This Week on perl5-porters - 8-14 May 2006
"Trust me on this one; I spent three years trying to kill v-strings
and all I managed to do was to defang them in the more recent 5.8.x
series." -- John Peacock
Topics of Interest
Putting "Tie::RefHash" on CPAN
One of the items on the list of things to do for Perl 5.10 is to "dual
life" everything. That is, give all core modules a counterpart on
CPAN, in order to correct bugs and introduce enhancements in a more
timely manner.
Yuval Kogman wanted to know how to go about doing just that for
"Tie::RefHash", a module he now maintains. Andreas Koenig performed
the required paper work on PAUSE, and it was done.
Module Liberation Front
http://xrl.us/mi6p
Things that go BOOM! on the context stack
Nicholas Clark started to work on the problem of warnings during the
constant folding optimisation step crashing due to internal panics.
The problem is that constant folding is a compile-time event that
appears to take effect at run-time. Unfortunately, the constant
folding code is not itself immune to run-time effects.
For instance, Perl being the dynamic language that it is, it is
possible for a "__WARN__" signal handler to be fired while the program
is being compiled. And depending on what that handler does, sometimes
that can really ruin your day.
Nicholas thought of a first approach, which Dave Mitchell identified
as being unworkable. So Dave solved the problem elegantly, by simply
switching off the "warn" and "die" handlers while the folding of
constants is taking place at compile time.
Preventing signal-to-noise
http://xrl.us/mi6q
Adding macros to Perl5
Another key milestone on the way to perl 5.10 is the notion of
assertions, that work like C's "assert"s. Nicholas Clark was
wondering, some time back, if it could be possible to implement
assertions outside of the core.
In order to *really* pull this off, Salvador Fandiño figured that Perl
needed real (Lisp-like) macros. And he did just that, by allowing a
":macro" attribute to be applied to a subroutine definition.
Rafael Garcia-Suarez wondered whether this could be dealt with by
"Attribute::Handlers". Salva pointed out that that module does its
magic far too late in the game, at "INIT" or "CHECK" time, at which
point it is too late to modify the op-tree, since it has already been
built.
Joshua ben Jore, as the *de facto* "B::" maintainer suggested that
Salva take "B::Generate", and enhance it do to the required footwork
instead of introducing a new B module.
Economic macro policy
http://xrl.us/mi6r
Removing "sv_derived_from" from the public API
chromatic wanted to remove the documentation (although not the
implementation) of "sv_derived_from"t, since using it on a blessed
"SV" can lead to incorrect results if "isa()" has been overloaded.
Jan Dubois is used to using this routine, and for him it works as
advertised, so he was a bit puzzled by chromatic's statement that
using it "is almost always wrong". John Peacock was also opposed to
the idea, commenting on how hard it is to get something out of perl
once it gets in.
It turns out that the heart of the matter is "Test::MockObject", which
cannot be made to work correctly if classes overload their own "isa"
functions. Jan Dubois identified the heart of the problem: squeezing
the information concerning implementation and interface inheritance
through the same mechanism, and went on to sketch out a technique to
deal with both situations cleanly.
Rafael liked Jan's idea, and asked for a more precise specification.
chromatic patched universal.c to push it in that direction, borrowing
the "does" method from Perl 6. Fergal Daly thought that perhaps there
might be code out there that already "does" stuff, and to play it safe
at this late stage of the game it might be wiser to name the method
"DOES".
In another sub-thread, chromatic thought that maybe the behaviour of
"sv_derived_from" could be changed to operate differently, and if XS
writers needed the old behaviour, they could pick it up by calling a
new method. Jan Dubois thought that that was exactly backwards.
Marvin Humphrey, who is heavily into this sort of stuff was interested
in a clean approach, but one that did not entail a performance hit,
since Perl methods calls are already expensive enough as it is.
In the end, chromatic supplied a documentation patch which clarified
how and when to use "sv_derived_from", which Rafael happily applied.
The patch at the end of the universe
http://xrl.us/mi6s
Getting "threads" to do your bidding
Last week, work on "threads" broke on Windows. Jerry D. Hedden
supplied a patch which straightened out that problem.
Windows repair man
http://xrl.us/mi6u
The real discussion, however, began when Jerry asked for some help in
adding the capability of cancelling a thread. He thought of an
approach that just might work, but wasn't too sure about the finer
details of diddling a thread's op-tree to cause it to terminate.
