将"多部分/表单数据"上传到 ASP.NET Web API 操作，在无缓冲模式下不可读

Question

我正在构建一个接受"多部分/form-data"请求的ASP.NET Web API端点。我使用.NETFramework4.5和Webneneneba API 2.1实现了本文中描述的它。我创建的操作方法的简化版本如下：

public async Task<HttpResponseMessage> PostFile()
{
    if (!Request.Content.IsMimeMultipartContent()) throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
    var rootPath = System.Configuration.ConfigurationManager.AppSettings["StorageLocation"].ToString();
    var provider = new MultipartFormDataStreamProvider(rootPath);
    var response = Request.CreateResponse(HttpStatusCode.OK);
    try
    {
        await Request.Content.ReadAsMultipartAsync(provider);
        // Imagine awesome logic here, unicorns and rainbows! Instead of that, we do the following:
        response.Content = new StringContent("You uploaded " + provider.FileData.Count.ToString() + " files.");
    }
    catch (Exception e) { throw new HttpResponseException(Request.CreateErrorResponse(HttpStatusCode.InternalServerError, e)); }
    return response;
}

因为上传的文件可能很大（高达2GiB），我希望我的请求不被ASP.NET缓冲，从而避免高内存使用率。为了实现这一点，我告诉Web API对传入请求进行流式处理，而不是缓冲它们，如本文所述。自定义WebHostBufferPolicySelector看起来像这样：

public class CustomWebHostBufferPolicySelector : WebHostBufferPolicySelector
{
    public override bool UseBufferedInputStream(object hostContext)
    {
        System.Web.HttpContextBase contextBase = hostContext as System.Web.HttpContextBase;
        if (contextBase != null && contextBase.Request.ContentType != null && contextBase.Request.ContentType.Contains("multipart")) return false;
        else return base.UseBufferedInputStream(hostContext);
    }
    public override bool UseBufferedOutputStream(System.Net.Http.HttpResponseMessage response)
    {
        return base.UseBufferedOutputStream(response);
    }
}

我在应用程序启动时将这个家伙加载到Global.asax中，如下所示：

protected void Application_Start(object sender, EventArgs e)
{
    // Here, other stuff got did.
    GlobalConfiguration.Configuration.Services.Replace(typeof(IHostBufferPolicySelector), new CustomWebHostBufferPolicySelector());
}

好了，棋盘已经装好了，让我们把棋子移动起来。如果我不使用CustomWebHostBufferPolicySelector，一切都会很好。然而，当它被使用时，我得到了以下异常：

Message: "An error has occurred."
ExceptionMessage: "Error reading MIME multipart body part."
ExceptionType: "System.IO.IOException"
StackTrace: " at System.Net.Http.HttpContentMultipartExtensions.<ReadAsMultipartAsync>d__0`1.MoveNext()  --- End of stack trace from previous location where exception was thrown ---  at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)  at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)  at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()  at ..."

以下内部异常：

Message: "An error has occurred."
ExceptionMessage: "Unable to read the entity body in Bufferless mode. The request stream has already been buffered."
ExceptionType: "System.InvalidOperationException"
StackTrace: " at System.Web.Http.WebHost.HttpControllerHandler.<>c__DisplayClass13.<GetStreamContent>b__10()  at System.Web.Http.WebHost.HttpControllerHandler.LazyStreamContent.get_StreamContent()  at System.Web.Http.WebHost.HttpControllerHandler.LazyStreamContent.CreateContentReadStreamAsync()  at System.Net.Http.HttpContent.ReadAsStreamAsync()  at System.Net.Http.HttpContentMultipartExtensions.<ReadAsMultipartAsync>d__0`1.MoveNext()"

看起来请求仍然被其他东西缓冲了。在ASP.NET管道中还有其他位置吗？甚至可能是IIS？在这个请求的生命周期中，还有哪些地方可以缓冲它，我该如何控制它们？

Answer 1

为了使问题更加清晰并与他人共享，我创建了一个简单的项目来尝试重现问题。在这样做的时候，我找到了答案：禁用所有类型的跟踪。

在我的案例中，我启用了ASP.NET自己的跟踪功能，还启用了Glimpse。这两者都在请求到达Web API操作之前对其进行缓冲。

为了完整起见，这里给出了在测试和生产时在Web.Config中关闭它们的正确方法。

<configuration>
  <system.web>
    <trace enabled="false" />
  </system.web>
  <glimpse defaultRuntimePolicy="Off">
  </glimpse>
</configuration>

就我而言，这两人是罪魁祸首，但我可以想象可能还有其他人，所以要警惕这一点。