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This Week on perl5-porters - 6-12 January 2008

posted by grinder on 2008.01.16 7:59   Printer-friendly

This Week on perl5-porters - 6-12 January 2008

"Um, I think you caused me negative work. I rather wanted dtrace support and it seemed that because of you I didn't have to write it." -- Nicholas Clark, on the joys of having other people do your work for you.

Topics of Interest

Ever more strict

In the continuing "strict by default in 5.12" thread, Glenn Linderman made a most pertinent remark: Perl 6 has staked the "strict be default" claim. Perl 5 may approach the same place, but will have to do so more slowly. Not that it shouldn't be done, we need not burden ourselves with bad practices forever, but deprecation cycles and dealing with dormant CPAN modules and code in the DarkPAN means it will take time.

The other contentious issue (should extra strictures be activated from a feature pragma) was addressed by Abigail, who put forward the argument that feature should be considered no more than a message from the programmer to the interpreter indicating that constraints regarding backwards compatibility are hereby waived.

  advancing, unmasked
  http://xrl.us/bec8z 
  the darkpan, defined
  http://xrl.us/bedds 

~~ changing behaviour after using ==

brian d foy was puzzled upon learning that smart matching suffered from action at a distance, and wondered if it was all documented. David Landgren explained that it was a problem due to the IOK flag being set, which caused the smart matching code to use numishness, rather than stringiness as a basis for matching.

Zefram rightly pointed out that this was exactly the sort of problem that prevented Perl from being able to be used in very large projects. Nicholas Clark asked himself what would happen if the IOK flag was not set during a numish comparison, were the number not truly numeric. And so now blead does just that.

  no more action at a distance
  http://xrl.us/bec83 

dtrace and Perl

Andy Armstrong added dtrace support to perl to probe subroutine entry and exit points and was happy to announce that it incurs no measurable performance hit.

Nicholas Clark made a number of comments regarding the patch: things that need to be done and the ways to do it. He went ahead and cleared up the nits and applied it all.

  the patch itself
  http://xrl.us/bec85 

The main issue with the patch was dealing with Configure, partly because not many people have strong Configure-fu (and H.Merijn Brand was busy using his fu to attend to some sorely needed work on it).

Andy Dougherty helped out with a rough sketch of what needed to be done. Bryan Allen of pobox.com generously provided shell access to a Solaris box to get the kinks sorted out.

  dtrace sees delight
  http://xrl.us/bec87 

Once that was more or less under control, the next question to ask was what probes should we have? The following items were put forward:

  • Creation and destruction of SVs.

  • Interpreter set-up and tear down.

  • Module loading (use and require).

  • Filesystem access.

  • opcode arguments

One of things that needs to be addressed is that in order to trace the stack (C's or Perl's, really), a so-called ustack helper is what's needed, but these are fiendishly difficult to write. For a start, much of dtrace runs in the kernel, not in userland.

Much of the more esoteric discussion was cross-posted from the dtrace-discuss mailing list, where talk of ustacks certainly appeared to give them The Fear.

  http://xrl.us/bec89 

In the process of producing dtrace support, Andy ran into trouble with getting the shellish idiom of if-not-condition-then correct.

H.Merijn Brand showed one way to do it, Zefram, another.

  http://xrl.us/beda3 

grep and smart match should warn in void context

The discussion continued this week as to whether or not this would be a good idea. Michael Schwern started to jot down the history of why "map in void context" played such an important part in the history of Perl.

  we have a wiki, man
  http://xrl.us/beddu 

Abigail maintained that it would be insane to start warning about a construct that has not previously triggered a warning, even if the construct was not considered to be a best practice (in which case a Perl::Critic policy would be more suitable).

Zefram put forward a suggestion that was endorsed by Larry Wall.

  should we, should we not
  http://xrl.us/bec9b 

feature.pm needs some version comparison code somewhere

Rafaël Garcia-Suarez waved his hands vaguely at the new error that comes out of bleadperl -e 'use 5.10.1' and hoped something could be done about it. Steffen Müller showed how difficult it would be to solve the general case.

