Parallel ForkManager error with run_on_wait()

[sojourn548]

When using Parallel ForkManager 0.7.5 (and the latest version 1.03) with run_on_wait(), I am seeing this error before the script terminates abnormally:

Use of uninitialized value in block exit at /lib/site_perl/5.8.7/Paral
+lel/ForkManager.pm line 365.
Use of uninitialized value in block exit at /lib/site_perl/5.8.7/Paral
+lel/ForkManager.pm line 365.
Unable to create sub named "" at /lib/site_perl/5.8.7/Parallel/ForkMan
+ager.pm line 365.
Good-Bye...

This is a constantly running process that seems to error out with the same message about once every 10 days or so, and it started occurring after I updated the script to use run_on_wait(). run_on_wait() is used to call a routine every second, and keeps track of forked processes and sends TERM signal to those child processes that have been running longer than 4 seconds. I am unsure how to go about debugging this, as this error seems to occur in rare occasions. I appreciate taking the time to review this, and thanks in advance.

our %procs;
use constant LIMIT => 4
$pm->run_on_wait(\&term_process, 1);

$pm->run_on_finish(
    sub {
        my ($pid, $exit_code, $ident) = @_;
        my ($check_id, $host) = $ident =~ /^(.*?) on (.*)/s;
        print("run_on_finish: $ident (pid: $pid) exited with code: [$e
+xit_code] host: [$host]\n");
        delete $procs{$pid};
        print("proc_mgmt: $ident: deleting (pid: $pid) from list\n");
    }
);

$pm->run_on_start(
    sub {
        my ($pid,$ident)=@_;
        print("** $ident started, pid: $pid\n");
        $procs{$pid} = time();
    }
);

sub term_process{
    my $debug_time;
    my $total_time;
    while (my ($pid, $started_at) = each %procs) {
       next unless time() - $started_at > LIMIT;
       $debug_time = time();
       $total_time = $debug_time - $started_at;
    print("[$pid] hung. time now: [$debug_time] - [$started_at]  = [$t
+otal_time] sending KILL.");
    kill TERM => $pid;
    delete $procs{$pid};
  }
}

Looking at ForkManager.pm:

    357 sub run_on_wait { my ($s,$code, $period)=@_;
    358   $s->{on_wait}=$code;
    359   $s->{on_wait_period} = $period;
    360 }
    361 
    362 sub on_wait { my ($s)=@_;
    363   if(ref($s->{on_wait}) eq 'CODE') {
    364     $s->{on_wait}->();
    365     if (defined $s->{on_wait_period}) {
    366         local $SIG{CHLD} = sub { } if ! defined $SIG{CHLD};
    367         select undef, undef, undef, $s->{on_wait_period}
    368     };
    369   };
    370 };

[Anonymous Monk]

When using Parallel ForkManager 0.7.5 ... and maybe I discovered a bug?

Try the latest, try  cpan SZABGAB/Parallel-ForkManager-1.03.tar.gz

0.7.5 is over 10 years old

[sojourn548]

That was one of my suspicions, so before I posted on PerlMonks, I downloaded and viewed the source for Parallel-ForkManager-1.03. The subs run_on_wait() and run_wait() hasn't changed, and I did not see any bug reports/changes related to the errors that I was seeing.. So I decided to post here to see if it was something that I was doing incorrectly.

[Anonymous Monk]

I downloaded and viewed the source

How about you try running the code?

[Anonymous Monk]

After having a look at the code and testing further, I think that the problem is the local scope of the $SIG{CHILD} in ForkManager's sub wait_on. Obviously, the empty subroutine is set in order to yank the process free of the following sleep (select) statement, otherwise IGNORE/undef would do just fine. When the process leaves the sub on_wait, then $SIG{CHILD} is reset to undef (likely meaning IGNORE). However, if more signals hit the process at just the right time (when exiting the sub/resetting $SIG{CHILD}), then the error is triggered. I can reproduce it outside ForkManager, so it is nothing special in ForkManager.

This error should only occur in forkmanager if the user has not set $SIG{CHILD}.

I see two possible solutions. Either remove the "local" clause on $SIG{CHLD} in ForkManager sub wait_on, OR simply set the following before you call ForkManager:

$SIG{CHLD} = sub { };

Best regards, /Bjarne