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Id: 54728
Status: open
Priority: 0/
Queue: perl5

Owner: Nobody
Requestors: jdb <jand [at] activestate.com>
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Operating System: (no value)
PatchStatus: (no value)
Severity: low
Type: unknown
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Fixed In: (no value)



Subject: PathTools-3.27 triggers a bug in Perl
Date: Fri, 23 May 2008 11:00:43 -0700
To: <perlbug [...] perl.org>
From: "Jan Dubois" <jand [...] activestate.com>
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The update to PathTools-3.27 triggers a bug in the localization of $1: #!perl use File::Spec::Win32 (); print File::Spec::Win32->catfile("c:/foo", "bar"), "\n"; "bar" =~ m/(.*)/; print File::Spec::Win32->catfile("c:/foo", $1), "\n"; "bar" =~ m/(.*)/; print File::Spec::Win32->catfile("c:/foo", "$1"), "\n"; __END__ With PathTools-3.27: C:\foo\bar C:\foo\c: C:\foo\bar With older version of PathTools: C:\foo\bar C:\foo\bar C:\foo\bar The problem seems to be that $1 isn't properly localized. A simple workaround in File::Spec::Win32 is to stringify @_ in _canon_cat(), e.g. sub _canon_cat(@) { @_ = map "$_", @_; my $first = shift; But it would be nicer if the problem can be solved in a more general manner. I encountered this problem when I was testing Pod::Simple with 5.8.9-tobe, which contains this code triggering the problem in Pod::Simple::HTMLBatch: if( ref($chunk->[-1]) and $url =~ m{^(_[-a-z0-9_]+\.css$)} ) { $outfile = $self->filespecsys->catfile( $outdir, $1 ); Cheers, -Jan
Subject: Re: [perl #54728] PathTools-3.27 triggers a bug in Perl
Date: Sat, 24 May 2008 10:42:46 +0200
To: perl5-porters [...] perl.org
From: "Rafael Garcia-Suarez" <rgarciasuarez [...] gmail.com>
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2008/5/23 via RT Jan Dubois <perlbug-followup@perl.org>: Show quoted text
> The update to PathTools-3.27 triggers a bug in the localization of $1: > > #!perl > use File::Spec::Win32 (); > > print File::Spec::Win32->catfile("c:/foo", "bar"), "\n"; > "bar" =~ m/(.*)/; > print File::Spec::Win32->catfile("c:/foo", $1), "\n"; > "bar" =~ m/(.*)/; > print File::Spec::Win32->catfile("c:/foo", "$1"), "\n"; > __END__ > > With PathTools-3.27: > C:\foo\bar > C:\foo\c: > C:\foo\bar
Does that happen also with older perls ?
Subject: RE: [perl #54728] PathTools-3.27 triggers a bug in Perl
Date: Sat, 24 May 2008 07:51:59 -0700
To: "'Rafael Garcia-Suarez'" <rgarciasuarez [...] gmail.com>, <perl5-porters [...] perl.org>
From: "Jan Dubois" <jand [...] activestate.com>
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On Sat, 24 May 2008, Rafael Garcia-Suarez wrote: Show quoted text
> > 2008/5/23 via RT Jan Dubois <perlbug-followup@perl.org>:
> > The update to PathTools-3.27 triggers a bug in the localization of $1: > > > > #!perl > > use File::Spec::Win32 (); > > > > print File::Spec::Win32->catfile("c:/foo", "bar"), "\n"; > > "bar" =~ m/(.*)/; > > print File::Spec::Win32->catfile("c:/foo", $1), "\n"; > > "bar" =~ m/(.*)/; > > print File::Spec::Win32->catfile("c:/foo", "$1"), "\n"; > > __END__ > > > > With PathTools-3.27: > > C:\foo\bar > > C:\foo\c: > > C:\foo\bar
> > Does that happen also with older perls ?
Yes, it happens with 5.6.1 and 5.8.0 too. All you have to do to reproduce it is copy the lib/File/Spec/Win32.pm into an older Perl to test this. The problem itself is not Windows specific; the test program above fails the same way on Linux too. Cheers, -Jan
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On Sat May 24 07:52:30 2008, jdb wrote: Show quoted text
> On Sat, 24 May 2008, Rafael Garcia-Suarez wrote:
> > > > 2008/5/23 via RT Jan Dubois <perlbug-followup@perl.org>:
> > > The update to PathTools-3.27 triggers a bug in the localization of $1: > > > > > > #!perl > > > use File::Spec::Win32 (); > > > > > > print File::Spec::Win32->catfile("c:/foo", "bar"), "\n"; > > > "bar" =~ m/(.*)/; > > > print File::Spec::Win32->catfile("c:/foo", $1), "\n"; > > > "bar" =~ m/(.*)/; > > > print File::Spec::Win32->catfile("c:/foo", "$1"), "\n"; > > > __END__ > > > > > > With PathTools-3.27: > > > C:\foo\bar > > > C:\foo\c: > > > C:\foo\bar
> > > > Does that happen also with older perls ?
