Why is calling a Python lambda expression from C# not thread-safe?

[ironpythonbot]

I define a side-effect-free (pure) lambda expression in IronPython and assign it to a C# delegate. When invoking the delegate simultaneously from multiple threads i get exceptions of type ** AccessViolationException ** , ** NullReferenceException ** and ** FatalEngineExecutionError ** .

The occurance of the error is non-deterministic and it mostly takes several million iterations to provoke it, which says "race condition" to me. How can i avoid it?

The exceptions are only raised when running the process with x64 (x86 does not crash) and outside of the debugger. The test system is a Core I7 (8 threads) on Windows 7, .NET Framework 4.0 and IronPython 2.7.1.

Here's the minimal code to produce the error:

    var engine = Python.CreateEngine();

    double a = 1.0;
    double b = 2.0;

    while (true)
    {
        Func<double, double, double> calculate = engine.Execute("lambda a,b : a+b");
        System.Threading.Tasks.Parallel.For(0, 1000, _ =>
        {
             for (int i = 0; i < 1000; i++) { calculate(a,b); }
        });

        Console.Write(".");   
    }

Error message:

FatalExecutionEngineError was detected

Message: The runtime has encountered a fatal error. The address of the error was at 0xf807829e, on thread 0x3da0. The error code is 0xc0000005. This error may be a bug in the CLR or in the unsafe or non-verifiable portions of user code. Common sources of this bug include user marshaling errors for COM- interop or PInvoke, which may corrupt the stack.

Even if the engine is declared as thread-local, it crashes after some time:

var calculate = new ThreadLocal<Func<double, double, double>>(() => Python.CreateEngine().Execute("lambda a,b : a+b"));

Work Item Details

Original CodePlex Issue: Issue 31924 Status: Proposed Reason Closed: Unassigned Assigned to: Unassigned Reported on: Dec 12, 2011 at 11:27 AM Reported by: Rauhotz Updated on: May 14, 2013 at 4:43 PM Updated by: Athari