Hi fellow Monks,

I tried to install AnyEvent::ForkManager on Cygwin. Both its main dependencies, AnyEvent and Parallel::ForkManager, installed without problems, but the module itself hung right after the first test in 001_basic.t: 

~/.cpan/build/AnyEvent-ForkManager-0.04-ZOUXbL$ ./Build test
t/000_load.t ...... 1/1 # Testing AnyEvent::ForkManager/0.04
t/000_load.t ...... ok
t/001_basic.t ..... 1/63

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I sprinkled the code with tracing warns to discover where exactly the code gets stuck. The following line never finished: 

    $pm->start(
        cb => sub {
            my($pm, $exit_code) = @_;
            local $SIG{USR1} = sub { $started_all_process = 1; };
            isnt $$, $pm->manager_pid, 'called by child';       # <<==
+ HERE
            until ($started_all_process) {}; # wait
            note "exit_code: $exit_code";
            $pm->finish($exit_code);
            fail 'finish failed';
        },
        args => [$exit_code]
    );

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At first, I though that's manager_pid that doesn't return, but after replacing the line with 

my $mpid = $pm->manager_pid;
isnt $$, $mpid, 'called by child';

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it became obvious it's the isnt line that causes the issue. I delved more deeply and found out it comes from Test::SharedFork. It uses flock to lock a file that shares the information between forks. The Store::Locker is constructed with the following: 

sub new {
    my ($class, $store) = @_;

    $store->_reopen_if_needed;

    if ($store->{lock}++ == 0) {
        flock $store->{fh}, LOCK_EX or die $!;  # <<== HERE
    }

    bless { store => $store }, $class;
}

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The code stops on the flock line and stays there forever (on Linux, it works correctly). I wanted to know more, so I prepended the following to the line: 

use Data::Dumper; $Data::Dumper::Deparse = 1; warn Dumper($store);

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Not only was I able to see the structure, but all the tests passed. "A race condition," though I and replaced the line with 

use Time::HiRes qw{ usleep }; usleep 200;

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Result: PASS. When lowering the value, the tests sometimes hung again.

The questions

  1. Can someone with a MSWin machine (non-cygwin) try the same? Is the behaviour similar?
  2. Can someone explain how exactly the race condition happens in this case?

Thanks. The issue lives outside of PM at github, too.

لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to AnyEvent::ForkManager fails tests on Cygwin by choroba

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