The only sane approach is for bleadperl to track feature sets in the current maintenance release. No new features could be added to 5.10.x after 5.12.0 comes out. Dave Mitchell thought add new features to a maintenance track was a bad idea anyway.

  still waiting for patches
  http://xrl.us/bec9d 

First pass at META.yml support for the core

Adam Kennedy took his YAML::Tiny module and made it even tinier by throwing everything that is not required to parse a META.yml file. He named it Parse::CPAN::Meta so that it doesn't say "YAML" on the tin. The idea being is that it will permit richer semantics in dealing with module requirements. This could come in handy for dual-lifed modules and modules that depend on external libraries.

Ævar Arnfjörð Bjarmason found a minor YAML remnant in the test suite.

  ain't what it used to be
  http://xrl.us/bec9f 

Segmentation violations with ithreads and regexps

Nicholas Clark was wondering what change caused a particular language construct to go boom! and asked Andreas König to fire up his magic binary search patch locator to find the offending change.

Andreas wasn't around, but Marcus Holland-Moritz was, and in about an hour had identified change #30755 as being the culprit. This enabled Nicholas to make the required correction.

  homing in
  http://xrl.us/bec9h 

Following that (if it's the same bug we're talking about) Dave Mitchell, Yves Orton and he kicked the problem around for a little while, and they appeared to arrive at the right solution by the end of the week.

  stirring up trouble with re 'eval'
  http://xrl.us/bec9j 

for (2..Inf)

Michael G. Schwern had a chance encounter with a C-style for loop, and realising that the range operation (..) used a lazy implementation, noted that it was a Simple Matter Of Programming to get for my $var (2..Inf) to Do The Right Thing. So he coded a quick and dirty patch (with a test!) to prove that it could be done. Much of the discussion that followed, apart from mentioning Ruby and bc involved what to do when adding 1 to an integer value overflows.

  from her to eternity
  http://xrl.us/bec9m 

Jerry D. Hedden picked up the patch and ran with it, delivering a whole pile of code, documentation and tests to implement the whole show. Michael made a number of good comments, so Jerry returned to the lab and cooked up a fresh patch.

One sticking point was whether the range should stop or not, when the value outgrew the maximum size of an integer. Yves thought that if that was a problem, people should really be using while (1).

But even that approach is flawed, since after a few billion or quintillion loops any integer-based counter will begin to lose precision (after it is upgraded to a float following the overflow an integer). And transparently switching over to Math::BigInt would be Really Hard.


  I do it my way
  http://xrl.us/bec9o 


What do we want to see in 5.12?

A number of porters described the innovations they would like to see in perl 5.12, the next major release after 5.10. Nicholas Clark commented that a lot of the suggestions had high niftiness values, but noted that the difference between suggesting and implementing will impose its own natural selection of what gets done.

Deprecated items removed in 5.12

Firstly, a number of items were marked as deprecated in 5.10 and thus may be removed from 5.12. All that could be recalled for the moment was the -P command-line switch and unnamed package; declarations (which actually disappeared in 5.10).

  all gone
  http://xrl.us/bec9q 

Am I a method call?

Ricardo Signes wanted support from the language to let him know if the routine had been called as a method. Currently, there is no mechanism that lets you do this in the general case. One possible approach is to do it by extending caller.

Given that caller already returns an impossible-to-remember list of values, Mark Jason Dominus wondered whether the last element should be a hashref, or whether an alternate hashified caller hashref should be made available. This suggestion attracted significant discussion, which evolved into calls for having autobox brought into core. Nicholas Clark warned against making that decision lightly.

  just so long as the hashref is purple
  http://xrl.us/bec9s 

Method declaration

Michael G. Schwern proposed a method foo declaration, that would work like a sub foo, except that it fetches the $self object from @_ for free.

