[Mono-list] HttpWebRequest.GetResponse() can trigger an uncatchable exception

Christopher David Howie me at chrishowie.com
Sun Jul 10 05:31:10 EDT 2011

 On Sat, 9 Jul 2011 11:50:34 -0400, Gonzalo Paniagua Javier wrote:
> Not much I can do without a test case.

 I know.  :(  The worst bug is the one that only happens once in a blue 

> I'd say that this is a problem reusing a connection and somehow 2
> threads "own" the same stream. Set KeepAlive to false (and increase
> the number of file descriptors! (ulimit)) and see if you can still
> reproduce the problem. If it does not happen, bingo! Reduce the
> 'maxconnections' setting via app.config file to 1 and begin as many
> requests as possible to the same URL. If you can reproduce it this
> way, it's easier to find when 2 threads are using the same stream.

 I'm using the following code:

 using System;
 using System.Linq;
 using System.Net;
 using System.Threading;

 internal class Program
     private static string url;

     private const int THREAD_COUNT = 1000;

     private static void Main(string[] args)
         ServicePointManager.DefaultConnectionLimit = 1;

         url = args.Length > 0 ? args[0] : "http://localhost/";

         var threads = new Thread[THREAD_COUNT];

         for (int i = 0; i < THREAD_COUNT; i++) {
             threads[i] = new Thread(WorkerThread);

         for (int i = 0; i < THREAD_COUNT; i++) {

     private static void WorkerThread()
         for (;;) {
             var request = (HttpWebRequest)WebRequest.Create(url);
             request.KeepAlive = false;

             try {
                 using (request.GetResponse())
             } catch {

 So far (10+ minutes) no exception has been thrown by this test.  I'd be 
 open to suggestions on how to tweak it.

> Also check the values of the variables involved in that BlockCopy 
> call
> and the values of readBuffer* before the call.

 I know that readBufferOffset is -1.  I'm not sure about the others.  If 
 we can get a consistent repro then I'll be able to check pretty easily.

 Chris Howie

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