#20643: $Error::THROWN takes precedence over $@

On Thu Aug 17 16:11:26 2006, PEVANS wrote:
> That die line is the magic that all __DIE__ handlers need to start with,
> so that they don't break during inner eval{} exception handling. I added
> that line to your supplied script, and I get the expected behaviour.

Is this bug all sorted now? Can I close it, or are there still
outstanding issues?

--

Paul Evans

I have played with this subject a little more - and I am afraid there
is no clean solution.

One general suggestion, that hopefully won't hurt anybody: The "die"
in Error::throw should be written as CORE::die, since at this point we
do not want any more interaction with other exception handling systems
etc.

Then I saw that CGI::Carp tries to do similar things as I do, and I
read this:

1.26 Replaced CORE::GLOBAL::die with the evil $SIG{__DIE__} because the
former isn't working in some people's hands. There is no such
thing
as reliable exception handling in Perl.

Looking at my test script again, one thing puzzles me:
CORE::GLOBAL::die cannot override the die in Carp::croak! Maybe this
is what Lincoln meant with the above...

-Marek

On So. 20. Aug. 2006, 15:37:47, PEVANS wrote:
> Is this bug all sorted now? Can I close it, or are there still
> outstanding issues?

Finally here is some news on this topic:
$^S unfortunately does not help at all - I found that during a more
thorough analysis. Since the try{} block in fact is an eval{}, $^S
is always true inside that block, and my original requirements are not
fulfilled at all. To remember, here they are again:
- I want to use Error exception handling throughout the application
- I want to force all modules throwing Error exceptions
- I want to have the possibility to debug stack traces (for that I
need the above).

Obviously I cannot force everybody to use Error exceptions in all
their CPAN modules. So the only way is to play with $SIG{__DIE__} or
CORE::GLOBAL::die. The former is deprecated in the manuals, the latter
seems to work OK. So I can do something like this:
BEGIN {
*CORE::GLOBAL::die = sub { Error::Simple->throw(shift) };
}
and all should be fine (note that this requires Error::throw to use
explicitely CORE::die, otherwise we end up in an endless loop).

But alas, there is a gotcha... when in some module there is an eval{}
around some code that does a die(), then $Error::THROWN is set - and
not cleared. And that masks any CORE::die exception (which may occur
e.g. when an XS module fails to load its binary *.so part - such an
exception does not go through CORE::GLOBAL::die). Try this script:

#!/opt/perl_5.8.8/bin/perl -w

use Error qw(:try);

BEGIN {
*CORE::GLOBAL::die = sub { Error::Simple->throw(shift) };
*Error::die = sub { CORE::die @_ };
}

try {
# this contains an eval { ... } around some code which
# throws an Error::Simple exception because of the
# above CORE::GLOBAL::die override. $Error::THROWN is
# set
require File::Temp;

# this is an alternative way to produce this situation:
#eval {
# Error::Simple->throw("This is caught by a simple eval");
#};

print "Error::THROWN = $Error::THROWN\n";

# this exception is caught nicely:
#Error::Simple->throw("thrown exception");

# also this is caught with the right text:
# since it triggers the CORE::GLOBAL::die
#die "ARGH";

# this one is masked by the $Error::THROWN
CORE::die "Do you see me?";
}
otherwise {
my $E = shift;
print "Caught exception:\n$E\n";
};

exit 0;
__END__

Uncomment and try the other exceptions - however, you won't see
the "Do you see me?" exception.
I looked at the code around line 358 in Error.pm:

else {
$err = defined($Error::THROWN)
? $Error::THROWN : $@;
$err = $Error::ObjectifyCallback->({'text' =>$err})
unless ref($err);
}

I am wondering, why $Error::THROWN is preferred over $@, which
actually contains the exception that caused the eval{} to abort. Why
not rewrite it like this (attention, also on lines 393 and 433 etc.):

else {
$err = $Error::THROWN = defined($@) ? $@ : $Error::THROWN;
$err = $Error::ObjectifyCallback->({'text' =>$err})
unless ref($err);
}

All the regression tests in the Error package pass OK with this
change, and my requirements would be fulfilled as well.

Thank you for your patience and best regards,

Marek