asynchronous - 如何在 F# Async.Parallel 中处理 HttpWebRequest 超时

Question

我只是花了很长时间才弄清楚为什么这段代码对于某些网址“挂起”:

let getImage (imageUrl:string) =
    async {
        try
            let req = WebRequest.Create(imageUrl) :?> HttpWebRequest
            req.UserAgent <- "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)";
            req.Method <- "GET";
            req.AllowAutoRedirect <- true;
            req.MaximumAutomaticRedirections <- 4;
            req.Timeout <- 3000; //HAHAHA, nice try!
            let! response1 = req.AsyncGetResponse()
            let response = response1 :?> HttpWebResponse
            use stream = response.GetResponseStream()
            let ms = new MemoryStream()
            let bytesRead = ref 1
            let buffer = Array.create 0x1000 0uy
            while !bytesRead > 0 do
                bytesRead := stream.Read(buffer, 0, buffer.Length)
                ms.Write(buffer, 0, !bytesRead)
            return SuccessfulDownload(imageUrl, ms.ToArray())

        with
            ex -> return FailedDownload(imageUrl, ex.Message)
    }

在设法追踪 3000 个 URL 中的哪个被挂起后，我了解到 AsyncGetResponse不注意 HttpWebRequest.Timeout .我做了一些搜索，提出了将异步请求包装在带有计时器的线程中的建议。这对 C# 来说很棒，但如果我通过 Async.Parallel |> Async.RunSynchronously 运行其中的 3000 个，处理这个问题的最佳方法是什么？

Answer 1

我只是粗略地测试了这个，但它应该有正确的行为:

type System.Net.WebRequest with
  member req.AsyncGetResponseWithTimeout () =
    let impl = async {
      let iar = req.BeginGetResponse (null, null)
      let! success = Async.AwaitIAsyncResult (iar, req.Timeout)
      return if success then req.EndGetResponse iar
             else req.Abort ()
                  raise (System.Net.WebException "The operation has timed out") }
    Async.TryCancelled (impl, fun _ -> req.Abort ())

在您的代码中，调用 req.AsyncGetResponseWithTimeout()而不是 req.AsyncGetResponse() .