I used utf8::upgrade() as a pure Perl example, so that I wouldn't have to resort to Inline::C or XS and more people would be able to run the sample code.
Perl_sv_utf8_upgrade_flags_grow is one of those root functions that's invoked via many wrappers, though, like Perl_do_openn or Perl_sv_setsv_flags. There are many ways to get at it.
As noted, I discovered the issue via the SvPVutf8 XS macro. The devel branch of one of my CPAN distros, KinoSearch, is a mostly-C library which uses UTF-8 strings exclusively internally. Therefore, I use SvPVutf8 rather than SvPV for accessing string pointers from arguments.
If anybody ever uses $1 as an argument to any XS library function which uses SvPVutf8, it will get upgraded, triggering the bug:
$category =~ /(\w+)/
my $term_query = KinoSearch::Search::TermQuery->new(
field => 'category',
term => $1,
Other libraries which use SvPVutf8 include Mail::SpamAssassin, Glib, Tk, etc. However, I suspect that the problem isn't limited to us. It's more that using m//g is a little esoteric, and many functions reset $1 by turning off the SVf_POK flag -- e.g. length($1) will do it. So the problem tends not to persist for very long -- but while it does, you can get some maddeningly subtle bugs!
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