the trick of using $SIG{__DIE__} with eval {} disregarding the $^S value

[Stas Bekman <st...@stason.org>]

This is an interesting approach to using $SIG{__DIE__} together with 
eval {} blocks, without relying on the value of $^S which sometimes goes 
broken. I guess we should add this "trick" to the guide.

-------- Original Message --------
Subject: Re: [ID 20020107.001] $^S is sticky
Date: Mon, 7 Jan 2002 15:15:58 -0500
From: Michael G Schwern <sc...@pobox.com>
To: chris@netmonger.net
CC: perl5-porters@perl.org
References: <20...@lion-around.at.yiff.net>

On Mon, Jan 07, 2002 at 02:52:02PM -0500, Christopher Masto wrote:
 > Is this supposed to happen?

$^S breaks a lot.  I found both 5.6.1 and 5.005_03 to have broken $^S,
though for different reasons.  I can't remember why 5.005_03's is
broken.  If you peek inside Test::Builder you'll find this:

$SIG{__DIE__} = sub {
     # We don't want to muck with death in an eval, but $^S isn't
     # totally reliable.  5.005_03 and 5.6.1 both do the wrong thing
     # with it.  Instead, we use caller.  This also means it runs under
     # 5.004!
     my $in_eval = 0;
     for( my $stack = 1;  my $sub = (CORE::caller($stack))[3];  $stack++ ) {
         $in_eval = 1 if $sub =~ /^\(eval\)/;
     }
     $Test_Died = 1 unless $in_eval;
};


 > chris@lion-around:~$ perl5.00503 -e '$SIG{__DIE__} = sub { die @_ if 
$^S; die "caught die: @_"; }; print "start: $^S\n"; eval { die "death in 
eval"; }; print "end: $^S\n"; die "death outside eval"'
 > start: 0
 > end: 0
 > caught die: death outside eval at -e line 1.
 >
 > chris@lion-around:~$ perl5.6.1 -e '$SIG{__DIE__} = sub { die @_ if 
$^S; die "caught die: @_"; }; print "start: $^S\n"; eval { die "death in 
eval"; }; print "end: $^S\n"; die "death outside eval"'
 > start:
 > end: 1
 > death outside eval at -e line 1.
 >
 > In 5.6.1, the value of $^S seems to get stuck once it's been set.
 > This only happens if the $SIG{__DIE__} handler actually looks at it.

t/op/magic.t appears to be the only thing which tests $^S, and it
doesn't get $SIG{__DIE__} involved.


bleadperl is back to:

start: 0
end: 0
caught die: death outside eval at -e line 1.

if modified a little, it shows its the correct behavior:

$ bleadperl -e '$SIG{__DIE__} = sub { die @_ if $^S; die "caught die: 
@_"; }; print "start: $^S\n"; eval { die "death in eval"; }; print $@; 
print "end: $^S\n"; die "death outside eval"'
start: 0
death in eval at -e line 1.
end: 0
caught die: death outside eval at -e line 1.


-- 

Michael G. Schwern   <sc...@pobox.com>    http://www.pobox.com/~schwern/
Perl Quality Assurance	    <pe...@perl.org>	       Kwalitee Is Job One
     And God created Cat to be a companion to Adam.  And Cat would not obey
Adam.  And when Adam gazed into Cat's eyes, he was reminded that he
was not the supreme being.  And Adam learned humility.
	-- http://www.catsarefrommars.com/creationist.htm

-- 


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