cannot weaken refs to read only values #6930

p5pRT · 2003-11-16T21:38:23Z

Migrated from rt.perl.org#24506 (status was 'resolved')

Searchable as RT24506$

p5pRT · 2003-11-16T21:38:24Z

From fergal@esatclear.ie

with 5.8.2 and 5.6.1 (and presumably 5.6.2)

perl -MScalar::Util=weaken -e '$a=\"h";weaken($a)'
Modification of a read-only value attempted at -e line 1

this means that you probaly should be doing

eval {
local $SIG{__DIE__};
weaken($a);
};

wherever you were using it before. Patch attached to fix this by adding
PERL_MAGIC_backref to the list of things that can be done to a readonly
value.

I added some tests, everything still passes, is there some other reason not to
allow this?

F

p5pRT · 2003-11-16T21:38:24Z

From fergal@esatclear.ie

weaken.patch

--- ./ext/List/Util/t/weak.t.orig	2003-11-16 20:05:28.000000000 +0000
+++ ./ext/List/Util/t/weak.t	2003-11-16 21:04:47.000000000 +0000
@@ -35,7 +35,7 @@
 
 eval <<'EOT' unless $skip;
 use Scalar::Util qw(weaken isweak);
-print "1..17\n";
+print "1..20\n";
 
 ######################### End of black magic.
 
@@ -44,6 +44,7 @@
 sub ok {
 	++$cnt;
 	if($_[0]) { print "ok $cnt\n"; } else {print "not ok $cnt\n"; }
+	return $_[0];
 }
 
 $| = 1;
@@ -205,6 +206,17 @@
 ok(isweak($x->{Y}));
 ok(!isweak($x->{Z}));
 
+#
+# Case 7: test weaken on a read only ref
+#
+
+$a = \"hello";
+$b = $a;
+eval{weaken($b)};
+# we didn't die
+ok($@ eq "") or print "#died with $@\n";
+ok(isweak($b));
+ok($$b eq "hello") or print "#b is '$$b'\n";
 
 package Dest;
 
--- ./lib/Exporter.pm.orig	2003-11-09 23:03:20.000000000 +0000
+++ ./lib/Exporter.pm	2003-11-16 20:53:12.000000000 +0000
@@ -30,6 +30,12 @@
   my $pkg = shift;
   my $callpkg = caller($ExportLevel);
 
+  if (@_ and $pkg eq "Exporter" and $_[0] eq "import")
+  {
+    *{$callpkg."::import"} = \&import;
+    return;
+  }
+
   # We *need* to treat @{"$pkg\::EXPORT_FAIL"} since Carp uses it :-(
   my($exports, $fail) = (\@{"$pkg\::EXPORT"}, \@{"$pkg\::EXPORT_FAIL"});
   return export $pkg, $callpkg, @_
@@ -103,6 +109,12 @@
   @ISA = qw(Exporter);
   @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request
 
+or
+
+  package YourModule;
+  use Exporter 'import'; # gives you Exporter's import() method directly
+  @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request
+
 In other files which wish to use YourModule:
 
   use ModuleName qw(frobnicate);      # import listed symbols
@@ -290,6 +302,19 @@
 - or people using your package will get very unexplained results!
 
 
+=head2 Exporting without inheriting from Exporter
+
+By including Exporter in your @ISA you inherit an Exporter's import() method
+but you also inherit several other helper methods which you probably don't
+want. To avoid this you can do
+
+  package YourModule;
+  use Exporter qw( import );
+
+which will export Exporter's own import() method into YourModule.
+Everything will work as before but you won't need to include Exporter in
+@YourModule::ISA.
+
 =head2 Module Version Checking
 
 The Exporter module will convert an attempt to import a number from a
--- ./lib/Exporter.t.orig	2003-11-09 23:03:30.000000000 +0000
+++ ./lib/Exporter.t	2003-11-16 21:13:15.000000000 +0000
@@ -21,7 +21,7 @@
 }
 
 
-print "1..26\n";
+print "1..28\n";
 require Exporter;
 ok( 1, 'Exporter compiled' );
 
@@ -196,3 +196,21 @@
 Moving::Target->import (bar);
 
 ::ok (bar eq "bar", "imported bar after EXPORT_OK changed");
+
+package The::Import;
+
+use Exporter 'import';
+
+eval { import() };
+::ok(\&import == \&Exporter::import, "imported the import routine");
+
+@EXPORT = qw( wibble );
+sub wibble {return "wobble"};
+
+package Use::The::Import;
+
+The::Import->import;
+
+my $val = eval { wibble() };
+::ok($val eq "wobble", "exported importer worked");
+
--- ./sv.c.orig	2003-11-16 20:01:12.000000000 +0000
+++ ./sv.c	2003-11-16 20:14:25.000000000 +0000
@@ -4649,6 +4649,7 @@
 	    && how != PERL_MAGIC_bm
 	    && how != PERL_MAGIC_fm
 	    && how != PERL_MAGIC_sv
+	    && how != PERL_MAGIC_backref
 	   )
 	{
 	    Perl_croak(aTHX_ PL_no_modify);

p5pRT · 2003-11-16T21:44:21Z

From fergal@esatclear.ie

Previous patch included an unrelated patch to Exporter.pm, this one is for
weaken only,

