具有无限超时的HttpClient会抛出超时异常

Question

我的HttpClient使用摘要式身份验证连接到服务器,并期望响应搜索查询.这些搜索查询可以随时进入,因此客户端应始终保持连接处于打开状态.

使用以下代码建立连接:

public static async void ListenForSearchQueries(int resourceId)
{
    var url = $"xxx/yyy/{resourceId}/waitForSearchRequest?token=abc";

    var httpHandler = new HttpClientHandler { PreAuthenticate = true };

    using (var digestAuthMessageHandler = new DigestAuthMessageHandler(httpHandler, "user", "password"))
    using (var client = new HttpClient(digestAuthMessageHandler))
    {
        client.Timeout = TimeSpan.FromMilliseconds(Timeout.Infinite);

        var request = new HttpRequestMessage(HttpMethod.Get, url);

        var tokenSource = new CancellationTokenSource();
            tokenSource.CancelAfter(TimeSpan.FromMilliseconds(Timeout.Infinite));

        using (var response = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, tokenSource.Token))
        {
            Console.WriteLine("\nResponse code: " + response.StatusCode);

            using (var body = await response.Content.ReadAsStreamAsync())
            using (var reader = new StreamReader(body))
                while (!reader.EndOfStream)
                    Console.WriteLine(reader.ReadLine());
         }
    }
}

这是在控制台应用程序的Main方法中使用该方法的方法.

private static void Main(string[] args)
{
   const int serviceId = 128;
   .
   .
   .
   ListenForSearchQueries(resourceId);
   Console.ReadKey();
}

这是控制台窗口上的输出如下所示:

Response code: OK
--searchRequestBoundary

即使客户端的超时设置为无穷大,连接在第一次输出后大约五分钟(这不是HttpClient的默认超时)后超时,抛出以下异常.

System.IO.IOException occurred
  HResult=0x80131620
  Message=The read operation failed, see inner exception.
  Source=System.Net.Http
  StackTrace:
   at System.Net.Http.HttpClientHandler.WebExceptionWrapperStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   at System.Net.Http.DelegatingStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   at System.IO.StreamReader.ReadBuffer()
   at System.IO.StreamReader.get_EndOfStream()
   at ConsoleTester.Program.<ListenSearchQueriesDigestAuthMessageHandler>d__10.MoveNext() in C:\Users\xyz\ProjName\ConsoleTester\Program.cs:line 270

Inner Exception 1:
WebException: The operation has timed out.

用于身份验证的DelegateHandler是对此代码的粗略修改(请参阅源代码部分).

为什么客户端超时,我该如何防止这种情况发生？

我的最终目标是打个电话并无限期地等待回应.当响应确实到来时,我不希望关闭连接,因为将来可能会有更多响应.不幸的是,我无法在服务器端更改任何内容.

Answer 1

尽管的默认值Stream.CanTimeout 是false，但通过返回流会response.Content.ReadAsStreamAsync()提供CanTimeout属性返回true的流。

该流的默认读写超时为5分钟。也就是说，闲置五分钟后，流将引发异常。与问题中显示的异常非常相似。

若要更改此行为，可以调整流的ReadTimeout和/或WriteTimeout属性。

下面是将ListenForSearchQueriesReadTimeout更改为Infinite 的方法的修改版本。

public static async void ListenForSearchQueries(int resourceId)
{
    var url = $"xxx/yyy/{resourceId}/waitForSearchRequest?token=abc";

    var httpHandler = new HttpClientHandler { PreAuthenticate = true };

    using (var digestAuthMessageHandler = new DigestAuthMessageHandler(httpHandler, "user", "password"))
    using (var client = new HttpClient(digestAuthMessageHandler))
    {
        client.Timeout = TimeSpan.FromMilliseconds(Timeout.Infinite);

        var request = new HttpRequestMessage(HttpMethod.Get, url);

        var tokenSource = new CancellationTokenSource();
            tokenSource.CancelAfter(TimeSpan.FromMilliseconds(Timeout.Infinite));

        using (var response = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, tokenSource.Token))
        {
            Console.WriteLine("\nResponse code: " + response.StatusCode);

            using (var body = await response.Content.ReadAsStreamAsync())
            {
                body.ReadTimeout = Timeout.Infinite;

                using (var reader = new StreamReader(body))
                    while (!reader.EndOfStream)
                        Console.WriteLine(reader.ReadLine());
            }
         }
    }
}

这修复了实际上由流引发但似乎由HttpClient引发的异常。