Multithread error not caught by catch

Question

Following is a complete console program that reproduces a strange error I have been experiencing. The program reads a file that contains urls of remote files, one per line. It fires up 50 threads to download them all.

static void Main(string[] args)
{
    try
    {
        string filePath = ConfigurationManager.AppSettings["filePath"],
            folder = ConfigurationManager.AppSettings["folder"];
        Directory.CreateDirectory(folder);
        List<string> urls = File.ReadAllLines(filePath).Take(10000).ToList();

        int urlIX = -1;
        Task.WaitAll(Enumerable.Range(0, 50).Select(x => Task.Factory.StartNew(() =>
          {
              while (true)
              {
                  int curUrlIX = Interlocked.Increment(ref urlIX);
                  if (curUrlIX >= urls.Count)
                      break;
                  string url = urls[curUrlIX];
                  try
                  {
                      var req = (HttpWebRequest)WebRequest.Create(url);
                      using (var res = (HttpWebResponse)req.GetResponse())
                      using (var resStream = res.GetResponseStream())
                      using (var fileStream = File.Create(Path.Combine(folder, Guid.NewGuid() + url.Substring(url.LastIndexOf('.')))))
                          resStream.CopyTo(fileStream);
                  }
                  catch (Exception ex)
                  {
                      Console.WriteLine("Error downloading img: " + url + "\n" + ex);
                      continue;
                  }
              }
          })).ToArray());
    }
    catch
    {
        Console.WriteLine("Something bad happened.");
    }
}

On my local computer it works fine. On the server, after downloading a few hundred images, it shows an error of either Attempted to read or write protected memory or Unable to read data from the transport connection: A blocking operation was interrupted by a call to WSACancelBlockingCall..

It seems to be a native error, because neither the inner nor the outer catch catches it. I never see Something bad happened..

I ran it in WinDbg, and it showed the following:

(3200.1790): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
LavasoftTcpService64+0x765f:
00000001`8000765f 807a1900        cmp     byte ptr [rdx+19h],0 ds:baadf00d`0000001a=??
0:006> g
(3200.326c): CLR exception - code e0434352 (first chance)
(3200.326c): CLR exception - code e0434352 (first chance)
(3200.2b9c): Access violation - code c0000005 (!!! second chance !!!)
LavasoftTcpService64!WSPStartup+0x9749:
00000001`8002c8b9 f3a4            rep movs byte ptr [rdi],byte ptr [rsi]

I just turned off Lavasoft, and now WinDbg shows this:

Critical error detected c0000374
(3c4.3494): Break instruction exception - code 80000003 (first chance)
ntdll!RtlReportCriticalFailure+0x4b:
00007fff`4acf1b2f cc              int     3
0:006> g
(3c4.3494): Unknown exception - code c0000374 (first chance)
(3c4.3494): Unknown exception - code c0000374 (!!! second chance !!!)
ntdll!RtlReportCriticalFailure+0x8c:
00007fff`4acf1b70 eb00            jmp     ntdll!RtlReportCriticalFailure+0x8e (00007fff`4acf1b72)
0:006> g
WARNING: Continuing a non-continuable exception
(3c4.3494): C++ EH exception - code e06d7363 (first chance)
HEAP[VIPJobsTest.exe]: HEAP: Free Heap block 0000007AB96CC5D0 modified at 0000007AB96CC748 after it was freed
(3c4.3494): Break instruction exception - code 80000003 (first chance)
ntdll!RtlpBreakPointHeap+0x1d:
00007fff`4acf3991 cc              int     3

your outside catch won't catch stuff in the thread, and there's a bunch of stuff in the thread not inside a catch — Keith Nicholas, Mar 23 '17 at 22:16
Do you need to go to the lavasoft ad aware program on the server and exclude your executable from the ad ware scanning. — Sql Surfer, Mar 23 '17 at 22:17
@SqlSurfer It is not Lavasoft - please see what I appended to the question. — wezten, Mar 23 '17 at 22:19
@KeithNicholas I just tried moving `try` 4 lines higher, but it's the same. — wezten, Mar 23 '17 at 22:21
@KeithNicholas: But shouldn't an exception thrown inside the lambda passed to `StartNew` become an error on the resulting task, which then causes an exception to be thrown by `Task.WaitAll`? — StriplingWarrior, Mar 23 '17 at 22:21
https://msdn.microsoft.com/en-us/library/system.accessviolationexception(v=vs.110).aspx it discusses how it won't get handled by try catch — Keith Nicholas, Mar 24 '17 at 00:23
Is that writing to the console in the exception handler permitted? That's not in the context of the primary thread and I know any attempt to mess with the UI would cause an error but I don't know if that also applies to the console. — Loren Pechtel, Mar 24 '17 at 02:06

score 6 · Answer 1 · answered Mar 24 '17 at 05:25

Your exception doesn't throw because you, well, do not attempt to get it. WaitAll method is basically a Barrier, which waits (haha) for all tasks to finish. It's void, so you have to save a reference for your tasks for futher actions, like this:

var tasks = Enumerable.Range(0, 50).Select(x => Task.Factory.StartNew(() =>
{
    while (true)
    {
        // ..
        try
        {
            // ..
        }
        catch (Exception ex)
        {
            // ..
        }
    }
})).ToArray();

Task.WaitAl((tasks);

// investigate exceptions here
var faulted = tasks.Where(t => t.IsFaulted);

According MSDN, exceptions are propagated when you use one of the static or instance Task.Wait or Task<TResult>.Wait methods, or .Result property. However, this is not an option for you, as you're using try/catch here. So you need to subscribe to TaskScheduler.UnobservedTaskException event:

TaskScheduler.UnobservedTaskException += TaskScheduler_UnobservedTaskException;

static void TaskScheduler_UnobservedTaskException(object sender, UnobservedTaskExceptionEventArgs e)
{
    Console.WriteLine("Error." + e);
    e.SetObserved();
}

Why does it run without throwing?

