Hello all. I'm using XML::LibXML to parse some HTML. Mostly it's working great - fast and very useful XPath support. My problem is that it's choking on some very bad HTML in a very bad way - it's sitting on the CPU until killed manually. I expected some HTML wouldn't parse, so this isn't such a tragedy. What is a big problem is that my attempt to work around this with alarm() aren't working!

Here's my code:

use strict;
use warnings;
use XML::LibXML;

my $html = do { local $/; <> };

my $libxml = XML::LibXML->new();
#$libxml->recover(2);

eval {
    local $SIG{ALRM} = sub { die "TIMEOUT\n" };
    alarm(10);
    $libxml->parse_html_string($html);
    alarm(0);
};
if ($@ and $@ eq "TIMEOUT\n") {
    warn "Timed out ok.\n";
} elsif ($@) {
    die $@;
}
[download]

If I replace the parse call with sleep(20) then it works as expected - the alarm triggers and the timeout is caught. If I run it as-is with my sample HTML then it never stops until killed. If you want to play along at home here's the test file:

http://sam.tregar.com/libxml-fail.html

BEWARE: that's some really bad HTML and it not only breaks XML::LibXML but it also crashed Firefox while I was writing this post the first time! You probably don't want to load it in your browser.

I've never had alarm() fail like this. Is there an alternative I can try? Any other ideas about how to handle this?

Thanks!

-sam

UPDATE: perrin reminded me about how safe-signals work in recent perls. That is indeed the problem - setting PERL_SIGNALS=unsafe makes my code DWIM, at the cost of a certain degree of safety. Ideas for alternatives are still welcome of course.

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.