F

p5pRT · 2003-11-16T21:44:21Z

From fergal@esatclear.ie

weaken.patch

+++ ./ext/List/Util/t/weak.t	2003-11-16 21:04:47.000000000 +0000
@@ -35,7 +35,7 @@
 
 eval <<'EOT' unless $skip;
 use Scalar::Util qw(weaken isweak);
-print "1..17\n";
+print "1..20\n";
 
 ######################### End of black magic.
 
@@ -44,6 +44,7 @@
 sub ok {
 	++$cnt;
 	if($_[0]) { print "ok $cnt\n"; } else {print "not ok $cnt\n"; }
+	return $_[0];
 }
 
 $| = 1;
@@ -205,6 +206,17 @@
 ok(isweak($x->{Y}));
 ok(!isweak($x->{Z}));
 
+#
+# Case 7: test weaken on a read only ref
+#
+
+$a = \"hello";
+$b = $a;
+eval{weaken($b)};
+# we didn't die
+ok($@ eq "") or print "#died with $@\n";
+ok(isweak($b));
+ok($$b eq "hello") or print "#b is '$$b'\n";
 
 package Dest;
 
--- ./sv.c.orig	2003-11-16 20:01:12.000000000 +0000
+++ ./sv.c	2003-11-16 20:14:25.000000000 +0000
@@ -4649,6 +4649,7 @@
 	    && how != PERL_MAGIC_bm
 	    && how != PERL_MAGIC_fm
 	    && how != PERL_MAGIC_sv
+	    && how != PERL_MAGIC_backref
 	   )
 	{
 	    Perl_croak(aTHX_ PL_no_modify);

p5pRT · 2003-12-02T23:19:16Z

From fergal@esatclear.ie

3rd attempt to get this mail through to the list...

With blead, 5.8.2 and probably all perls that have weak refs

perl -MScalar::Util=weaken -e '$a=\"h";weaken($a)'
Modification of a read-only value attempted at -e line 1

Any good reason for this? It's not documented and it's a pain.

If you want to weaken refs of unknown origin (eg in a generic cache module)
then you actually have to do

eval {
local $SIG{__DIE__};
weaken($ref);
};

And also you end up with read only refs that don't get garbage collected when
they should [1].

Patch for blead attached to fix this by adding PERL_MAGIC_backref to the list
of things that can be done to a readonly value. Also applied to 5.8 and maybe
5.6.

I added some tests, everything still passes, is there some other reason not to
allow this?

F

[1] It's rare that you get a GC'able read only ref but

$r = eval '\"hello"'

produces one.

p5pRT · 2003-12-02T23:19:16Z

From fergal@esatclear.ie

weaken.patch

--- ./ext/List/Util/t/weak.t.orig	2003-12-02 23:13:36.000000000 +0000
+++ ./ext/List/Util/t/weak.t	2003-12-02 23:13:47.000000000 +0000
@@ -35,7 +35,7 @@
 
 eval <<'EOT' unless $skip;
 use Scalar::Util qw(weaken isweak);
-print "1..17\n";
+print "1..22\n";
 
 ######################### End of black magic.
 
@@ -44,6 +44,7 @@
 sub ok {
 	++$cnt;
 	if($_[0]) { print "ok $cnt\n"; } else {print "not ok $cnt\n"; }
+	return $_[0];
 }
 
 $| = 1;
@@ -205,6 +206,20 @@
 ok(isweak($x->{Y}));
 ok(!isweak($x->{Z}));
 
+#
+# Case 7: test weaken on a read only ref
+#
+
+$a = eval '\"hello"';
+ok(ref($a)) or print "# didn't get a ref from eval\n";
+$b = $a;
+eval{weaken($b)};
+# we didn't die
+ok($@ eq "") or print "# died with $@\n";
+ok(isweak($b));
+ok($$b eq "hello") or print "# b is '$$b'\n";
+$a="";
+ok(not $b) or print "# b didn't go away\n";
 
 package Dest;
 
--- ./sv.c.orig	2003-12-02 23:13:37.000000000 +0000
+++ ./sv.c	2003-12-02 23:13:47.000000000 +0000
@@ -4896,6 +4896,7 @@
 	    && how != PERL_MAGIC_bm
 	    && how != PERL_MAGIC_fm
 	    && how != PERL_MAGIC_sv
+	    && how != PERL_MAGIC_backref
 	   )
 	{
 	    Perl_croak(aTHX_ PL_no_modify);

p5pRT · 2003-12-02T23:30:16Z

From @rgs

Fergal Daly wrote:

With blead, 5.8.2 and probably all perls that have weak refs

perl -MScalar::Util=weaken -e '$a=\"h";weaken($a)'
Modification of a read-only value attempted at -e line 1

Any good reason for this? It's not documented and it's a pain.