The open issues are, can you call it as a bare subroutine, what if you don't like using the name $self, what about class methods, and should $self be removed from @_ or should it remain?

  http://xrl.us/bec9u 
  and it's on the wiki
  http://xrl.us/beddw 

Lexical named subroutines

Ricardo also suggested named lexical subroutines, to be used in situations such as comparator routines for sort, while avoiding namespace pollution.

Dave Mitchell pointed out the problems this would have in terms of closures. He sketched out two possible implementations and pointed out the flaws in both of them.

Nicholas Clark recalled that Mark Jason Dominus was believed to be in the possession of a list of examples of the corner cases that would need to be addressed in any such proposal. Dave thought he could come up with one of his own at a pinch.

  just a spoonful of sugar
  http://xrl.us/bec9w 

Anonymous packages

Ricardo also mentioned a conversation with Matt Trout concerning the issue of creating anonymous packages. Currently it can be hacked together using Package::Generator or Class::Mix. In any event, more thought needs to go into what the syntax should be.

  heh, inherit from *that*
  http://xrl.us/bec9y 

Named parameters

Michael then kicked off a very large thread by p5p standards, proposing real honest-to-goodness named parameters. One point he stressed was that what ever solution is implemented, it doesn't have to be perfect. We could apply the 80/20 rule and strive for a simple solution that is satisfactory 80% of the time.

Then the difficulties began to surface: should the parameters that are named be removed (or not) from @_ (see also: the method declaration thread). Should the parameters be aliases (fast, but dangerous) or copies (slow and safe, and conforms to the Principle of Least Surprise). What happens if not enough parameters are passed. Can we have defaults? How does this tie in with prototype specifications?

One useful item to come out of the discussion was a post from Juerd Waalboer who stepped of with a review of subroutine signatures in Perl 6, and is worth stealing from it for Perl 5. Another point is that, one way or another, we'll need better support for aliases in the language. Solve that problem, and named parameters are a piece of cake.

  http://xrl.us/bec92 

  also wikified
  http://xrl.us/beddy 

Saner behaviour for length

Aristotle Pagaltzis put forward an elegant suggestion to deal with length(undef). Rather than issuing a warning, it should quietly return undef. Rafaël Garcia-Suarez liked the idea so much that he tried to implement, but was stumped by a number of strange failures in the test suite.

Nicholas Clark picked up where he left off, and discovered that it is horrendously hard to get it to work correctly with tied variables. After a number of patches, the implementation was satisfactory, and in the process, Nicholas uncovered a bug involving (drum roll) UTF-8.

  how long is a string
  http://xrl.us/bec94 


Patches of Interest

Marcus Holland-Moritz's refactoring of PL_opargs generation in opcode.pl from last week was applied. As was Jerry Hedden's File::Temp cleanup and his ~~ is not a feature documentation tweak. The perlcommunity.pod patch from brian d foy was also applied, despite being in a yucky diff format. \X is equivalent to an atomic group made it too.

  see last week for URLs

Change backticks into more readable qx()

Jari Aalto put forward the sensible suggestion of using qx() instead of backticks (`) since it's to confuse with an apostrophe, and some non-English keyboards make it quite difficult to generate.

Tels fell of his chair since he didn't even know that qx() existed, but made up for it by spotting an leftover ` in Jari's patch.

  told you they were hard to see
  http://xrl.us/bec96 

No more big slowdown in 5.10 @_ parameter passing

Rick Delaney picked up the gauntlet and delivered a patch to fix the issue identified by Dave Mitchell, where the OPpASSIGN_COMMON wound up being set in error. The amazing part was that Rick didn't even believe there was a performance problem, but fixed it anyway. When the benchmarks were run, it was clear there was a net gain.

  Dave the M, right as usual
  http://xrl.us/bec98 

Getting Data::Dumper to dump the new-fangled REGEXPs

Yves updated Data::Dumper to handle blessed regexes in blead, but cautioned that the code was probably still too raw for previous releases.

  a blessing in disguise
  http://xrl.us/bedaa 

That gave Jerry D. Hedden some grief, until he tracked down the reason for the failure.

  that's not supposed to happen
  http://xrl.us/bedac 

Jerry has been building perls for quite a while without including the XS component of re. Yves Orton thought a perl without re, since it was a pragma.

