[doc] Bug in Perl Threads tutorial section "A Thread Pitfall: Deadlocks"

[mvanle]

Where

https://perldoc.perl.org/perlthrtut#A-Thread-Pitfall:-Deadlocks

Description

The following example code in the Perl Threads tutorial section "A Thread Pitfall: Deadlocks" does not work (v5.40.0 et al):

use threads;

my $x :shared = 4;
my $y :shared = 'foo';
my $thr1 = threads->create(sub {
    lock($x);
    sleep(20);
    lock($y);
});
my $thr2 = threads->create(sub {
    lock($y);
    sleep(20);
    lock($x);
});

=comment
Expected: Thread deadlock.

Actual: Script terminates with:

    Perl exited with active threads:
            2 running and unjoined
            0 finished and unjoined
            0 running and detached 

and/or does not lock when threads are join()'d.
=cut

Suggested fix:

use threads;
use threads::shared;

my $x :shared = 4;
my $y :shared = 'foo';
my $thr1 = threads->create(sub {
    lock($x);
    sleep(20);
    lock($y);
});
my $thr2 = threads->create(sub {
    lock($y);
    sleep(20);
    lock($x);
});

$thr1->join();
$thr2->join();