> > Yes, it happens with 5.6.1 and 5.8.0 too. > > All you have to do to reproduce it is copy the lib/File/Spec/Win32.pm > into an older Perl to test this. The problem itself is not Windows > specific; the test program above fails the same way on Linux too. > > Cheers, > -Jan >
Path-Tools is a CPAN distribution. In the Perl 5 core distribution, File::Spec appears under 'dist/Cwd/'. So I don't see why we should be concerned with Path-Tools per se. Be that as it may, I can't seem to reproduce this on Perl 5.18.0 on Unix-ish systems (Darwin/PPC; Linux/i386): ##### $ cat 54728_catfile.pl use File::Spec (); print File::Spec->catfile("c:/foo", "bar"), "\n"; "bar" =~ m/(.*)/; print File::Spec->catfile("c:/foo", $1), "\n"; "bar" =~ m/(.*)/; print File::Spec->catfile("c:/foo", "$1"), "\n"; ##### $ perl 54728_catfile.pl c:/foo/bar c:/foo/bar c:/foo/bar ##### Is this ticket closable? Thank you very much. Jim Keenan
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On Sat Aug 03 17:05:29 2013, jkeenan wrote: Show quoted text
> Path-Tools is a CPAN distribution. In the Perl 5 core distribution, > File::Spec appears under 'dist/Cwd/'. So I don't see why we should be > concerned with Path-Tools per se.
dist/Cwd *is* PathTools. blead is upstream. The CPAN distribution name happens to be different from the directory name in blead. I don’t know the history of that. Bugs relating to PathTools modules in general can go in this tracker. Bugs relating to the CPAN (e.g., the Makefile.PL, which is not in blead) packaging go in the CPAN queue. At least that’s how I report them. In actuality, both categories of bugs end up in both queues. Yes, it’s a mess. (And I don’t know why the CPAN Makefile.PL cannot be in core.) Show quoted text
> Is this ticket closable?
I don’t know the answer to that. -- Father Chrysostomos
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On Sat Aug 03 17:29:47 2013, sprout wrote: Show quoted text
> On Sat Aug 03 17:05:29 2013, jkeenan wrote:
> > Path-Tools is a CPAN distribution. In the Perl 5 core distribution, > > File::Spec appears under 'dist/Cwd/'. So I don't see why we should be > > concerned with Path-Tools per se.
> > dist/Cwd *is* PathTools. blead is upstream. The CPAN distribution name > happens to be different from the directory name in blead. I don’t know > the history of that. > > Bugs relating to PathTools modules in general can go in this tracker. > Bugs relating to the CPAN (e.g., the Makefile.PL, which is not in blead) > packaging go in the CPAN queue. At least that’s how I report them. In > actuality, both categories of bugs end up in both queues. > > Yes, it’s a mess.
Sorry, that was meant to be an informative note, but it turned into a rant. -- Father Chrysostomos
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Show quoted text
> > Is this ticket closable?
> > I don’t know the answer to that. >
tested on linux, perl 5.10.1 bug persists with PathTools 3.27 3.26 no bug in 3.2501 3.29 diff: https://metacpan.org/diff/release/KWILLIAMS/PathTools-3.27/SMUELLER/PathTools-3.29#lib/File/Spec/Win32.pm seems bug is fixed, there is changelog entry: https://metacpan.org/source/SMUELLER/PathTools-3.29/Changes#L24 related tickets: http://perlmonks.org/?node_id=671025 https://rt.cpan.org/Public/Bug/Display.html?id=33675
CC: pp Porters <perl5-porters [...] perl.org>
Subject: Re: [perl #54728] PathTools-3.27 triggers a bug in Perl
Date: Sun, 4 Aug 2013 16:12:46 +0300
To: perlbug-followup [...] perl.org
From: Alexandr Ciornii <alexchorny [...] gmail.com>
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This ticket can be viewed as two tickets: 1. Bug in PathTools (fixed) 2. This problem: #!perl "2" =~ m/(.*)/; print test1("1", $1), "\n"; sub test1 { 'aa' =~ m/(.)/; return $_[0].'/'.$_[1]; } prints "1/a" on perl 5.18.0. Can it be considered a bug? 2013/8/4 James E Keenan via RT <perlbug-followup@perl.org>: Show quoted text
> On Sat May 24 07:52:30 2008, jdb wrote:
>> On Sat, 24 May 2008, Rafael Garcia-Suarez wrote:
>> > >> > 2008/5/23 via RT Jan Dubois <perlbug-followup@perl.org>:
>> > > The update to PathTools-3.27 triggers a bug in the localization of $1: >> > > >> > > #!perl >> > > use File::Spec::Win32 (); >> > > >> > > print File::Spec::Win32->catfile("c:/foo", "bar"), "\n"; >> > > "bar" =~ m/(.*)/; >> > > print File::Spec::Win32->catfile("c:/foo", $1), "\n"; >> > > "bar" =~ m/(.*)/; >> > > print File::Spec::Win32->catfile("c:/foo", "$1"), "\n"; >> > > __END__ >> > > >> > > With PathTools-3.27: >> > > C:\foo\bar >> > > C:\foo\c: >> > > C:\foo\bar
>> > >> > Does that happen also with older perls ?
