From: Hugh Sasse <hgs@...>
Date: 2005-11-03T19:52:53+09:00
Subject: Re: Trapping errors.

On Thu, 3 Nov 2005, Robert Klemme wrote:

> Hugh Sasse wrote:
> > On Thu, 3 Nov 2005, Robert Klemme wrote:
> >
> >> Hugh Sasse <hgs@dmu.ac.uk> wrote:
        [...]
> >>> Then it's already caught, and I should never see the error.
> >>
> >> Why not?  If it's printed in the other rescue clause you would see
> >> it.  Or am I missing something here?
> >
> > I wouldn't see it as an error which kills my program.  It would have
> > been caught, and dealt with, or caught and re-raised, in which case
> > the outer block would catch it.
> 
> Well, there's at least the theoretical chance that this block just calls
> #exit... :-)

It died with a stack trace.  exit doesn't do that.
> 
> > 1 We have established that a begin...rescue...end block
> >   will rescue any errors raised inside it, provided the rescue
> >   clause matches that error type.
> >
> > 2 We have established that if a begin ... rescue ... raise ... end
> >   block re-raises an error, it can be caught by a surrounding
> >   begin...rescue...end block
> >
> > 3 My case is that I have a
> >    begin
> >       part1
> >    rescue Exception, SystemCallError, Errno::EACCES, Errno::EBUSY => e
> >       part2
> >    end
> >
> >   block and it is not catching a Errno::EBUSY error from part1: The
> >   error crashes the program.  So:
> >     If that error has been raised in part1, and not caught in part1
> >       then I should be able to catch it.
> >     If that error has been raised in part1, and caught in part1,
> >       part2 should never come into play, and the program should
> >       continue
> >
> > 4 How can a subclass of Exception not be caught by Exception?
> > 5 Why doesn't SystemCallError catch it?
> > 6 Why doesn't the Errno::%s form catch it?
> >
> > i.e everything between "Exception" and " =>" should be unnecessary,
> > but even with those it doesn't catch the error.
> 
> This sounds really strange.  Here's what I'd do: use set_trace_func to
> write out all method invocations, returns and thread ids and try to see
> where it goes.  HTH

Good job I tried to add docs to profiler.rb in stdlib yesterday.
I'll have a go at that.  I've not played with set_trace_func before,
so it should be interesting.
> 
> Kind regards
> 
>     robert
> 
        Hugh