Liz Mattijsen was intrigued by the possibility and commented that if
Jerry was able to succeed, it would be nice to extend the work in
order to add some sort of suspend/resume functionality, then she could
put a thread on ice for a while, which would be very useful.
Jan pointed out that it was possible to deliver a signal from C to a
thread. Jerry wondered if that could be used to deliver a "__DIE__"
signal to a thread. The main problem with this approach is that
"die()" and "warn()" are not true signals, they just use %SIG as a
convenient place for pointing to their exception handlers.
To use this approach, one would need to invent a new signal name with
an unused numeric value. As this is guaranteed to be highly
platform-dependent, a certain amount of "Configure" work would be
required. After that, one would be able to deliver a new pseudo-signal
to the interpreter, and thereby kill, or otherwise incapacitate, your
thread.
Jerry sketched out some sample code to show what the implementation
would look like. This elicited a few remarks, but no show-stoppers, so
he started to work on it in more detail.
Signal processing
http://xrl.us/mi6v
The discussion then segued in more detail into the issue of suspending
and resuming threads. Liz explained that her "Thread::Suspend" module,
that currently only works on certain vintage Linux kernels could be
refitted to use this functionality. And it would be Good.
Jan lent his deep understanding of Perl internals to help flesh out
the idea of using a fake signal. He noted that there used to be a
"Perl_raise_signal" function that would have been very useful in this
context, but in "blead" is has become a private static function. This
made Rafael wonder whether the privatisation had been the right move,
and that maybe it should be made public again.
It turns out that the function had been zinged by Andy's wand of const
refactoring goodness, largely because poor internals documentation had
not identified its possible public use.
Dave Mitchell cautioned that it is deceptively easy to forget or drop
pending requests if the proper safeguards are not in place to ensure
atomic updates to the C variables that control signal delivery. Worse,
if the signals come too closely together, it may not be possible to
determine in which order to apply them unless a full-blown action
queue is implemented.
Jan Dubois proposed an alternate algorithm using a signal and a
semaphore, for which Dave, at least for the time being, could not find
fault with.
Holding the flag
http://xrl.us/mi6w
Jerry delivered a first cut at signalling, which Jan commented on,
explaining that the approach would not work on Windows, simply due to
the nature of the beast, and that the code could probably be
simplified with little risk.
http://xrl.us/mi6x
Jerry then delivered a more complete patch to allow a kill() method to
be used on a thread, to send it signals. Rafael made a couple of
suggestions, to improve consistency in the way it deals with things
such as signal names and error messages.
http://xrl.us/mi6y
So Jerry took these considerations on board and revised the patch.
Nicholas had to tweak it slightly in order to get it to pass its test
suite on FreeBSD. Trying to do this stuff portably is a nightmare. Who
knows what Unicos or VMS are going to make of it.
Jan Dubois took issue with the suspend/resume semantics, arguing that
if a thread receives two suspend signals, when it receives a
subsequent resume signal, it should revive immediately, rather than
continuing to wait for a second resume signal. Jerry thought that
these different strategies should be handled by other modules.
A question of semantics
http://xrl.us/mi6z
Winding up for the week, Jerry improved the documentation to describe
the issues surrounding safe signals and unsafe signals with respect to
threads.
http://xrl.us/mi62
Patches of Interest
Proper use of static functions in toke.c and pp_sys.c
Andy Lester applied some elementary preprocessor magic to a static
"printf"-like function, which means that the function signature could
be changed.
When compiled with "gcc", the function signature can then supply the
necessary information to have attribute checking performed on the
format string passed to the underlying "printf" code. This identifies
things like '%s' is being used to print an "int" at compile-time,
thereby avoiding considerable grief and heartache at run-time.
http://xrl.us/mi63
He then spotted a problem with embed.fnc, that was not declaring a
function as being able to accept a "NULL".
http://xrl.us/mi64
And a tweaklet to speed up utf8.c
http://xrl.us/mi65
Andy then tinkered with "bytes_to_uni", bringing it into line with
f<embed.fnc> best practices. In doing so, this brought the function up
on Sadahiro Tomoyuki's radar, who realised that "S_bytes_to_uni" in
pack.c is nearly the same as utf.c's "Perl_bytes_to_utf8" function.