>> >> Yes, it happens with 5.6.1 and 5.8.0 too. >> >> All you have to do to reproduce it is copy the lib/File/Spec/Win32.pm >> into an older Perl to test this. The problem itself is not Windows >> specific; the test program above fails the same way on Linux too. >> >> Cheers, >> -Jan >>
> > > Path-Tools is a CPAN distribution. In the Perl 5 core distribution, > File::Spec appears under 'dist/Cwd/'. So I don't see why we should be > concerned with Path-Tools per se. > > Be that as it may, I can't seem to reproduce this on Perl 5.18.0 on > Unix-ish systems (Darwin/PPC; Linux/i386): > > ##### > $ cat 54728_catfile.pl > use File::Spec (); > > print File::Spec->catfile("c:/foo", "bar"), "\n"; > "bar" =~ m/(.*)/; > print File::Spec->catfile("c:/foo", $1), "\n"; > "bar" =~ m/(.*)/; > print File::Spec->catfile("c:/foo", "$1"), "\n"; > ##### > $ perl 54728_catfile.pl > c:/foo/bar > c:/foo/bar > c:/foo/bar > ##### > > Is this ticket closable? > > Thank you very much. > Jim Keenan > > --- > via perlbug: queue: perl5 status: open > https://rt.perl.org:443/rt3/Ticket/Display.html?id=54728
-- Alexandr Ciornii, http://chorny.net
Subject: Re: [perl #54728] PathTools-3.27 triggers a bug in Perl
Date: Sun, 04 Aug 2013 15:51:31 +0200
To: perl5-porters [...] perl.org
From: Lukas Mai <plokinom [...] gmail.com>
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On 04.08.2013 15:12, Alexandr Ciornii wrote: Show quoted text
> This ticket can be viewed as two tickets: > 1. Bug in PathTools (fixed) > 2. This problem: > #!perl > > "2" =~ m/(.*)/; > print test1("1", $1), "\n"; > > sub test1 { > 'aa' =~ m/(.)/; > return $_[0].'/'.$_[1]; > } > prints "1/a" on perl 5.18.0. > > Can it be considered a bug?
I don't see it as a bug. 1) "2" =~ m/(.*)/ sets $1 to "2". 2) test1("1", $1) calls test1 with "1" and $1. 3) 'aa' =~ m/(.)/ sets $1 to "a". 4) $_[0].'/'.$_[1] builds a string from the arguments passed to test1. Perl passes arguments by alias. $_[0] is an alias to "1". $_[1] is an alias to $1, which currently contains "a". 5) Thus test1 returns "1/a". 6) Match variables are implicitly localized, so $1 reverts to "2" here. 7) "1/a\n" is printed. -- Lukas Mai <plokinom@gmail.com>
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On Sun Aug 04 06:14:16 2013, chorny wrote: Show quoted text
> This ticket can be viewed as two tickets: > 1. Bug in PathTools (fixed) > 2. This problem: > #!perl > > "2" =~ m/(.*)/; > print test1("1", $1), "\n"; > > sub test1 { > 'aa' =~ m/(.)/; > return $_[0].'/'.$_[1]; > } > prints "1/a" on perl 5.18.0. > > Can it be considered a bug? > >
No, I don't think so. With this line: Show quoted text
> 'aa' =~ m/(.)/;
... you have made a new assignment to global variable $1. Hence, you have changed the value to which $_[1] is aliased. From 'pod/perlsub.pod' (blead): ##### Any arguments passed in show up in the array @_. Therefore, if you called a function with two arguments, those would be stored in $_[0] and $_[1]. The array @_ is a local array, but its elements are aliases for the actual scalar parameters. In particular, if an element $_[0] is updated, the corresponding argument is updated (or an error occurs if it is not updatable). ##### I'll take this ticket for the purpose of closing it within 7 days unless someone thinks we still have a bug in Perl. Thank you very much. Jim Keenan
