Handling threading in IIS ISAPI extensions

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C++

I wrote an ISAPI extension in C++ whose purpose is to run PowerBuilder code in IIS. PowerBuilder has an interface to allow C++ code to call directly into PowerBuilder code.

One of the features allows it to kick off a background thread using CreateThread and return to the client before the background thread is finished.

One of the users is having a problem where if two client apps request the same thing at about the same time, one of them aborts at a certain point in the PowerBuilder code executed by the background thread.

In IIS the AppPool is a process and each request is a thread. If two request threads can successfully run PowerBuilder code at the same time, why can't threads created by the request threads run successfully?

What I have tried:

This is the code that kicks off the background thread:

if (RunBackground) {
       // create the background thread
       hThread = CreateThread(NULL, 0, BackgroundThread, strArgument, 0, &dwThreadID);
       if (hThread != NULL) {
           // wait for background thread to finish setup
           if (strArgument->hEvent != NULL) {
               WaitForSingleObject(strArgument->hEvent, INFINITE);
           }
           // cleanup
           delete strArgument;
           CloseHandle(hThread);
           CloseHandle(strArgument->hEvent);

           // set the HTTP status
           pECB->dwHttpStatusCode = HTTP_STATUS_OK;

           return HSE_STATUS_PENDING;
       }
   }

The BackgroundThread function initializes the PowerBuilder runtime and before calling the user code, does the following:

// signal that initialization is done
if (strArgument->hEvent != NULL) {
    SetEvent(strArgument->hEvent);
}

// release the session
pECB->ServerSupportFunction(pECB->ConnID, HSE_REQ_DONE_WITH_SESSION, NULL, NULL, NULL);