c# 为 multipartReader.ReadNextSectionAsync() 设置超时

Question

我正在使用 C# .net 核心从发送多个文件的多部分发布用户读取上传数据。在

中读取最后一个文件后，如何防止使用无限等待

try
{
  var cancellationTokenSource = new CancellationTokenSource();
  cancellationTokenSource.CancelAfter(1000);
  section = await multipartReader.ReadNextSectionAsync(cancellationTokenSource.Token);
}
catch (Exception ex)
{
  throw;
}

虽然我已经设置了 1 秒的 cancelationToken，但它会变得无限，并且不会去下一行，如果我将发送另一个请求。

public static async Task<HttpRequest> FromHttpContextAsync(HttpContext httpContext)
        {
            bool multipart = false;
            HttpRequest retVal = new HttpRequest(httpContext);
            var sb = new StringBuilder();         
            var sr = new StreamReader(httpContext.Stream, Encoding.UTF8);
            {
                var line1 = await sr.ReadLineAsync();
                sb.AppendLine(line1);
                var Line1Parts = (line1).Split(' ');
                retVal.Methode = Line1Parts[0].ToLower();
                retVal.RawUrl = System.Net.WebUtility.UrlDecode(Line1Parts[1]).Replace("&amp;", "&");
                var urlPart = retVal.RawUrl.Split('?');
                retVal.Url = urlPart[0];
                if (urlPart.Length > 1)
                {
                    foreach (var part in urlPart[1].Split(new[] { '&' }, StringSplitOptions.RemoveEmptyEntries))
                    {
                        var tmp = part.Split(new[] { '=' }, StringSplitOptions.RemoveEmptyEntries);
                        try
                        {
                            retVal.QueryStrings.Add(tmp[0], tmp[1]);
                        }
                        catch (Exception ex)
                        {
                        }
                    }
                }
            string line = await sr.ReadLineAsync();
            sb.AppendLine(line);
            int contentLength = 0;
            while (!string.IsNullOrEmpty(line))
            {
                var tmp = line.Split(':');
                var key = tmp[0].Trim().ToLower();
                retVal.Header.Add(new KeyValuePair<string, string>(tmp[0], tmp[1]));

                switch (key)
                {

                    case "cookie":
                        {

                            foreach (var part in tmp[1].Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries))
                            {
                                var pares = part.Split('=');
                                if (pares.Length == 2)
                                {
                                    retVal.Cookies.Add(new KeyValuePair<string, string>(pares[0], pares[1]));
                                }
                            }
                            break;
                        }
                    case "content-length":
                        {
                            contentLength = int.Parse(tmp[1]);
                            break;
                        }
                }
                line = await sr.ReadLineAsync();
                sb.AppendLine(line);
            }
if (sb.ToString().Contains("Content-Type: multipart/form-data"))
{
  string boundary = FindBoundary(sb.ToString());
  MultipartReader multipartReader = new MultipartReader(boundary, httpContext.Stream);
  var section = await multipartReader.ReadNextSectionAsync();
  while (section != null)
  {
    // process each image
    const int chunkSize = 1024;
    var buffer = new byte[chunkSize];
    var bytesRead = 0;
    var fileName = GetFileName(section.ContentDisposition);

    using (var stream = new MemoryStream())
    {
      do
      {
        try
        {
          bytesRead = await section.Body.ReadAsync(buffer, 0, buffer.Length);
        }
        catch (Exception ex)
        {
          Console.Write(ex);
        }

        stream.Write(buffer, 0, bytesRead);
      } while (bytesRead > 0);
      retVal.Files.Add(new Tuple<string, string, byte[]>("", fileName, stream.ToArray()));
    }

    try
    {
      var cancellationTokenSource = new CancellationTokenSource();
      cancellationTokenSource.CancelAfter(1000);
      section = await multipartReader.ReadNextSectionAsync(cancellationTokenSource.Token);
    }
    catch (Exception ex)
    {
      throw;
    }


  }
  foreach (var file in retVal.Files)
  {
    File.WriteAllBytes("d:\\" + file.Item2, file.Item3);
  }
}

HttpContext 是这个项目的内联类，这是 HttpContext 的来源:

public class HttpContext
    {
        public HttpRequest Request { get; private set; }
        public HttpResponse Response { get; private set; }
        public Stream Stream { private set; get; }
        private HttpContext(Stream networkStream)
        {
            Stream = networkStream;
        }

        public async static Task<HttpContext> FromHttpContextAsync(Stream networkStream)
        {
            var retVal = new HttpContext(networkStream);
            retVal.Request = await HttpRequest.FromHttpContextAsync(retVal);
            retVal.Response = HttpResponse.FromHttpContext(retVal);
            return retVal;
        }
    }

Answer 1

虽然缺乏详细信息和上下文使得重现此问题变得非常困难，但我怀疑这里的问题是由于 NetworkStreams(在幕后由您的 MultipartReader 实例使用)尚未完全支持 CancellationTokens。事实上，.NET Core 上几乎所有与 Socket 相关的操作都只是预先检查最终传递的 CancellationToken - 在我看来这是无用的。

好消息是 .NET Core 团队正在积极解决这个问题，我相信这个问题将在 .NET Core 3.0 中得到彻底解决:

https://github.com/dotnet/corefx/issues/24430

作为一个临时的丑陋解决方法，您可以更改您的代码以等待伪造的延迟任务和您对 ReadNextSectionAsync() 的调用，假设您不想在此之后重新使用该停滞的套接字/NetworkStream:

var timeoutTask = Task.Delay(TimeSpan.FromSeconds(1)).ConfigureAwait(false);
var readNextSectionTask = multipartReader.ReadNextSectionAsync().ConfigureAwait(false);

if (await Task.WhenAny(timeoutTask, readNextSectionTask).ConfigureAwait(false) == timeoutTask)
{
    // TODO: Handle the timeout
}
else
{
    section = await readNextSectionTask;
}

此外，您没有配置可等待对象这一事实可能会成为死锁源(我不清楚您是在 ASP.NET Core 本身还是在其他同步上下文提供程序上运行此代码)。为了排除这种可能性，我建议您在调用 await 之后立即调用 ConfigureAwait(false)，如您在我之前的代码块中所见。