Unable to func-eval using Mdbg due to "code is optimized" exception - debugging

Question

We are using MdbgCore.dll to evaluate a property off a parameter on a thread callstack.
To do this, we are performing a func-eval.
Unfortunately, all our attempts to perform the func-eval are failing with CORDBG_E_ILLEGAL_IN_OPTIMIZED_CODE, which seems to be due to the thread being used for the func-eval not being in a GC-safe point.
This is documented here: http://blogs.msdn.com/b/jmstall/archive/2005/11/15/funceval-rules.aspx.
We tried scanning all threads in the process to find a thread that is in a GC-safe point, but they all appear to have UserState marked with USER_UNSAFE_POINT.
There is very scarce documentation on the subject, and we are pulling our hair out trying to figure out if there is a way to get a thread in a GC-safe point so we can do the func-eval. We would consider anything that allows us to deterministically break into the process with a thread to do the func-eval with.
Disclaimer: we are trying to evaluate a method on a class that resides in an optimized assembly, so not sure if this is maybe also causing an issue.
The sample code follows:
if (argument.TypeName.EndsWith(
"WorkerRequest", StringComparison.OrdinalIgnoreCase)
&& !argument.IsNull)
{
try
{
// Invoke the "GetUriPath()" function to obtain the URI
string functionName = "System.Web.HttpWorkerRequest.GetUriPath";
MDbgFunction func = debugger.Processes.Active.ResolveFunctionNameFromScope(
functionName,
thread.CorThread.AppDomain
);
if (null == func)
{
throw new InvalidOperationException(
String.Format("Could not resolve {0}", functionName));
}
// Setup the eval
CorEval eval = threadForFuncEvals.CorThread.CreateEval();
// Setup the function parameters
List<CorValue> values = new List<CorValue>();
// Add the worker request "this" pointer
values.Add(
argument.CorValue
);
// resume the thread being used to do the func-eval
threadForFuncEvals.CorThread.DebugState = CorDebugThreadState.THREAD_RUN;
// Queue the function for execution
// EXCEPTION THROWN BELOW
// EXCEPTION THROWN BELOW
// EXCEPTION THROWN BELOW
eval.CallFunction(func.CorFunction, values.ToArray());
// BUGBUG: Should we pause all other threads to prevent them from moving?
// Continue the process to execute the function
if (!proc.Go().WaitOne(settings.BreakTimeout))
{
throw new InvalidOperationException("Timeout while evaluating function");
}
// get the returned string
var result = eval.Result;
if (result != null)
{
MDbgValue mv = new MDbgValue(proc, result);
string returnedValue = mv.GetStringValue(false);
threadInfo.Url = returnedValue;
}
}
catch (Exception e)
{
// BUGBUG: Ignoring exception
}
finally
{
// suspend the thread again
if (threadForFuncEvals != null)
{
threadForFuncEvals.CorThread.DebugState =
CorDebugThreadState.THREAD_SUSPEND;
}
}
}
Microsoft / Mdbg team, can you help?
Best,
Mike

Answer 1

Is this something to do with JIT optimization?
In my program, I turn JIT optimization off (for technical reasons, I think you can only do this with CreateProcess() and not using Attach()).
proc = m_Debugger.CreateProcess(ProcessName, ProcessArgs, DebugModeFlag.Default, DebugEngineUtils.GetAssemblyRuntimeVersion(ProcessName,DefaultNetVersion));
if (proc!=null) proc.CorProcess.OnCreateProcess += new Microsoft.Samples.Debugging.CorDebug.CorProcessEventHandler(CorProcess_OnCreateProcess);
if (proc!=null) proc.CorProcess.OnModuleLoad += new Microsoft.Samples.Debugging.CorDebug.CorModuleEventHandler(CorProcess_OnModuleLoad);
void CorProcess_OnModuleLoad(object sender, Microsoft.Samples.Debugging.CorDebug.CorModuleEventArgs e)
{
e.Module.JITCompilerFlags = Microsoft.Samples.Debugging.CorDebug.CorDebugJITCompilerFlags.CORDEBUG_JIT_DISABLE_OPTIMIZATION;
}
void CorProcess_OnCreateProcess(object sender, Microsoft.Samples.Debugging.CorDebug.CorProcessEventArgs e)
{
//try to disable optimization
((Microsoft.Samples.Debugging.CorDebug.CorProcess)sender).DesiredNGENCompilerFlags = Microsoft.Samples.Debugging.CorDebug.CorDebugJITCompilerFlags.CORDEBUG_JIT_DISABLE_OPTIMIZATION;
}