Jerry revised the patch two more times to get it into shape.

  running light
  http://xrl.us/bedae 

During the discussion, Yves Orton noticed that regexp_pattern really belonged in universal.c.

So Jerry made the move, wrote the appropriate changes and their attendant tests and this enabled other tests to be moved around and simplified some additional housework in Data::Dumper. All applied by Rafaël.

  moving to new digs
  http://xrl.us/bedag 

ARRAY(0x...) is not very helpful in sprintf.t diagnostics

Yitzchak Scott-Thoennes noticed that op/sprintf.t could produce meaningless messages on failure, and changed this to make it start making sense again. Applied by Nicholas Clark.

  http://xrl.us/bedai 

Smoke report for 5.11.0 (delivered manually...)

Steve Hay upgraded his smoking kit to the latest CPAN version, and discovered to his dismay that it could no longer mail the smoke results. He proffered a patch to get things going again.

  still smoking
  http://xrl.us/bedap 

B::Deparse fixes for implicit smart matching in given/when

Florian Ragwitz thought that the results of deparsing a switch statement were suboptimal and offered a patch to make things better. Rafaël applied the patch, and added a test too.

  given a patch
  http://xrl.us/bedar 

Remove AutoLoader::can

A thread in late November about slowness in can for AutoLoader was put on hold during the freeze for 5.10.0. Rafaël dug out the final patches and applied them. Steffen Müller said he'd push a new version out to CPAN.

  picking up where we left off
  http://xrl.us/bedat 

Exclude .svn and _darcs from IPC::SysV's Makefile.PL libscan

Andy Armstrong had been working on IPC::SysV and noticed that the droppings left over by revision control messed things up and proposed a patch to fix things up again.

A debate followed as to how, when and where to solve the problem and at the end of the day, the appropriate repositories received the appropriate patches.

  repository droppings
  http://xrl.us/bedav 

Move the reg_stringify logic to Perl_sv_2pv_flags

Ævar Arnfjörð Bjarmason moved the stringification of regexps into sv.c following Nicholas Clark's work of raising regexps into first class objects. Ævar explained that this would not break the pluggable interface for the regexp engine.

Nicholas smoothed over some of the rough bits and applied it to blead.

  just another SV
  http://xrl.us/bedax 

Porting/manicheck

Robin Barker noticed that the last patch to manicheck broke its -x and -m command line switches. So he fixed it. Applied by Rafaël.

  switched on
  http://xrl.us/bedaz 

tru64: poison stack, write protect string constants

Jarkko Hietaniemi desired bad code to die a quick death by way of a segmentation fault, Steve Peters complied.

  0xfff58005fff58005: segmentation fault *and* NaN
  http://xrl.us/beda5 


Watching the smoke signals

Smoke [5.11.0] 32864 FAIL(F) MSWin32 Win2003 SP2 (x86/1 cpu)

There was a problem with platforms that were able to fork, or merely pretend to, or cannot. This was corrected, and Steve Hay recommended that for dual-lifed modules that need to test forking, the best practices may be found in Test/Simple/t/fork.t and IO/t/io_pipe.t.

  there is no fork
  http://xrl.us/beda7 


New and old bugs from RT

forking on Tru64 (alpha-dec-osf) (#3387)

A bug involving a child reading from a file and looping back to the beginning and reading again, and again was posted nearly eight years ago (hence the low bug id). The problem was kicked around for a while, but no-one was able to figure out what was going wrong.

And there the bug slumbered from many years, until Franz Fischer revived it a few days ago, having run into the same behaviour on HP-UX.

  better luck this time
  http://xrl.us/bedbb 
  let's do the time warp
  http://rt.perl.org/rt3//Ticket/Display.html?id=3387 

Segfault with complicated regex inside map (#24898)

Abigail posted this bug four years ago, and noting that the problem was still present, created a TODO test for it.