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Let's try replace "$1" with $x in your example and explanation (and localize it manually): ######## our $x; $x = 2; print test1("1", $x), "\n"; sub test1 { local $x; $x = 'a'; return $_[0].'/'.$_[1]; } ######## 1) $x=2 sets $x to "2". 2) test1("1", $1) calls test1 with "1" and $x. 3) $x = 'a' sets $x to "a". 4) $_[0].'/'.$_[1] builds a string from the arguments passed to test1. Perl passes arguments by alias. $_[0] is an alias to "1". $_[1] is an alias to $x, which currently contains "a". 5) Thus test1 returns "1/a". 6) $x is explicitly localized, so $x reverts to "2" here. 7) "1/a\n" is printed. ######### However it's not true. This example with "$x" prints 1/2 Anyway, even if we find it's not a bug, it's unique situation and can be documented (btw who is responsible for that kind of side effects - caller or subroutine?) On Sun Aug 04 06:52:21 2013, plokinom@gmail.com wrote: Show quoted text
> On 04.08.2013 15:12, Alexandr Ciornii wrote:
> > This ticket can be viewed as two tickets: > > 1. Bug in PathTools (fixed) > > 2. This problem: > > #!perl > > > > "2" =~ m/(.*)/; > > print test1("1", $1), "\n"; > > > > sub test1 { > > 'aa' =~ m/(.)/; > > return $_[0].'/'.$_[1]; > > } > > prints "1/a" on perl 5.18.0. > > > > Can it be considered a bug?
> > I don't see it as a bug. > > 1) "2" =~ m/(.*)/ sets $1 to "2". > 2) test1("1", $1) calls test1 with "1" and $1. > 3) 'aa' =~ m/(.)/ sets $1 to "a". > 4) $_[0].'/'.$_[1] builds a string from the arguments passed to test1. > Perl passes arguments by alias. > $_[0] is an alias to "1". > $_[1] is an alias to $1, which currently contains "a". > 5) Thus test1 returns "1/a". > 6) Match variables are implicitly localized, so $1 reverts to "2" here. > 7) "1/a\n" is printed. >
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So, it looks like a bug to me. 1) two explanation above, with aliasing and localized global vars does not apply for real localized glabal vars. 2) Sometimes subroutines just cannot copy arguments ( my ($a, $b) = @_ ) due to performance reasons (COW can help here?) And if it uses regexps, there is no way to prevent such issues, except documenting this in subroutine interface documentation. (even my ($m1, $m2) = /(...)(.)/ doest not help. enclosing each regexp in { } probably helps, unless it uses one of aliased arguments ) On Sun Aug 04 11:13:19 2013, vsespb wrote: Show quoted text
> Let's try replace "$1" with $x in your example and explanation (and > localize it manually): > > ######## > our $x; > > $x = 2; > print test1("1", $x), "\n"; > > sub test1 { > local $x; > $x = 'a'; > return $_[0].'/'.$_[1]; > } > ######## > > 1) $x=2 sets $x to "2". > 2) test1("1", $1) calls test1 with "1" and $x. > 3) $x = 'a' sets $x to "a". > 4) $_[0].'/'.$_[1] builds a string from the arguments passed to test1. > Perl passes arguments by alias. > $_[0] is an alias to "1". > $_[1] is an alias to $x, which currently contains "a". > 5) Thus test1 returns "1/a". > 6) $x is explicitly localized, so $x reverts to "2" here. > 7) "1/a\n" is printed. > > ######### > > However it's not true. This example with "$x" prints 1/2 > > Anyway, even if we find it's not a bug, it's unique situation and can be > documented (btw who is responsible for that kind of side effects - > caller or subroutine?) > > On Sun Aug 04 06:52:21 2013, plokinom@gmail.com wrote:
> > On 04.08.2013 15:12, Alexandr Ciornii wrote:
> > > This ticket can be viewed as two tickets: > > > 1. Bug in PathTools (fixed) > > > 2. This problem: > > > #!perl > > > > > > "2" =~ m/(.*)/; > > > print test1("1", $1), "\n"; > > > > > > sub test1 { > > > 'aa' =~ m/(.)/; > > > return $_[0].'/'.$_[1]; > > > } > > > prints "1/a" on perl 5.18.0. > > > > > > Can it be considered a bug?
