNames(entityFieldNames);

            // Create the dictionary of properties by field name
            var serializablePropertiesByFieldName = serializableProperties.ToDictionary(prop => prop.GetCustomAttribute<CsvFieldNameAttribute>().FieldName, prop => prop, StringComparer.OrdinalIgnoreCase);

            var fields = headerRow.Split(delimiter);

            for (var i = 0; i < fields.Length; i++)
            {
                // If any field in the CSV is unknown as a serializable property, we return a failed result
                if (!serializablePropertiesByFieldName.TryGetValue(fields[i], out var foundProperty))
                {
                    result.Invalidate($"The header row contains a field that does not match any of the serializable properties - {fields[i]}.",
                        DataflowErrorSeverity.Critical);
                    return result;
                }

                // Perform a bunch more validation

                fieldIndexesByPropertyName.Add(foundProperty.Name, i);
            }

            result.SetFieldIndexesByName(fieldIndexesByPropertyName);
            return result;
        }, dataflowBlockOptions ?? new ExecutionDataflowBlockOptions());
    }
}

ParsedHeaderAndRecordJoiner

对于通过管道的每个后续记录，此块用于检索已解析的头数据并将其添加到记录中。

public class ParsedHeaderAndRecordJoinerFactory
{
    public TransformBlock<string, HeaderAndRecordJoiningResult> CreateParsedHeaderAndRecordJoiner(WriteOnceBlock<HeaderParsingResult> parsedHeaderContainer, 
        ExecutionDataflowBlockOptions dataflowBlockOptions = null)
    {
        return new TransformBlock<string, HeaderAndRecordJoiningResult>(async csvRecord =>
            {
                var headerParsingResult = await parsedHeaderContainer.ReceiveAsync();

                // If the header couldn't be parsed, a critical error is already on its way to the failure logger so we don't need to continue
                if (!headerParsingResult.WasSuccessful) return new HeaderAndRecordJoiningResult(identifier: "N.A.", wasSuccessful: false, null, null);

                // The entity parser can't do anything with the header record, so we send a message with wasSuccessful false
                var isHeaderRecord = true;
                foreach (var entityFieldName in headerParsingResult.EntityFieldNames)
                {
                    isHeaderRecord &= csvRecord.Contains(entityFieldName);
                }
                if (isHeaderRecord) return new HeaderAndRecordJoiningResult(identifier: "N.A.", wasSuccessful: false, null, null);

                return new HeaderAndRecordJoiningResult(identifier: "N.A.", wasSuccessful: true, headerParsingResult, csvRecord);
            }, dataflowBlockOptions ?? new ExecutionDataflowBlockOptions());
    }
}

问题细节

对于当前的实现，ParsedHeaderAndRecordJoiner从对ParsedHeaderContainer的ReceiveAsync()调用中正确地接收数据并按预期返回，但是没有消息到达EntityParser。

另外，当一个完整的信号被发送到流的前端( ProducerQueue)时，它会传播到RecordDistributor，然后停止在ParsedHeaderAndRecordJoiner (它从HeaderRowContainer开始继续，所以RecordDistributor正在传递它)。

如果我自己移除ReceiveAsync()调用并模拟数据，则该块的行为与预期相同。

Answer 1

我认为这部分是关键

但是，没有消息到达EntityParser。

根据示例，EntityParser不接收ParsedHeaderAndRecordJoiner输出的消息的唯一方法是当WasSuccessful返回false时。链接中使用的谓词排除了失败的消息，但这些消息没有去处，因此它们会在ParsedHeaderAndRecordJoiner输出缓冲区中生成，并且还会阻止Completion传播。您需要链接一个空目标来转储失败的消息。

parsedHeaderAndRecordJoiner.LinkTo(DataflowBlock.NullTarget<HeaderParsingResult>());

此外，如果模拟数据总是带着WasSuccessful true返回，那么这可能会指向await ...ReceiveAsync()

不一定是冒烟的枪，但是个很好的起点。当管道卡住时，您能确认ParsedHeaderAndRecordJoiner输出缓冲区中所有消息的状态吗？