Yves Orton wondered if the problem occurred outside of a map but Abigail listed a number of variations on theme that produced an interesting variety of segfaults, valgrind errors or memory exhaustion, and some that worked just fine.

  I told you map in void context was evil
  http://xrl.us/bedbd 

CHECK{}, INIT{} in sitecustomize.pl (#45627)

Peter Dintelmann reported a bug that was actually due to a misunderstanding of the documentation, so Rafaël tried to improve that.

  it's the way it is
  http://xrl.us/bedbh 

pp_ftrread appears to use the wrong access mode for -x (#49003)

Jason Hord filed a bug last month the behaviour of -x when using the filetest pragma and attached a patch to fix it up. This week, Rafaël applied the patch.

  hooray for bug reports with patches
  http://xrl.us/bedbj 

B::Deparse fails to deparse a reference to an anonymous hash (#49298)

Rafaël committed a fix that he thought should fix the bug that David Leadbeater reported last week.

  anon deparsing is go
  http://xrl.us/bedbm 

Scalar::Util documentation problem (#49434)

Ben Okopnik pointed out that the documentation in this module refers to a book that never has, and probably never will be published.

  a tad optimistic
  http://xrl.us/bedbo 

Attributes and Unknown Error (#49472)

"mz" reported a significant regression in the clarity of error messages between 5.8.8 and 5.10.0 when attributes are involved. No takers as yet.

  http://xrl.us/bedbq 

Outdated Test::Harness::Straps (#49504)

Imacat from Taiwan, who runs a very tight testing ship, reported that the Test::Harness::Straps shipped with 5.10.0 is quite far behind the current CPAN version. Furthermore, upgrading the module copies it to the site directory, hence shadowed by the older core version. Michael G. Schwern fixed this.

Imacat also asked Michael to bundle a Makefile.PL, to enable a vanilla 5.8.8 distribution (sans Module::Build to install THS automatically in a CPAN shell.

Michael was a little more reluctant to take on this additional technical debt, but Andreas König thought he had little choice.

  http://xrl.us/bedbs 

state variable not available (#49522)

Abigail uncovered a curious omission in the implementation of state variables, and wrote a TODO test to help with the search for the solution.

  here you see it, there you don't
  http://xrl.us/bedbu 

syswrite layer forgets to encode (#49548)

Mark Overmeer wrote a small test case showing that syswrite failed to honour encoding directives.

  http://xrl.us/bedbw 

constant overloading (#49594)

Goro Fuji noted that overload::constant doesn't work if the constant in question contains escaped characters. Michael G. Schwern hypothesised that the presence of a backslashed character caused the code to run down the wrong path.

  the high road be shorter, but the low road prettier
  http://xrl.us/bedb2 

open ':locale' does not work under locale with modifier (#49646 and #49648)

Mashrab Kuvatov filed a bug regarding problems with locales, UTF-8 and Cyrillic. Rafaël trimmed down the test case, saw the problem, but wanted to obtain some reference material on how to read encoding definitions.

  http://xrl.us/bedb4 

Use of uninitialized value in die at /opt/perl/lib/perl5/File/Copy.pm line 224. (#49660)

Marc Lehmann ran into a bug with File::Copy but couldn't reproduce it (although unfortunately the effect was to remove the file being copied).

  ouch
  http://xrl.us/bedb6 

Perl5 Bug Summary

310 new + 1470 open = 1780 (8 created, 4 closed). Robin Barker wondered where his #49302 bug went.

  http://xrl.us/bedb8 
  overview
  http://rt.perl.org/rt3/NoAuth/perl5/Overview.html 


New Core Modules

Math-Complex 1.43

Black smoke emerged from 1.42, reasons why ok($a == $b) may be needed instead of cmp_ok($a, '==', $b) were given, long doubles were overlooked and so Jarkko Hietaniemi pushed out a new version.