If you want to weaken refs of unknown origin (eg in a generic cache module)
then you actually have to do

eval {
local $SIG{__DIE__};
weaken($ref);
};

And also you end up with read only refs that don't get garbage collected when
they should [1].

Yes.

Patch for blead attached to fix this by adding PERL_MAGIC_backref to the list
of things that can be done to a readonly value. Also applied to 5.8 and maybe
5.6.

This makes me a bit nervous, but I think it's correct ; in fact I was looking
at it currently.

I added some tests, everything still passes, is there some other reason not to
allow this?

I noticed a Point of Style : you patched the core but added a test in
a CPAN module. This test will obviously fail when backported to CPAN. There
are two solutions -- either put the test in a separate file, or skip it for
$] < 5.009 or $] == 5.009 && ! $ENV{PERL_CORE}.

Graham, any comments on this ?

p5pRT · 2003-12-03T22:19:46Z

From @gbarr

On 2 Dec 2003, at 23:31, Rafael Garcia-Suarez wrote:

I added some tests, everything still passes, is there some other
reason not to
allow this?

I noticed a Point of Style : you patched the core but added a test in
a CPAN module. This test will obviously fail when backported to CPAN.
There
are two solutions -- either put the test in a separate file, or skip
it for
$] < 5.009 or $] == 5.009 && ! $ENV{PERL_CORE}.

Graham, any comments on this ?

I would make it so that the test is skipped for $] < 5.009

Graham.

p5pRT · 2003-12-04T20:17:15Z

From fergal@esatclear.ie

On Wednesday 03 December 2003 22:19, Graham Barr via RT wrote:

I would make it so that the test is skipped for $] < 5.009

Graham.

Before I do that, any chance I could convert the script to use Test::More as
the testing framework? It makes everything a bit easier. I'll even convert
the other test scripts for free,

F

p5pRT · 2003-12-25T19:20:24Z

From @rgs

Graham Barr wrote:

On 2 Dec 2003, at 23:31, Rafael Garcia-Suarez wrote:

I added some tests, everything still passes, is there some other
reason not to
allow this?

I noticed a Point of Style : you patched the core but added a test in
a CPAN module. This test will obviously fail when backported to CPAN.
There
are two solutions -- either put the test in a separate file, or skip
it for
$] < 5.009 or $] == 5.009 && ! $ENV{PERL_CORE}.

Graham, any comments on this ?

I would make it so that the test is skipped for $] < 5.009

OK. I thus applied Fergal's patch (thanks) as:

Change 21955 by rgs@rgs-home on 2003/12/25 18:59:54

Subject: [perl #24506] [PATCH] cannot weaken refs to read only values
From: Fergal Daly <fergal@esatclear.ie>
Date: Tue, 2 Dec 2003 23:18:18 +0000
Message-Id: <200312022318.18353.fergal@esatclear.ie>

(tweaked so the test is skipped on perls < 5.9.0)

Affected files ...

... //depot/perl/ext/List/Util/t/weak.t#3 edit
... //depot/perl/sv.c#707 edit

p5pRT · 2003-12-25T19:37:49Z

@rgs - Status changed from 'new' to 'resolved'

p5pRT · 2003-12-29T08:55:14Z

From @gbarr

On 25 Dec 2003, at 19:25, Rafael Garcia-Suarez wrote:

Graham Barr wrote:

On 2 Dec 2003, at 23:31, Rafael Garcia-Suarez wrote:

I added some tests, everything still passes, is there some other
reason not to
allow this?

I noticed a Point of Style : you patched the core but added a test in
a CPAN module. This test will obviously fail when backported to CPAN.
There
are two solutions -- either put the test in a separate file, or skip
it for
$] < 5.009 or $] == 5.009 && ! $ENV{PERL_CORE}.

Graham, any comments on this ?

I would make it so that the test is skipped for $] < 5.009

OK. I thus applied Fergal's patch (thanks) as:

Change 21955 by rgs@rgs-home on 2003/12/25 18:59:54
    Subject&#8203;: \[perl \#24506\] \[PATCH\] cannot weaken refs to read only 
values
From: Fergal Daly <fergal@esatclear.ie>
Date: Tue, 2 Dec 2003 23:18:18 +0000
Message-Id: <200312022318.18353.fergal@esatclear.ie>
    $tweaked so the test is skipped on perls \< 5\.9\.0$

I saw that, Thanks.

I am a bit tied up with family stuff over the holidays, but I will do a
CPAN release when I can.

Graham.

p5pRT closed this as completed Dec 25, 2003

p5pRT added Severity Low distro-All hasPatch labels Oct 18, 2019

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cannot weaken refs to read only values #6930

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cannot weaken refs to read only values #6930

cannot weaken refs to read only values #6930

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p5pRT commented Nov 16, 2003

p5pRT commented Nov 16, 2003

From fergal@esatclear.ie

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