Tomoyuki reworked "S_bytes_to_uni" to get it to do its job without
requiring a scratch buffer. He also pointed out a missing gap in the
orthogonality of functions converting back and forth between bytes and
UTF-8, although this elicited no comments.
http://xrl.us/mi66
Various t/op/*.t files using test.pl
The summariser, finding the traffic a little light this week, sent in
a patch to add shiny test.pl goodness to a number of files in the test
suite. Unapplied.
Obviously some sort of oversight
http://xrl.us/mi67
"sprintf" and variadic macros
Robin Barker spotted a problem with patch #27987, in that it created a
pair of source code fragments that must be kept in sync. He proposed a
patch that used variadic macros (as per the C 89 standard). An old Sun
compiler that Andy Dougherty had at hand choked on this.
http://xrl.us/mi68
Since the porters want Perl to run on as many platforms as is
reasonably possible, and since using variadic macros (macros that may
take a varying number of parameters, in case you were wondering) would
wipe out a number of older platforms in one fell swoop, Jarkko
Hietaniemi proposed an alternate take, using variadic functions (which
have been available in one form or another in C since just about
forever).
Third time lucky
http://xrl.us/mi69
Nicholas Clark thought that, all the same, if a platform did have
usable variadic macros, it would be nice to be able to use them, thus
avoiding the cost of plunging through a function call to do the deed.
If 'twere done, when 'tis done
http://xrl.us/mi7a
New and old bugs from RT
What Steve Peters did this week
Steve closed out bug (#7014), since these days, an embedded "\0" in a
"die" or "warn" string no longer causes the following characters in
the string to be discarded,
http://xrl.us/mi7b
noted that when a "make test" hangs on AIX 4.3.3 and cannot be killed
(#21328), upgrading the compiler is as good a fix as any.
http://xrl.us/mi7c
pointed out that it is all very well to install a free .NET compiler,
(#21607), but to get anywhere, you also need to install the Windows
SDK.
http://xrl.us/mi7d
closed an old bug by noting that it had been fixed by an old change
(#21944).
http://xrl.us/mi7e
explained that trying to use a "gcc" compiler built for AIX 4.3.2 on
an AIX 4.3.3 will only end in tears. (#23111)
http://xrl.us/mi7f
tickled an three year old bug concerning leading spaces and "Inf"
infinity (#23731)
http://xrl.us/mi7g
made a brief addition to perlop.pod based in the the suggestion that
"qx/STRING/" should mention "STDIN" (#34288)
http://xrl.us/mi7h
and another note in the configuration hints for Linux, pertaining to
the IA64 platform (#37156)
http://xrl.us/mi7i
Perl is not flexible when selecting vendor install paths (#39069)
Andy Dougherty showed another simple trick to do at configuration time
in order to rebuild perl with the same settings as a vendor-supplied
perl.
http://xrl.us/mi7j
Regexp optimizer loses its hopes too soon (#39096)
Ruud Affijn showed how the "/m" flag, "^" and "$" can conspire to drag
the performance of the regexp engine down into the abyss of
exponential backtracking.
http://xrl.us/mi7k
"Pod::Html" error stops CPAN install/test of "Pod::Readme" (#39098)
Randy W. Sims returned to the problem of modules not being installed
simply because the build failed to generate HTML from the POD. Since
this behaviour was judged to be a little too drastic, Randy outlined
some possible scenarios to resolve the situation.
http://xrl.us/mi7m
Perl on AIX 64 bit problem (#39100)
Daljit Singh has an IBM AIX computer, but no IBM C compiler, and
wanted to install a 64-bit perl to speak to his 64-bit Oracle
instance. This meant using "gcc". He was, however, getting some
compile errors, for which H.Merijn Brand offered a diagnosis.
http://xrl.us/mi7n
"pod2man" omits ".SH NAME" section (#39106)
Ben Okopnik noticed that "pod2man" does not create a ".SH NAME"
section in the man pages it generates. Consequently, programs like
"mandb" produce copious amounts of warnings when the man page index is
rebuilt.
Rafael Garcia-Suarez explained that "pod2man" does, except when the
original POD file lacks a "=head1" section in the first place, so he
documented the offending files correctly.
http://xrl.us/mi7o
Internal error in Bytecode.pm (#39110)
Bart Bartholomew discovered using "-MO=Bytecode" to emit bytecode no
longer works. Joshua ben Jore wasn't sure that anyone was even
maintaining "B::Bytecode" any more, and he should know, since he is
about the only person poking around in "B::" these days.
http://xrl.us/mi7p
"sprintf" with UTF-8 format string and ISO-8859-1 variables (#39126)
Willem-Jan Veen discovered that if you have a format string for
"sprintf" encoded in UTF-8, and use it to format a string encoded in
ISO-8859-1, the program dies a horrible death. Willem-Jan was the
first to admit that wanting to do such a thing was rather strange, but
thought that the resulting core dump was not the most useful result.
