Reporting Crashes in IMVU: Structured Exceptions

Previously, we discussed the implementation of automated reporting of
unhandled C++ exceptions
. However, if you’ve ever programmed in C++,
you know that C++ exceptions are not the only way your code can fail.
In fact, the most common failures probably aren’t C++ exceptions at
all. You know what I’m referring to: the dreaded access violation
(sometimes called segmentation fault).

Access Violation

How do we detect and report access violations? First, let’s talk
about what an access violation actually is.

Your processor has a mechanism for detecting loads and stores from
invalid memory addresses. When this happens, it raises an interrupt,
which Windows exposes to the program via Structured Exception Handling
(SEH). Matt Pietrek has written an excellent article on how
SEH works
, including a description of C++ exceptions implemented
on top of SEH. The gist is that there is a linked list of stack
frames that can possibly handle the exception. When an exception
occurs, that list is walked, and if an entry claims it can handle it,
it does. Otherwise, if no entry can handle the exception, the program
is halted and the familiar crash dialog box is displayed to the user.

OK, so access violations can be detected with SEH. In fact, with the
same mechanism, we can detect all other types of structured
exceptions, including division by zero and stack overflow. What does
the code look like? It’s approximately:

bool handle_exception_impl_seh(function f) {
    __try {
        // This is the previously-described C++ exception handler.
        // For various reasons, they need to be in different functions.
        // C++ exceptions are implemented in terms of SEH, so the C++
        // exception handling must be deeper in the call stack than
        // the structured exception handling.
        return handle_exception_impl_cpp(f);
    }
    // catch all structured exceptions here
    __except (EXCEPTION_EXECUTE_HANDLER) {
        PyErr_SetString(PyExc_RuntimeError, "Structured exception in C++ function");
        return true; // an error occurred
    }
}

Note the __try and __except keywords. This means we’re using
structured exception handling, not C++ exception handling. The filter
expression in the __except statement evaluates to
EXCEPTION_EXECUTE_HANDLER, indicating that we always want to handle
structured exceptions. From the filter expression, you can optionally
use the GetExceptionCode
and GetExceptionInformation
intrinsics to access information about the actual error.

Now, if you write some code like:

Object* o = 0;
o->method(); // oops!

The error will be converted to a Python exception, and reported
with our existing mechanism. Good enough for now! However, there are
real problems with this approach. Can you think of them?

Soon, I’ll show the full implementation of the structured
exception handler.

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