Linux::Inotify2 and threads

Question

I try to use Linxu::Inotify2 - but not in a thread.

If I have threads inside the program then I get a crash with Inotifys read. If I do not use threads all is fine.

Here is a little sample that should show the problem.

use strict;
use warnings;
use threads;
use Linux::Inotify2;

my $X = 0;  # terminate the thread: 0=nothing, 1=detach, 2=join

STDOUT->autoflush ();
my $ino = new Linux::Inotify2 ();

$ino->blocking (0);
$ino->watch (".", IN_MODIFY | IN_ATTRIB | IN_CREATE) or die "error";

my @ls = ();   # for save threads if $X == 2
for (my $i=0;;$i++)
{
    my $th = threads->create (sub { print "\nTH". shift; }, $i);  # just to have threads; do nothing
    if ($X == 1)
    {
        $th->detach ();     # detach the thread
    }
    elsif ($X == 2)
    {
        push (@ls, $th);    # save for joining after thread finished
    }

    my @events = $ino->read ();   # *** CRASH if $X = 1 or 2
    if ($X == 2)
    {
        foreach (@ls)
        {
            if ($_->is_joinable ())
            {
                $_->join ();
            }
        }
    }

    sleep (1);
}

If I set $X to 0 it will work. If I set it to 1 or 2 then I get a runtime-error.

Linux::Inotify2: read error while reading events at /usr/local/lib/x86_64-linux-gnu/perl/5.26.0/Linux/Inotify2.pm line 266.

Why is that?

EDIT

I simplified my sample to give a better view of the problem.

use strict;
use warnings;
use threads;
use Linux::Inotify2;

STDOUT->autoflush ();
my $ino = new Linux::Inotify2 ();

$ino->blocking (0);
$ino->watch (".", IN_CREATE | IN_DELETE) or die "error";

my $th = threads->create (sub {  });
$th->join ();

print "\nINO: ".$ino->poll();

Remove the join it will be ok, keep it and the poll will fail.

Answer 1

The following example implements the idea of zdim who proposed to use an event loop with poll() instead of read(). It's developed on CentOS 7 with Perl 5.16.2 and Linux-Inotify2 v1.22.

The program spawns a thread that watches the /tmp directory. You can test it by touching and removing files: touch /tmp/1 /tmp/2 && rm /tmp/1 /tmp/2

#!/usr/bin/env perl

use warnings FATAL => 'all';
use strict;
use threads;

threads->new(\&_eventListener, '/tmp')->join();

sub _eventListener {
  my ($path) = @_;

  require Linux::Inotify2;
  Linux::Inotify2->import(qw(IN_CREATE IN_DELETE IN_DELETE_SELF));

  my $fsEventListener = Linux::Inotify2->new() or 
    die('Cannot register Linux::Inotify2, stopped');

  $fsEventListener->watch(
    $path, 
    IN_CREATE() | IN_DELETE() | IN_DELETE_SELF(), 
    \&_watchCallback
  ) or die("Watch creation failed. '$!'");

  $fsEventListener->blocking(undef);

  while(1) {
    $fsEventListener->poll();
    sleep 1;
  }

  return;
}

sub _watchCallback {
  my ($e) = @_;

  printf STDOUT "Receive event '0x%04x' for file '%s'\n", $e->mask(), $e->fullname();

  return;
}