  http://xrl.us/bedca 


In Brief

Andreas König identified the source of H.Merijn Brand's double free error in 5.10.0

  it says "rgs@stencil" on the tin
  http://xrl.us/bedcc 

And Moritz Lenz's bugs in extended regexp features were Warnocked.

  perlbug it
  http://xrl.us/bedce 

H.Merijn continued to plough through his Configure and metaunits work and was happy to report that he was down to a mere 193 warning messages.

  a mere SMOP
  http://xrl.us/bedcg 

Jari Aalto wanted an URL for Perl documentation. Two were given.

  a vast program
  http://xrl.us/bedci 

Nicholas Clark was Warnocked over a parallel make bug in SDBM_File

  because no-one uses it?
  http://xrl.us/bedck 

Craig Berry noticed that the latest REGEXP patches produced black smoke on VMS. Robin Barker and Nicholas Clark vied to produce the best patch to resolve the problem and at the end of the day VMS and Craig were happy again.

  http://xrl.us/bedcn 

Yves Orton complained that development version numbers, warnings, XS and MakeMaker don't mix. The concensus is that using underscore in a version number to signify the beta release of a module to PAUSE blows chunks, and a much better idea would be to have something in META.yml.

  a call for metadata
  http://xrl.us/bedcp 

Nicholas welcomed Marcus Holland-Moritz's correction of an 8 year old bug, and wondered how much perl 4 code still remained in the code base, as this could lead to the possibility of correcting an 18 year old bug.

Andy Dougherty thought that much of doio.c dated back to Perl 4 and significant parts of sv.c could be traced all the way back to Perl 1.

  layer upon layer upon layer
  http://xrl.us/bedcr 

Coverity was a great help to the development of perl 5.10, helping pin down a number of questionable C practices. Andy Lester asked for volunteers to say good things about the product.

  scratch my back
  http://xrl.us/bedct 

Max "Corion" Maischein made perl build again on Win32 when the path contains whitespace.

  under program files
  http://xrl.us/bedcv 

Patrick Rutkowski ran into a problem with typedef definitions in XS code written in C++. Jan Dubois had already dealt with the problem in the past and show Patrick what to do.

  http://xrl.us/bedcx 

Ask Bjørn Hansen encountered some weird behaviour when @INC contained ./ (that is, with a trailing slash). Andy Armstrong noted that the perl in question was from a Red Hat distribution, and wondered if one of their own patches was interfering with normal @INC behaviour.

  http://xrl.us/bedcz 

The latest version of the smoking harness for the perl core fell apart, and some fixes were made. Further improvements may be necessary.

  sync early, sync often
  http://xrl.us/bedc3 

Gerard Goossen released kurila 1.7, an experimental fork of the perl codebase, the aim of which is wto see what would happen if one were to free one's self completely of any notions of backwards compatibility.

  where does a kurila sit?
  http://xrl.us/bedc5 

Rick Delaney's patch from last week to allow the clearing of @ISA from XS was applied.

  assign with impunity
  http://xrl.us/bedc7 

Robin Barker had some more consting goodness applied to POSIX.xs

  strtoul() and helper
  http://xrl.us/bedc9 

as well as Compress::Raw::Zlib and Filter::Util::Call from a short time back.


  http://xrl.us/beddb 

Nicholas Clark's change to assert that these are the regexps you were looking for came to grief on Jerry D. Hedden's machine. A revised patch appears to have fixed the problem.

  http://xrl.us/beddd 

About this summary

This summary was written by David Landgren.

  last week's
  http://xrl.us/beddf 

Weekly summaries are published on http://use.perl.org/ and posted on a mailing list, (subscription: perl5-summary-subscribe@perl.org ). The archive is at http://dev.perl.org/perl5/list-summaries/ . Corrections and comments are welcome.

If you found this summary useful, please consider contributing to the Perl Foundation to help support the development of Perl.

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