> > > > I don't see it as a bug. > > > > 1) "2" =~ m/(.*)/ sets $1 to "2". > > 2) test1("1", $1) calls test1 with "1" and $1. > > 3) 'aa' =~ m/(.)/ sets $1 to "a". > > 4) $_[0].'/'.$_[1] builds a string from the arguments passed to test1. > > Perl passes arguments by alias. > > $_[0] is an alias to "1". > > $_[1] is an alias to $1, which currently contains "a". > > 5) Thus test1 returns "1/a". > > 6) Match variables are implicitly localized, so $1 reverts to "2" here. > > 7) "1/a\n" is printed. > >
> >
CC: perl5-porters [...] perl.org
Subject: Re: [perl #54728] PathTools-3.27 triggers a bug in Perl
Date: Mon, 5 Aug 2013 11:50:54 +0100
To: Victor Efimov via RT <perlbug-followup [...] perl.org>
From: Dave Mitchell <davem [...] iabyn.com>
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On Sun, Aug 04, 2013 at 04:28:32PM -0700, Victor Efimov via RT wrote: Show quoted text
> So, it looks like a bug to me. > > 1) two explanation above, with aliasing and localized global vars does > not apply for real localized glabal vars.
Except that $1 etc are *not* localised. The thing that is localised is the current match (PL_curpm internally). $1 et al are just magic vars that retrieve values from the current match on request. -- Spock (or Data) is fired from his high-ranking position for not being able to understand the most basic nuances of about one in three sentences that anyone says to him. -- Things That Never Happen in "Star Trek" #19
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On Mon Aug 05 03:51:38 2013, davem wrote: Show quoted text
> On Sun, Aug 04, 2013 at 04:28:32PM -0700, Victor Efimov via RT wrote:
> > So, it looks like a bug to me. > > > > 1) two explanation above, with aliasing and localized global vars does > > not apply for real localized glabal vars.
> > Except that $1 etc are *not* localised. The thing that is localised is the > current match (PL_curpm internally). $1 et al are just magic vars that > retrieve values from the current match on request. > >
Isn't it unclearly documented? (both cases declared as "dynamically-scoped") http://perldoc.perl.org/perlvar.html ######## Most of the special variables related to regular expressions are side effects. Perl sets these variables when it has a successful match, so you should check the match result before using them. For instance: These variables are read-only and dynamically-scoped, unless we note otherwise. The dynamic nature of the regular expression variables means that their value is limited to the block that they are in, as demonstrated by this bit of code: ######## http://perldoc.perl.org/perlsub.html ######## A local modifies its listed variables to be "local" to the enclosing block, eval, or do FILE --and to any subroutine called from within that block. A local just gives temporary values to global (meaning package) variables. It does not create a local variable. This is known as dynamic scoping. ######## Also, isn't this a misfeature? Seems we're going drop things like empty regexps https://rt.perl.org/rt3/Ticket/Display.html?id=119095 (although it is documented). And this issue much more useless and unclear imho.
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Subject: Re: [perl #54728] PathTools-3.27 triggers a bug in Perl
Date: Tue, 6 Aug 2013 12:24:04 +0100
To: Victor Efimov via RT <perlbug-followup [...] perl.org>
From: Dave Mitchell <davem [...] iabyn.com>
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On Mon, Aug 05, 2013 at 08:42:48AM -0700, Victor Efimov via RT wrote: Show quoted text
> On Mon Aug 05 03:51:38 2013, davem wrote:
> > On Sun, Aug 04, 2013 at 04:28:32PM -0700, Victor Efimov via RT wrote:
> > > So, it looks like a bug to me. > > > > > > 1) two explanation above, with aliasing and localized global vars does > > > not apply for real localized glabal vars.
> > > > Except that $1 etc are *not* localised. The thing that is localised is the > > current match (PL_curpm internally). $1 et al are just magic vars that > > retrieve values from the current match on request. > > > >
> > Isn't it unclearly documented?
Quite possibly Show quoted text
> (both cases declared as "dynamically-scoped")
That's because they're both dynamically (as opposed to lexically) scoped. Show quoted text
> Also, isn't this a misfeature?
I don't see why. Show quoted text
> > Seems we're going drop things like empty regexps > https://rt.perl.org/rt3/Ticket/Display.html?id=119095 (although it is > documented). And this issue much more useless and unclear imho.
If we were to set every capture var ($1,$2,...) on every match, rather than just on demand, regexes would be an awful lot slower. -- Never work with children, animals, or actors.
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On Tue Aug 06 04:24:55 2013, davem wrote: Show quoted text
> On Mon, Aug 05, 2013 at 08:42:48AM -0700, Victor Efimov via RT wrote: >
> > Also, isn't this a misfeature?
> > I don't see why.
