scope(exit) and scope(failure) on Errors

[ZombineDev via Digitalmars-d]

On Monday, 25 April 2016 at 08:42:33 UTC, Shachar Shemesh wrote:
> There is a problem with scope(exit) and scope(failure) running 
> when Error types exceptions are thrown. It throws the program 
> into code paths that are really not healthy.
>
> Imagine, for example, code handling linked lists. We do 
> manipulations on linked lists, and put a scope(exit) that 
> clears stuff up.
>
> Now imagine that somewhere inside, we have an assert that makes 
> sure that everything is still in order. If it's not, then an 
> AsserError is thrown, which derive from Error. The scope(exit) 
> runs, but since things are not where they should be (hence the 
> assert), we segfault. All the useful stuff that the assert was 
> supposed to provide us is now gone, replaced by cleanup code 
> that had no business running under those circumstances.
>
> Even when not harmful, this is, often, useless. Even if the 
> cleanup proceeds correctly, what is the sense of cleaning up 
> when the program is in serious trouble?
>
> The above is not categorically always true. In Weka, for 
> example, we are using exceptions derived from Error to force 
> all fibers of a certain logical component to exit. It is useful 
> that these exceptions are not caught. In those cases, there are 
> some types of cleanups that we do want to take place, but not 
> others.
>
> Some way to control this would be appreciated.
>
> Shachar

So, if I understand correctly, you want scope(exit) and 
scope(failure) code to run only when an exception not derived 
from Error is thrown (because otherwise the program is in an 
undefined state), but you need an uncatchable exception, so you 
can't derive from Exception?

Then why don't you use this:

class Fault : Throwable
{ /* ... */ }

// Error: can only catch class objects derived from Exception in 
@safe code, not 'Fault':
try
     faultThrowingFunc()
catch (Fault f) //
     // ...

And change `assert (expr, "Error msg")` to `expr || abort("Error 
msg")`, which will cleanly terminate the program immediately, 
instead of throwing AssertError and choking on scope blocks.

Example: http://dpaste.dzfl.pl/5a557d982230