[omniORB] Processing DII Exceptions

[Duncan Grisby]

On Tuesday 3 July, "Peter Chase" wrote:

> We seem to be experiencing a change of behaviour from OmniORB, regarding 
> exceptions that happen in DII calls. I would like to check whether my 
> guesses are correct.

Sorry for the long delay in replying to this...

[...
> Using the debugger, I can see that, when the operation invoked throws a 
> COMM_FAILURE SystemException, the above code to process a UserException is 
> running. It crashes with AccessViolation somewhere in "CORBA::Any 
> any(pEx->exception())".
> 
> Presumably, the crash is due to our casting a CORBA::SystemException* to a 
> CORBA::UnknownUserException*.

Yes, I'm sure it is.

> This crash occurs in OmniORB 4.1.0, but did not occur in OmniORB 3.0.4.
> 
> I notice that OmniORB 4.1.0 has a configuration parameter 
> diiThrowsSysExceptions default = 0. This suggests that OmniORB 4 will, by 
> default, do exactly what I am observing; that is, SystemExceptions do not 
> get thrown but instead end up in the exception().
> 
> I guess I can make the crash go away by changing this configuration 
> parameter. But I am curious to know why it did not crash in OmniORB 3.

The change is because at some point the CORBA spec was clarified to say
that system exceptions should be handled in the same way as user
exceptions in DII, where before omniORB threw system exceptions as
normal exceptions. Newer versions of omniORB adhere to the clarified
spec, but keep the option to throw system exceptions as exceptions for
backwards compatibility.

> Looking at Pure CORBA book, it suggests using _downcast() on the 
> CORBA::Exception, to try safely casting it to SystemException and then to 
> UserException. This looks like a better approach than twiddling the 
> configuration parameter, as the configuration parameter is Omni-specific, 
> isn't it?

Yes, that's the best thing to do. _downcast will do the right thing.

Cheers,

Duncan.

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