Debugging unobserved concurrency runtime task exceptions

The C++ Concurrency runtime (AKA PPLX) is the unmanaged answer to the Task Parallel Library (TPL), and it works surprisingly well. It is even cross platform by way of the C++ Rest SDK (codename Casablanca).

I work at Microsoft and in our group we are making extensive use of this library (we develop an iOS application). Recently, I encountered an interesting crash due to an unobserved exception. An unobserved exception is basically an exception thrown from a task, which no other entity (be it the caller or some later continuation) observed (typically by calling task::wait or task::get). In PPLX, such exceptions crash the process (in .NET they used to, and still may depending on configuration).

When an unobserved PPLX exception occurs, the debugger will break in the following location inside pplxtasks.h:

// If you are trapped here, it means an exception thrown in task chain didn’t get handled.
// Please add task-based continuation to handle all exceptions coming from tasks.
// this->_M_stackTrace keeps the creation callstack of the task generates this exception.
_REPORT_PPLTASK_UNOBSERVED_EXCEPTION(); // <– debugger will break here

Your mileage may vary, but I wasn’t able to inspect the _M_stackTrace variable in XCode’s debugger.

However, using the lldb console (you can bring it up with ⌘+Shift+C) I was able to inspect its value:

(lldb) expr _M_stackTrace

Here is a sample output:

(pplx::details::_TaskCreationCallstack) $1 = {
_M_SingleFrame = 0x00c0935d
_M_frames = size=0 {}
}

In this case, the frame of interest is stored in _M_SingleFrame (otherwise the list of frames would have been stored in _M_frames and _M_SingleFrame would have been null). Of course, “0x00c0935d” is not a terribly useful piece of data for root cause analysis, and I wasn’t even sure what that address represented! Fortunately, following macro in the same header clarified that mystery:

#define _CAPTURE_CALLSTACK() ::pplx::details::_TaskCreationCallstack::_CaptureSingleFrameCallstack(_ReturnAddress())

So now we know it is a return address, pointing to the code creating the offending exception. Fortunately, lldb can resolve that address to source:

(lldb) so l -a 0x00c0935d

Sample output:

/Users/ohads/Library/Developer/Xcode/DerivedData/ios-gdhrlqjldqgqktgtpdnpssahaqme/Build/Products/OurApp`pplx::task<void> pplx::task_from_exception<void, std::exception>(std::exception, pplx::task_options const&) + 62 at …

It’s not perfect, but it should be enough to narrow the search down (in this case, there was only one place in our code using task_from_exception).

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