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This Week on perl5-porters - 3-9 October 2005

posted by rafael on 2005.10.11 7:56   Printer-friendly
A quiet week on perl5-porters, in which we learn that if we diddle with the environment, we eventually pay for the consequences, and Schwern saves the day by preventing the module testing infrastructure from falling apart.

Inline broken on blead

David Dyck wrote in to say that Inline's make test was failing in blead, due to a construct that applied the defined operator to a hash. Rafael Garcia-Suarez summed up the problem succinctly:

  $ perl5.8.7 -wle 'print defined %foo::'
  $ bleadperl -wle 'print defined %foo::'

and opined that the use of defined(%hash) was discouraged anyway. Dave Mitchell pointed out that for lexicals, a compile-time warning is issued:

  $ perl -wce 'my %foo; print defined %foo'
  defined(%hash) is deprecated at -e line 1.
  (Maybe you should just omit the defined()?)

Rafael traced the change in behaviour to patch #24660, in which Nicholas Clark shaved four bytes of the size of an HV body on 32 bit platforms. And the patch comment even stated that one side effect was to make defined %symbol_table:: always true.

Alas, it may be that this breaks Tk if the source code comments are to be believed. So far, it doesn't appear that anyone has investigated this possibliity. 

Enhancing Data::Dumper

Curtis "Ovid" Poe worked out how to make Data::Dumper do something clever like:

  print Dumper($foo, @bar);
  # prints something like
  $foo = 'stuff';
  @bar = ( 3, 17 );

(Data::Dumper::Simple already does this, albeit with a source filter). This time, no source filter. The biggest problem is that this would require bringing PadWalker into the core. And then...

Steve Peters related his difficulty in getting PadWalker to pass its tests on Linux, which made him wonder what things might be like on more exotic platforms. Robin Houston (the author of PadWalker) mentioned that this has been fixed in version 0.13. Dave Mitchell noted how PadWalker, relying as it does on undocumented perl internals, is very sensitive to breakages between perl versions, is not robust, and should not be included in core.

Rafael was more concerned about teaching Data::Dumper new tricks, a justifiably ancient module which is probably better left alone.

Robin answered that the Perl debugger will make use of PadWalker if it find it, and was concerned by the porters' view of its apparent fragility. He admitted that this perception might be due to the fact that he let it slide from August 2003 to August 2005, but that he is now working hard to make it as robust and reliable as possible. He didn't see any fundamental reason as to why robustness could not be achieved.

Dave said that he wasn't criticising PadWalker per se, but that the goal it is trying to achieve is fraught with peril -- source filters, undocumented internal interfaces, lexicals, closures and other subtleties -- such that even perl itself doesn't always get right.

Robin countered again, with a new version that resolved nearly all of Dave's problems and then asked for a better name for the module, perhaps something in the Devel or Lexical namespaces.

If you only have time to read one thread this week, this is a good 'un.

  Ovid's journal entry discussing the Data::Dumper hack 
  The thread on p5p 

Taint checks in the test suite

There are lots of warnings issued when one runs

  C<make test.taintwarn>

(Gee, I didn't know that existed). Steven Schubiger noted that many of them come from using $^X to determine the name of the perl executable. Steven wondered what the best way to fix that would be (such as a hardcoded ../perl) and also said, in fine understatement, that some of the code seems rather difficult to untaint.

Steve Peters mentioned that searching for perl will be difficult when your executable is named ponie. Rafael thought that $^X being tainted was a good thing, and that it was not sufficient to untaint it with a brute-force /(.*)/.

After taking a second look at the problem, Rafael came back and said that he didn't really care about the warnings. The test.taintwarn is there more to test the tainting mechanism itself, that is, tests totally unrelated to tainting that begin to fail when tainting is enabled. If it warns, but runs ok, that's enough.

The thread then went on a couple of tangents, from bringing File::Which into core, to better deal with tainting, to the fact that MacPerl was no longer supported.

The latter remark stung Chris Nandor into stating that a platform is not declared unsupported until such time as those developers who care about supporting Perl on that platform decide to call it a day. He stated that he did intend to release another version of MacPerl at some point in the future. 5.8.x came very close at one time, but then Real Life intervened. Nevertheless, another release is still quite possible.

John Malmberg, ever a glutton for punishment, said that he was trying to determine whether or not the taint code was being aggressive enough in tracking VMS-ish things susceptible to taintedness. Craig Berry said that a certain number of things were already being done, such as dropping image privileges (my VMS is too rusty to summarise what that means).