> >
a) is there a valid case when user indeed wants 'aa' =~ m/(.)/ to modify $_[1], in code like this? "2" =~ m/(.*)/; print test1("1", $1), "\n"; sub test1 { 'aa' =~ m/(.)/; return $_[0].'/'.$_[1]; } b) I think using regexps with capture groups, before using $_[], or @_, or &othersub; without expecting side effects is much more usable than this feature.
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I've tried to grep CPAN to find examples of use of regexps before @_ so here they are: ## bug: https://metacpan.org/source/ADAMK/Params-Util-1.07/lib/Params/Util.pm#L191 (https://rt.cpan.org/Public/Bug/Display.html?id=87649 https://rt.cpan.org/Public/Bug/Display.html?id=81276) ## possible bug: https://metacpan.org/source/FLORA/NEXT-0.65/lib/NEXT.pm#L147 https://metacpan.org/source/ADAMK/pip-1.19/lib/pip.pm#L99 ## not affected, because no capture groups: https://metacpan.org/source/PJF/autodie-2.20/lib/Fatal.pm#L194 https://metacpan.org/source/MELO/Redis-1.961/lib/Redis.pm#L101 https://metacpan.org/source/FLORA/bignum-0.32/lib/bigint.pm#L34 https://metacpan.org/source/TOBYINK/Test-Tabs-0.003/lib/Test/Tabs.pm#L148 https://metacpan.org/source/GAAS/Data-Dump-1.22/lib/Data/Dump.pm#L86 https://metacpan.org/source/STBEY/Date-Calc-6.3/lib/Date/Calendar/Year.pm#L87 ## not affected: private subroutine, no possibility to call with $1, etc https://metacpan.org/source/ADAMK/pip-1.19/lib/pip.pm#L99 https://metacpan.org/source/FLORA/bignum-0.32/lib/bigint.pm#L205 https://metacpan.org/source/GAAS/HTTP-Cookies-6.01/lib/HTTP/Cookies.pm#L598 On Tue Aug 06 04:42:19 2013, vsespb wrote: Show quoted text
> On Tue Aug 06 04:24:55 2013, davem wrote:
> > On Mon, Aug 05, 2013 at 08:42:48AM -0700, Victor Efimov via RT wrote: > >
> > > Also, isn't this a misfeature?
> > > > I don't see why.
> > >
> > a) is there a valid case when user indeed wants 'aa' =~ m/(.)/ to modify > $_[1], in code like this? > > "2" =~ m/(.*)/; > print test1("1", $1), "\n"; > > sub test1 { > 'aa' =~ m/(.)/; > return $_[0].'/'.$_[1]; > } > > b) I think using regexps with capture groups, before using $_[], or @_, > or &othersub; without expecting side effects is much more usable than > this feature. >
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Subject: Re: [perl #54728] PathTools-3.27 triggers a bug in Perl
Date: Tue, 6 Aug 2013 14:31:39 +0100
To: Victor Efimov via RT <perlbug-followup [...] perl.org>
From: Dave Mitchell <davem [...] iabyn.com>
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On Tue, Aug 06, 2013 at 04:42:20AM -0700, Victor Efimov via RT wrote: Show quoted text
> On Tue Aug 06 04:24:55 2013, davem wrote:
> > On Mon, Aug 05, 2013 at 08:42:48AM -0700, Victor Efimov via RT wrote: > >
> > > Also, isn't this a misfeature?
> > > > I don't see why.
> > >
> > a) is there a valid case when user indeed wants 'aa' =~ m/(.)/ to modify > $_[1], in code like this? > > "2" =~ m/(.*)/; > print test1("1", $1), "\n"; > > sub test1 { > 'aa' =~ m/(.)/; > return $_[0].'/'.$_[1]; > }
Probably not, but in the same way that the user probably doesn't want sub f { $_ = 1 } to modify $_[0], but it will if called as f($_); In what ever way that $1 et al are implemented, there will be strange edge cases where it doesn't do what people expect. If we change it, it will just swap one set of edge cases for another. By not swapping, we at least a) don't break code which relies on the current behaviour; b) mange to do implement it in an efficient way. Show quoted text
> b) I think using regexps with capture groups, before using $_[], or @_, > or &othersub; without expecting side effects is much more usable than > this feature.