Dominic Dunlop was not able to reproduce the failure, except by using
an custom "malloc" library that contains aggressive debugging checks.
In the meantime, Willem-Jan had found a workaround, which simply
consisted of ensuring that the format string is never encoded in
UTF-8.
http://xrl.us/mi7q
Failure not always detected in "IPC::Open2::open2" (#39127)
Vincent Lefevre wondered why opening a program that doesn't actually
exist does not always give rise to a trappable error.
http://xrl.us/mi7r
h2ph generates incorrect code for '#if defined A || defined B' (#39130)
Jason Vas Dias noted that "h2ph" can emit code that cannot be compiled
on Linux PPC64 platforms and proposed a patch to make it emit saner
code.
Say can you C
http://xrl.us/mi7s
Perl5 Bug Summary
14 new + 18 closed = 1523
http://xrl.us/memk
Respectable bug, seeks friendly owner
http://rt.perl.org/rt3/NoAuth/perl5/Overview.html
New Core Modules
* Encode version 2.17 was released by Dan Kogai. This followed
hot on the heels of 2.16, but since one of the changes was a
code adjustment to remove a Coverity issue he decided to
hasten the release.
http://xrl.us/mi7t
* Test-Harness version 2.58 was unleashed upon an unsuspecting
world by Andy Lester.
http://xrl.us/mi7u
* Following changes to perly.y, H.Merijn Brand uploaded a new
version of defined-or ("//=") for "maint".
http://xrl.us/mi7v
In Brief
Larry explained the meaning of the MAD keys. (This reply appeared the
week before, but I managed to delete the item in last week's summary).
http://xrl.us/mi7w
(as did a patch to squelch the use of "chop" in perlport.pod).
http://xrl.us/mi7x
Nicholas Clark announced the arrival of new Coverity findings. Andy
Dougherty noticed that many of them had already been analysed, which
means that the information was not being retained.
http://xrl.us/mi7y
The ext/IPC/SysV/t/ipcsysv.t patch caused a bit of trouble, but was
eventually beaten into submission.
http://xrl.us/mi7z
A minor typo in perlfaq4.pod was cleared up, but a more minor (or more
important, if you are into typography) issue concerning em dashes was
left in, because life is too short.
http://xrl.us/mi72
Vadim continued on the Win32/Wince merger, and supplied a "Wince
crosscompile patch" to keep things happy.
http://xrl.us/mi73
Nicholas Clark found an off-by-one in "no" error, and suggested a
token fix in pp_ctl.c to make it do the right thing. Yitzchak
Scott-Thoennes wondered if the fix was appropriate for "maint".
http://xrl.us/mi74
Nicholas was also surprised that parsing changes in blead caused "Tk"
to fail, since [EMAIL PROTECTED] now fails with a run-time error. (One
should of course be using "$#$arrayref").
Anthony Heading wondered whether the bogus "gcc" behaviour seen by
Nicholas Clark was not simply due to confusion between C and C++ on
the part of the compiler authors.
http://xrl.us/mi75
Jarkko sent in a small patch for sv.c, since "printf %d" wants an
"int" not "size_t".
http://xrl.us/mi76
About this summary
This summary was written by David Landgren.
Last week's summary
http://xrl.us/mi77
garnered one response, concerning "Data::Dumper"'s propensity to quote
numeric values needlessly. Brad Baxter followed up on the thread,
saying that the underlying problem, of which "Data::Dumper" was only a
symptom, was that if the operand on the right hand side of a numeric
comparison is "undef", the left hand side is not marked as having been
used in numeric context (technically, the "SvIOK" flag is not set).
Sounds like a bug
http://xrl.us/mi78
If you want a bookmarklet approach to viewing bugs and change reports,
there are a couple of bookmarklets that you might find useful on my
page of Perl stuff:
http://www.landgren.net/perl/
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--
hope still, a little resistance always maybe stubborn tiny lights vs.
clustering darkness forever ok?