  From a small patch, a mighty thread grows 

Localising %Config

Benjamin Franz was trying to install Test::Plan, and getting strange failures:

  %Config::Config is read-only

After looking at the code, he noticed simplified the problem down to the following, and posted it to the list:

  use Config;
  local *Config::STORE = sub { 1; };

(%Config is actually a tied hash). He went to the trouble of digging through RT (the bug tracking system) and unearthed #35865 and #35865.

Michael Schwern replied that since Test::Plan is digging around in Config's internals, this hardly constitutes a bug on the part of Config, and invoked Tom Christiansen:

  You are wicked and wrong to have broken inside and peeked at the
  implementation and then relied upon it.
    -- tchrist in <31832.969261130@chthon>

... and then showed how he solved the problem in ExtUtils::MakeMaker. (Basically, one takes a copy of %Config). Benjamin forwarded the information to the author of Test::Plan.

  The lethal problem: 
  How EU:MM does it: 

Wrapping up untested builtins

Steve Hay reported back to say that the protocol files on NT4 Workstation, 2000 Server, 2003 Server and XP Pro are all identical, save for copyright dates. So Steven Schubiger's test is in as change #25696. But then Rafael immediately had problems on Linux smokes and pulled it out. Steven went back to the drawing boards with a less ambitious test. Applied by Steve Peters.

  It started in August 
  And continued in September 
  Which was summarised here 
  And then this week 

Return value for Perl_io_close

Robin Barker noted that Perl_io_close is used twice in the perl source. In one location the return value is used, in the second, it is ignored. So he proposed fixing embed.fnc to fix the warning when it is not used. Rafael noted that it was invoked during SV destruction, and thought it might be nice to do something if the close fails, but what?

Andy Lester though we should do something with the return value, otherwise it means that "a close will never fail". Dave Mitchell, ever the voice of reason, pointed out that when the file handle is a lexical, and is being closed because the lexical is going out of scope, as in

   open my $fh, ....;
 # implicit close here

there is no possible way, short of dying, to tell the program that Something Bad happened. 

Environmental damage on Solaris 10

Alan Burlison was Warnocked over a question of how to get MakeMaker deal with the tricky symlinks necessary to build DBD::Oracle, where the symlink hackery would let one build a perl that speaks to Oracle without a Oracle client install, thereby saving 400 megabytes of disk space.

He must have figured a way forward, because he later wrote back to say that his compile of DBD::Oracle on Solaris 10 had uncovered horrors lurking in the dim, rarely visited corners of the perl source. It's basically a problem with %ENV. Perl_my_setenv can actually reach out and directly manipulate the memory pointed to by the environ pointer that the system passes to the perl executable.

The new, improved method Solaris 10 employs for managing the environment breaks an assumption long held by the perl5 porters. And results in bizarre "Out of memory" errors during perl_destruct. After having looked at the code that deals with getenv and putenv, and having considered it to be "pretty vile", Alan asked what it was all supposed to achieve.

After discussing the issue a while with Steve Peters, Steve suddenly made the connection between what Alan was seeing and what happens on the Cygwin platform. In a nutshell, does delete $ENV{foo} just delete the value, but leave foo= in the environment table, or does it delete the key and value altogether?

Andy Dougherty pointed out that much of the code dates back to when no standards existed for these matters. These days, however, it's probably safe to assume that the vendor's library is sane and should be used by default, and only invoke work-arounds when there is good reason to.

All summed up in bugs #37376 and #37377.

  Managing synlinks with MakeMaker 
  The stench begins 
  The link with Cygwin 
  The source code for Solaris's getenv 

New Core Module Releases

Michael Schwern released a new version of Test::Simple. The thread on p5p is rather succinct. To get the full picture, you had to be following perl-qa. Over there, Adam Kennedy noticed that the September 23 release of Test::Simple/More broke Test::Builder::Tester (that scraped STDERR to pick up the failure messages). Which in turn is used by about 26 other testing modules. So Adam asked Michael to back out the changes until T::B::T could be updated.

Given the number of fixes that went into that release, Michael quite naturally refused to back the changes out. The main problem is that people had ample time to try out the betas to see if the changes would cause problems. Unfortunately, Mark Fowler, author of T::B::T was holed up in bed, nursing a ferocious cold.