This application domain-wide event provides a mechanism to prevent exception escalation policy (which, by default, terminates the process) from triggering.

To make it easier for developers to write asynchronous code based on tasks, the .NET Framework 4.5 changes the default exception behavior for unobserved exceptions. Although unobserved exceptions still raise the UnobservedTaskException exception, the process does not terminate by default. Instead, the exception is handled by the runtime after the event is raised, regardless of whether an event handler observes the exception. This behavior can be configured. Starting with the .NET Framework 4.5, you can use the configuration element to revert to the behavior of the .NET Framework 4 and terminate the process:
<configuration> 
 <runtime> 
  <ThrowUnobservedTaskExceptions enabled="true"/> 
 </runtime> 
</configuration>

Now, back to your code. Consider using a static HttpClient instance instead of HttpWebRequest, as you simply need a result string. This class was designed to be used in multithreaded code, so it's methods are thread-safe.

Also, you should provide a TaskCreationOptions.LongRunning flag to your StartNew method (which is dangerous, by the way, but you still need it):

Specifies that a task will be a long-running, coarse-grained operation involving fewer, larger components than fine-grained systems. It provides a hint to the TaskScheduler that oversubscription may be warranted.

Oversubscription lets you create more threads than the available number of hardware threads. It also provides a hint to the task scheduler that an additional thread might be required for the task so that it does not block the forward progress of other threads or work items on the local thread-pool queue.

Thanks, however I just tried TaskScheduler.UnobservedTaskException, and it didn't catch it. I can't see an advantage with HttpClient, and it's a shame to create a string, when currently I am streaming the result directly to a file. I should be using TaskCreationOptions.LongRunning though (although it doesn't fix the issue). — wezten, Mar 24 '17 at 09:23
Also this doesn't explain why the inner catch doesn't catch it. — wezten, Mar 24 '17 at 09:43
@wezten your error is about infrastructure, so it can happen inside some system code out of scope `try`. `HttpClient` works with the streams too, you didn't read the docs: http://www.tugberkugurlu.com/archive/streaming-with-newnet-httpclient-and-httpcompletionoption-responseheadersread — VMAtm, Mar 24 '17 at 14:13
@wezten Also, did you inspect the faulted tasks? Your exception must be either on `AppDomain.UnhandledException` or `TaskScheduler.UnobservedTaskException` or in `task.Exception` if it is `IsFaulted`. If you can't find this exception, it means that it do occur not in your app. — VMAtm, Mar 24 '17 at 19:43
I cannot inspect the faulted tasks, since it never gets past the WaitAll. — wezten, Mar 25 '17 at 17:31
You may try `WaitAny` in a loop so examine step by step the tasks — VMAtm, Mar 25 '17 at 17:43
When copying from a book, it should add the nutshell book as a reference? :) — dragonfly02, Apr 27 '17 at 09:12
I did provide the links from MSDN. What book are you talking about? — VMAtm, Apr 27 '17 at 12:05

score 4 · Accepted Answer · answered Apr 20 '17 at 20:15

4

The issue was with Lavasoft Web Companion after all. Even though I had disabled it, there was still something from it running in the background. Uninstalling it, fixed the issue.

answered Apr 20 '17 at 20:15

wezten

2,126
3
25
48

Rickey · Answer 3 · 2017-03-26T15:13:00.870

0

You could add a continuation on your task.

 Task.Factory.StartNew(() => ...)
    .ContinueWith (task => 
    {
       If (task.isFaulted)
       {
           //task.Exception
           //handle the exception from this context
       }
    });

edited Mar 26 '17 at 15:13

answered Mar 25 '17 at 20:47

Rickey

7,830
1
19
12

score 0 · Answer 4 · answered Apr 25 '17 at 21:09

There is code in your sample that is actually unprotected by try/catch. An error there would thrown an uncaught exception on a thread.

Here is a refactor (comments by previously unprotected code). The outer try would not catch these, as it is on the starting thread. Therefore it would be unhandled on the sub threads

static void Main(string[] args)
{
    try
    {
        string filePath = ConfigurationManager.AppSettings["filePath"],
            folder = ConfigurationManager.AppSettings["folder"];
        Directory.CreateDirectory(folder);
        List<string> urls = File.ReadAllLines(filePath).Take(10000).ToList();

        int urlIX = -1;
        Task.WaitAll(Enumerable.Range(0, 50).Select(x => Task.Factory.StartNew(() =>
          {
              try
              {
                  while (true) // ** was unprotected
                  {
                      int curUrlIX = Interlocked.Increment(ref urlIX);  // ** was unprotected
                      if (curUrlIX >= urls.Count)   // ** was unprotected
                          break;                    // ** was unprotected
                      string url = urls[curUrlIX];  // ** was unprotected
                      try
                      {
                          var req = (HttpWebRequest)WebRequest.Create(url);
                          using (var res = (HttpWebResponse)req.GetResponse())
                          using (var resStream = res.GetResponseStream())
                          using (var fileStream = File.Create(Path.Combine                    (folder, Guid.NewGuid() + url.Substring(url.LastIndexOf('.')))))
                              resStream.CopyTo(fileStream);
                      }
                      catch (Exception ex)
                      {
                          Console.WriteLine("Error downloading img: " + url + "\n" + ex);
                          continue;
                      }
                  } // while
              } // try
          })).ToArray());
    }
    catch
    {
        Console.WriteLine("Something bad happened.");
    }
}

I had tried that - please see the fifth comment on the question. — wezten, Apr 27 '17 at 05:41

Multithread error not caught by catch

4 Answers4