I think any code that passes raw global 'magic' vars as args to subs (e.g. without stringifying them a la "$1"), is asking for trouble: foo($1, $+[0], $_, $., $!); -- The Enterprise is involved in a bizarre time-warp experience which is in some way unconnected with the Late 20th Century. -- Things That Never Happen in "Star Trek" #14
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On Tue Aug 06 06:32:12 2013, davem wrote: Show quoted text
> Probably not, but in the same way that the user probably doesn't want > sub f { $_ = 1 } > to modify $_[0], but it will if called as > f($_);
if $_ is localized, then code sub f { local $_ = 1; print $_[0] } $_=2; f($_); print $_; prints 22, so $_[0] is not modified. is subroutine intention is to modify global $_, it should be documented, and modified $_ and even $_[0] is expected behaviour. subroutine that does not localize $_ and does not document this, subroutine is buggy imho. Show quoted text
> I think any code that passes raw global 'magic' vars as args to subs (e.g. > without stringifying them a la "$1"), is asking for trouble:
Ok, get it. So caller responsible for this. Then it's probably thing to report as feature request for perlcritic.
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On Sun Aug 04 06:56:58 2013, jkeenan wrote: Show quoted text
> On Sun Aug 04 06:14:16 2013, chorny wrote:
> > This ticket can be viewed as two tickets: > > 1. Bug in PathTools (fixed) > > 2. This problem: > > #!perl > > > > "2" =~ m/(.*)/; > > print test1("1", $1), "\n"; > > > > sub test1 { > > 'aa' =~ m/(.)/; > > return $_[0].'/'.$_[1]; > > } > > prints "1/a" on perl 5.18.0. > > > > Can it be considered a bug? > > > >
> > No, I don't think so. > > With this line: >
> > 'aa' =~ m/(.)/;
> > ... you have made a new assignment to global variable $1. Hence, you > have changed the value to which $_[1] is aliased. From > 'pod/perlsub.pod' (blead): > > ##### > Any arguments passed in show up in the array @_. > Therefore, if you called a function with two > arguments, those would be stored in $_[0] and > $_[1]. The array @_ is a local array, but its > elements are aliases for the actual scalar > parameters. In particular, if an element $_[0] is > updated, the corresponding argument is updated (or > an error occurs if it is not updatable). > ##### > > I'll take this ticket for the purpose of closing it within 7 days unless > someone thinks we still have a bug in Perl. >
Since there has been extensive back-and-forth in this RT since Sunday, I can no longer take the responsibility of closing it. So I'm reassigning it to the ever-popular Nobody. Nonetheless, I still don't believe there is a bug in Perl here. Thank you very much. Jim Keenan
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1) How about notice in perlvar: "It's not advised to pass directly regular expressions variables to function as arguments, because function can modify it before use. Use stringification instead: somefunc("$1")" before this paragraph: "Due to an unfortunate accident of Perl's implementation, use English imposes a considerable performance penalty on all regular expression matches" (I am not submitting patch, because almost sure exact wording can be improved) 2) Would it be useful if I try to improve tests for this (probably op/local.t, op/magic.t) to test current behaviour and submit a patch? On Tue Aug 06 06:44:56 2013, vsespb wrote: Show quoted text
> On Tue Aug 06 06:32:12 2013, davem wrote: >
> > Probably not, but in the same way that the user probably doesn't want > > sub f { $_ = 1 } > > to modify $_[0], but it will if called as > > f($_);
> > if $_ is localized, then code > > sub f { local $_ = 1; print $_[0] } > $_=2; > f($_); > print $_; > > prints 22, so $_[0] is not modified. > > is subroutine intention is to modify global $_, it should be documented, > and modified $_ and even $_[0] is expected behaviour. > > subroutine that does not localize $_ and does not document this, > subroutine is buggy imho. >
> > I think any code that passes raw global 'magic' vars as args to subs
(e.g. Show quoted text
> > without stringifying them a la "$1"), is asking for trouble:
> > Ok, get it. So caller responsible for this. Then it's probably thing to > report as feature request for perlcritic.