So the new release of Test::Simple embraces and extends T::B::T and includes it as its own, with the amended screen scraping in place.

I believe that in times like these, one is supposed to say:

  You are wicked and wrong to have broken inside and peeked at the
  implementation and then relied upon it.

In the meantime, imminent meltdown of the global infrastructure for testing Perl modules was averted.

I suppose the moral of the story is that if you write a module that has a another module as a prerequisite, you should make sure you are aware of all changes made to the prerequisite, whether it be by watching new beta uploads (or writing a program to do the watching for you), or subscribing to the right mailing list or something else. Doubly so if you're walking around with a shot-gun in the lounge, or whatever the saying is. Nothing is ever carved in stone.

  The thread on perl-qa 
  Test/Simple/More/Builder is released 

In brief

The Perl5 bug summary has 1512 open items 

Sébastien Aperghis-Tramoni asked for an addition to Devel::PPPort, to make life easier for Net::Pcap, for much the same thing that Ken Williams is already doing in Cwd.xs. 

Paul D. Lalli found that:

  my $i;
  my @a = (1..10);
  my $last = $#{@a};

produced the message "Bizarre copy of ARRAY in leave". Then again, if I'm not mistaken, I think that code wants:

  my $last = $#a;
  Bug #37350 

H.Merijn Brand added -C as an allowed flag to the PERL5OPT environment variable. (Which you can use to set command-line switches for all invocations of perl, so one would hope that -u is not allowed). 

Andy Lester checked in with some more const work, and cleaned up printf. Applied by H.Merijn. 

Rajarshi Das patched utf8.c on EBCDIC platforms. Sadahiro Tomoyuki asked for regression tests, and provided four, two that currently fail and two that pass. Rajarshi's fix should ensure that all four pass. Rajarshi ran the tests and all passed. 

Mohammad Yaseen asked about ext/B/t/optree_specials.t when running on z/OS (an EBCDIC platform), and was Warnocked. 

"jafoc" said (in #37355) that bad things happen when you try to run

  perl -c -e 'my $xyz; undef xyz'

on perl 5.6.1. Yves "demerphq" Orton replied that it was fixed in 5.8. 

Following on from Michael Schwern's announcement of EU::MM 6.30_01, Peter Prymmer came up with some patches for VMS: 

Tassilo von Parseval reported in #36875 (August 2005) that for perls between 5.5.4 (did he not mean 5.8.4?) and 5.9.3 that

  perl -we 'print lc(undef)'

would not emit a warning about uninitialised values. Steve Peters confirmed the bug, and had a preliminary patch beginning to take shape. 

In #37384, Dan Dascalescu reported that

  C<perl -we '$a=qr//; $b=qr//x; 1 =~ ($a|$b)'>

fails with a 'Sequence (?}...) not recognized in regex'. Dave Mitchell pointed out that bitwise-oring two string is liable to make for a pretty odd regular expression...

About this summary

This summary was written by David Landgren.

Information concerning bugs referenced in this summary (as #nnnnn) may be viewed at

Information concerning patches to maint or blead referenced in this summary (as #nnnnn) may be viewed at

Weekly summaries are published on and posted on a mailing list, (subscription: ). The archive is at . Corrections and comments are welcome.

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  • I believe that in times like these, one is supposed to say: You are wicked and wrong to have broken inside and peeked at the implementation and then relied upon it.

    Let me say that the so-called 'screen-scraping' idea for T::B::T was entirely the author of Test::Builder's idea [] and uses only documented interfaces. There is no 'breaking inside' or 'peeking' going on. If you can think of any better way to determine if the diagnostics that are output from your new testing module matches what you expecte

    •   I believe that in times like these, one is supposed to say: You are wicked
        and wrong to have broken inside and peeked at the implementation and then
        relied upon it.
      Let me say that the so-called 'screen-scraping' idea for T::B::T was entirely the author of Test::Builder's idea [] and uses only documented interfaces.

      The method of grabbing the screen output is documented, but exactly what gets output is not. The way I put it was:

      You are wicked and wrong to have scraped the

      • How could so many people miss such a widespread failure? What can be done to fix that?

        Let's face facts, we're busy, busy people. Rafael's point about monitoring releases (particularly beta ones) is a good one. I should have been doing that. But it's very hard - I don't have time to read the perl-qa list every week, and it's sometimes months between when I blast though each of my mailing lists. Why don't we look at what steps can be taken to make it easier to monitor what releases are coming out?

        I li