[Issue 1513] try/catch/finally misbehavior on windows

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=1513


Don <clugdbug at yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|DMD                         |druntime
         AssignedTo|nobody at puremagic.com        |sean at invisibleduck.org


--- Comment #11 from Don <clugdbug at yahoo.com.au> 2010-12-15 17:22:58 PST ---
This is definitely a druntime issue (though it could be an issue with the C
stdlib). The bug must lie in the druntime/src/rt/deh.c

In the test case, here's my understanding of what happens: 

The first exception is thrown. 
Every try block is queried (in _d_framehandler() )to see if it's handled.
Nothing in main() handles it, but something else does. (I think in this case,
it's a catch-all block around main).
Then, the unwinding starts. _d_framehandler() runs this:
    // Have system call all finally blocks in intervening frames
    int retval = _global_unwind((ESTABLISHER_FRAME *)frame, exception_record);

_global_unwind is a wrapper around the Windows system function RtlUnwind.
That calls _dframehandler() again, with EXCEPTION_UNWIND this time.
This calls _d_local_unwind(), to run all of the finally blocks

_d_local_unwind() is also in druntime/deh.c. 

In the test case, the finally block contains a throw, so the procedure is
repeated. Again it doesn't get caught until the catch-all.
As before, global_unwind and then local_unwind get called.

The first thing _d_local_unwind() does is set up a double-fault exception
handler.

When it begins the unwind, the double-fault handler gets triggered.
The double-fault handler basically just does this:

     if (!(ExceptionRecord->ExceptionFlags & EXCEPTION_UNWIND)) 
        return ExceptionContinueSearch;
     *((ESTABLISHER_FRAME **)DispatcherContext) = EstablisherFrame;
     return ExceptionCollidedUnwind;

Because it returns ExceptionCollidedUnwind, Windows is supposed to abandon the
original unwinding, and the new unwinding
should take over. But that's doesn't seem to be what happens.
No further unwinding occurs. To my suprise, it returns from _global_unwind
into _d_framehandler, back in the original catch handler, and all the variables
seem to be unchanged (so you have no way of detecting what happened).

Dunno where the problem is. Maybe it could be in _global_unwind in the C
runtime?
This stuff is amazingly undocumented by Microsoft.

This is the most detailed info that I've found:
http://www.nynaeve.net/?p=106
See point 7.
And here is more detail about collided unwinds.
http://www.nynaeve.net/?p=107

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