Subject: Re: [perl #54728] PathTools-3.27 triggers a bug in Perl
Date: Fri, 9 Aug 2013 10:26:37 +0000 (UTC)
To: perl5-porters [...] perl.org
From: Ed Avis <eda [...] waniasset.com>
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IMHO, this is a longstanding misfeature in Perl. It is easy to write our $options = 'debug'; sub foo { say 'debug flag set' if $options =~ /debug/; say $_[0] + $_[1]; } And it is also easy to write my $x = '123'; $x =~ /(\d+)/ or die; foo(5, $1); No warning is given in either case. So, depending on which way you look at it, you have a subroutine foo() which is buggy if called with $1, or you have some buggy user code calling it, depending on the details of how foo() is implemented. It would be far better if $1 were truly localized, so that the regexp match inside foo() did not have any effect on the $1 used by the calling code (which has been passed by reference). It would even be better, or at least easier to understand, if $1 were global. Then the puzzled programmer could see that calling foo() has modified $1 as a side effect. But at present $1 exists in a strange halfway house where it is neither truly global nor truly local. It's a global variable where Perl shuffles around the value behind the scenes; but this does not give the same behaviour as: our $o; sub foo { local $o = 'a'; say $_[0] + $_[1]; } local $o = 123; foo(5, $o); Even though foo() is setting the value of $o, this doesn't break the argument passing because $o is localized. If $1 were localized properly then it wouldn't break either. -- Ed Avis <eda@waniasset.com>
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Subject: Re: [perl #54728] PathTools-3.27 triggers a bug in Perl
Date: Mon, 12 Aug 2013 15:34:55 +0100
To: Ed Avis <eda [...] waniasset.com>
From: Dave Mitchell <davem [...] iabyn.com>
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On Fri, Aug 09, 2013 at 10:26:37AM +0000, Ed Avis wrote: Show quoted text
> But at present $1 exists in a strange halfway house where it is neither truly > global nor truly local. It's a global variable where Perl shuffles around the > value behind the scenes;
In this respect, $1 is no different than for example $. : sub f { print "$_[0]\n"; # prints 1 open my $fh, $0 or die; <$fh>; <$fh>; print "$_[0]\n"; # prints 2 } open my $fh, $0 or die; <$fh>; f($.); Show quoted text
> but this does not give the same behaviour as: > > our $o; > sub foo { > local $o = 'a'; > say $_[0] + $_[1]; > } > local $o = 123; > foo(5, $o); > > Even though foo() is setting the value of $o, this doesn't break the > argument passing because $o is localized. If $1 were localized properly > then it wouldn't break either.
Well yes, but for a successful match, this would involve: * for $1: * save the existing scalar slot of *1 on the savestack; * create a new SV and copy the relevant substring of the match string to it; * put that new SV in the scalar slot of *1; * on scope exit, undo the above and free the SV; * ditto for $2, and any other captures; * possibly ditto for $`, $&, $&; * possibly ditto for ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH}; * for @-: similarly localise, creating a new array with new indices; * something similar for @+, %-, %+; * possibly something similar for $+, $^N, $^R. Which collectively would impose a significant performance penalty for each successful match. Potentially these vars could remain magic, but when localised, a new var with the same magic attached could be created, where the magic points to a different match object (i.e. PL_curpm). This would avoid having to copy the relevant substring to each var until needed, but would still involve lots of SV creation and destruction and typeglob+savestack activity for each successful match. -- "Strange women lying in ponds distributing swords is no basis for a system of government. Supreme executive power derives from a mandate from the masses, not from some farcical aquatic ceremony." -- Dennis, "Monty Python and the Holy Grail"
Subject: Re: [perl #54728] PathTools-3.27 triggers a bug in Perl
Date: Tue, 20 Aug 2013 10:19:54 +0000 (UTC)
To: perl5-porters [...] perl.org
From: Ed Avis <eda [...] waniasset.com>
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Dave Mitchell <davem <at> iabyn.com> writes: Show quoted text
>>But at present $1 exists in a strange halfway house where it is neither >>truly global nor truly local.
You are right to note that $. and other punctuation variables have the same behaviour. I suggested that $1 should be 'localized properly' but you noted that this might involve quite a performance penalty for every regexp match. A simpler answer, I suggest, is that when variables like $1 and $. are passed as arguments to subroutines, a copy be made in a read-only temporary scalar. That would get rid of these perennial bug reports or surprises. How about it? -- Ed Avis <eda@waniasset.com>
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Subject: Re: [perl #54728] PathTools-3.27 triggers a bug in Perl
Date: Tue, 20 Aug 2013 14:32:30 +0400
To: Ed Avis <eda [...] waniasset.com>
From: Victor Efimov <victor [...] vsespb.ru>
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It looks to me that example with $. is irrelevant here. $. is not localized (unlike $1)

sub f {
    open my $fh, $0 or die;
    <$fh>; <$fh>;
    print $.;
}

open my $fh, $0 or die;
<$fh>;
print $.;
f();
print $.;

__END__
prints 122


2013/8/20 Ed Avis <eda@waniasset.com>
Show quoted text
Dave Mitchell <davem <at> iabyn.com> writes:

>>But at present $1 exists in a strange halfway house where it is neither
>>truly global nor truly local.

You are right to note that $. and other punctuation variables have the
same behaviour.

I suggested that $1 should be 'localized properly' but you noted that this
might involve quite a performance penalty for every regexp match.

A simpler answer, I suggest, is that when variables like $1 and $. are passed
as arguments to subroutines, a copy be made in a read-only temporary scalar.
That would get rid of these perennial bug reports or surprises.  How about it?

--
Ed Avis <eda@waniasset.com>








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