This Week on p5p 2001/03/05



Notes

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We're trying something new this week, and providing brief biographies of some of the more prolific porters. If you have anything to add to this, or object to something I've said about you, email me as above.

Locale Support

Andrew Pimlott filed a bug report related to POSIX::tolower; basically, it is not as locale-aware as Perl's own lc. He also found that lc failed to be locale-aware while in the debugger. Nick Ing-Simmons pointed out that use locale is lexically scoped, and the debugger is in a different scope, meaning that it won't pick up on the pragma. Andrew thought this was probably a bug (the debugger not assuming the debuggee's scope) but it is unclear as to how that could be fixed. Nick wondered if the first was due to not calling setlocale, but Andrew reported that this didn't help anything.

Andrew then went digging around in POSIX.pm and found that isalpha is perfectly locale aware, but tolower is not - this is because isalpha is written in XS, but tolower simply calls lc; because it's in a different lexical context, it doesn't pick up on the use locale. It transpires that XS code does execute in the same lexical context as the caller, which is quite strange. Andrew pointed out that there's no way to make a pragma dynamically scoped. He said:

I think this raises some fundamental issues, but I'm not sure exactly which. It seems clear that one would like to be able to write a correct tolower (ie, exactly equivalent to lc, as per the POSIX documentation) in pure Perl. One possibility is a TCL-like "uplevel", but I desperately hope that doesn't turn out to be the best option.

I asked why POSIX functions were being implemented in Perl instead of C, and Andrew replied that in some cases it already works merely by magic: Perl doesn't correctly turn on and off locale support lexically, so some functions inherit the support for free. Jarkko grumbled about locales in his customary manner, and said he'd take a look at the areas which needed setlocale calling, but then Andrew had a revelation:

Hmm, looks like I missed an essential point: locale support is not dependent on 'use locale' at all! This seems to be intentional. perl unconditionally calls setlocale() on startup, and never calls it again (unless you use POSIX::setlocale() explictly). So POSIX::isalpha() respects $LANG by default, even if you never mention the locale pragma.

It is only where (core) perl has a choice between calling a locale-sensitive libc function, and doing things its own way (eg, hard-coding character semantics), that the locale pragma currently matters. Since the string value of $! requires calling a locale-sensitive function (strerror()), $! always respects locale.

And you wonder why Jarkko throws his hands in the air at the mention of anything locale-related...

Coderef @INC

Nicholas Clark provided a patch which extended the little-known coderef-in- @INC feature to allow passing an object; if you pass an object instead of a coderef, the INC method will be called on it. This has allowed him to create an experimental pragma, ex::lib::zip which lets you put a module tree inside a ZIP archive and Perl will extract the modules it needs from it.

He then also explained what it was all about, in the hope that someone would write some proper documentation. Nobody did so, (my fault, I promised to but didn't get around to it) but his extremely helpful explanation of the coderef-in- @INC API, and the cheap source filter API it allows can be found here.

Briefly, you can do

    BEGIN {
        push @INC, \&somesub;
        sub somesub {
            my ($owncoderef, $wanted_file) = @_;
            # Produce a filehandle somehow
            if ($got_a_handle) {
                return *FH;
            } else {
                returm undef;
            }
        }
    }

and have your subroutine intercept calls to use. The ByteCache module on CPAN makes use of this to cache just-in-time compiled versions of modules.

More Memory Leak Hunting

Some people started complaining about the lateness of 5.6.1, and Alan mentioned the he probably wouldn't be able to ship 5.6.1 in Solaris yet because of what he saw as "the large number of leaks". Sarathy disagreed:

I strongly suspect you'll end up shipping no version of Perl with Solaris, then. Every single version of Perl out there has more unfixed "leaks" than 5.6.1-to-be, and some "real" ones to boot.

I say "leaks" because these are still totally hypothetical, given your vantage point of a -DPURIFY enabled build with the arena cleanup repressed (which is the right vantage point for someone who has set out to clean up all leaks, I should add). However, this is not the real world. In the real world, the arena cleanup is enabled and appears to do its thing (however ugly you or I say it is).

There was then some similarly ugly debate about what actually constitutes a leak: Alan considered a leak anything which allocated memory and lost the pointer to it; Sarathy was only considering monotonic process growth. Nick Clark suggested that he could trigger a leak in the Sarathy sense by repeatedly useing modules and clearing out %INC, but this wasn't the case; the leak is due to some ugly fakery that goes on when the compiler sees use Module;. Alan could, however, trigger a leak with

    sub X { sub {} }

as the inner subroutine wasn't being properly reference counted. Alan and I scrambled around looking for the use leak, and Alan found that the two were related. I'm not aware as to whether or not he's fixed it.

In other news, Nicholas Clark is a wicked, wicked man and managed to compile the Boehm Garbage Collector into Perl. (As Randal pointed out off-list, no more Boehm garbage!) and found a way to use it as a memory leak detector.

Weird Memory Corruption

Weird bug of the week came from Jarkko, who found that

    $ENV{BAR} = 0;
    reset;
    if (0) {
      if ("" =~ //) {
      }
    }

caused all kinds of merry hell - on some platforms, it ran fine, on some it segfaulted; the problems were not consistent across platforms, meaning that some machines with identical setups produced differing results. This is obviously maddening. Nick Clark made it even weirder:

./perl will pass. /usr/local/bin/perl will SEGV. They are byte for byte identical.

Jarkko thought this was a recent problem, but Nick managed to reproduce it in 5.005_02. Alan produced an impressive explanation of what was going on, which I greatly encourage you to read if you want to learn how to track this sort of thing down, but stopped short of an actual fix.

There was, of course, the usual discussion of how useless reset was anyway, including one suggestion of rewriting the op in pure Perl.

Yet More Unicode Wars

87 messages this week were spent attempting to formulate a sensible and acceptable Unicode policy. The attempt failed. If you really want to jump in and have a look, this is as good a place to start as any.

Switch is broken

Jarkko reported that for some reason, Switch 2.01 from CPAN has suddenly started failing tests on bleadperl. It would be really, really, really great if someone out there could look into why this is happening and try to come up with an isolated bug report. Or even better, fix it.

Various

Olaf Flebbe chimed in a bunch of EPOC fixes, for those of you running Perl on your Psions; Sarathy fixed a long-standing parser bug. Michael Stevens did some sterling work clearing up the POD markup of the documentation. Craig Berry turned in some updates to VMS's configure.com. Daniel Stutz and Ed Peschko both rewrote perlcc.

David Mitchell deserves an honourable mention for a really useful first patch, which lets perl -Dt tell you which variables are being accessed, as well as another debug option, -DR which tells you the reference counts of SVs in the stack. Very cool stuff, David, thanks.

Someone reported that ExtUtils::Install is naughty and doesn't check the return values of File::Copy::copy; this would be easy enough to fix up if anyone out there is interested. (That's bug ID 20010227.005, by the way)

Until next week I remain, your humble and obedient servant,


Simon Cozens
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