anirudh2290 commented on issue #9681: Better Exception Handling for Operators URL: https://github.com/apache/incubator-mxnet/pull/9681#issuecomment-364031160 After discussion with @piiswrong , we came to a conclusion that in an execution graph once an exception is thrown, the same exception should not be thrown again. For example: ``` x = None y = None try: x, y = op1() # Fails x.asnumpy() # Throw exception except: handle_exc(x, y) y.asnumpy() # Should execute fine z = op2(x) # Should execute fine, expectation is that user modified the value of x from garbage, when handling exception ``` You can see that op1 throws an exception and it may end up writing garbage values to x and y. The line x.asnumpy() throws exception. Once this is done, user may handle the exception or keep the garbage values as it is. Any consequent usage of x or op2 should not throw the same exception, since user is not expecting and it is already handled. One challenge during the implementation was that dereferencing exception_ptr in C++ will cause undefined behavior. So there is no way to modify state of the exception_object that exception_ptr points to, just by using exception_ptr itself. To workaround this limitation, we are holding the exception_ptr itself in a shared_ptr object. We decided to remove global_exception thrown in WaitForAll, since it adds unnecessary complexity and is not really used much except during benchmarking.
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