leaking_promises.pl

use Mojo::Base -strict;
use Devel::Cycle;
use Mojo::UserAgent;
use Promises 'deferred';

my $ua = Mojo::UserAgent->new;

sub fetch_it {
  my $url = shift;
  my $d   = deferred();
  $ua->get(
    $url => sub {
      my ($ua, $tx) = @_;
      $d->resolve($tx);
    }
  );
  return $d;
}

my $promise1 = fetch_it('mojolicio.us');
my $promise2 = $promise1->then(sub {
  my $tx = shift;
  say $tx->res->code;
  fetch_it('mojolicio.us');
});
$promise2->then(sub {
  my $tx = shift;
  say $tx->res->code;
  Mojo::IOLoop->stop;
});
find_cycle $ua;
Mojo::IOLoop->start;

# $ perl leaking_promises.pl
# Cycle (1):
# 	$Mojo::UserAgent::A->{'connections'} => \%B                           
# 	$B->{'92724e8de92d121700aa39b721792a89'} => \%C                           
# 	                    $C->{'cb'} => \&D                           
# 	                $D variable $d => \$E                           
# 	                           $$E => \%Promises::Deferred::F       
# 	$Promises::Deferred::F->{'rejected'} => \@G                           
# 	                       $G->[0] => \&H                           
# 	                $H variable $d => \$I                           
# 	                           $$I => \%Promises::Deferred::J       
# 	$Promises::Deferred::J->{'rejected'} => \@K                           
# 	                       $K->[0] => \&L                           
# 	             $L variable $self => \$M                           
# 	                           $$M => \%Promises::Deferred::J       
#
# Cycle (2):
# 	$Mojo::UserAgent::A->{'connections'} => \%B                           
# 	$B->{'92724e8de92d121700aa39b721792a89'} => \%C                           
# 	                    $C->{'cb'} => \&D                           
# 	                $D variable $d => \$E                           
# 	                           $$E => \%Promises::Deferred::F       
# 	$Promises::Deferred::F->{'rejected'} => \@G                           
# 	                       $G->[0] => \&H                           
# 	             $H variable $self => \$N                           
# 	                           $$N => \%Promises::Deferred::F       
#
# Cycle (3):
# 	$Mojo::UserAgent::A->{'connections'} => \%B                           
# 	$B->{'92724e8de92d121700aa39b721792a89'} => \%C                           
# 	                    $C->{'cb'} => \&D                           
# 	                $D variable $d => \$E                           
# 	                           $$E => \%Promises::Deferred::F       
# 	$Promises::Deferred::F->{'resolved'} => \@O                           
# 	                       $O->[0] => \&P                           
# 	                $P variable $d => \$Q                           
# 	                           $$Q => \%Promises::Deferred::J       
# 	$Promises::Deferred::J->{'rejected'} => \@K                           
# 	                       $K->[0] => \&L                           
# 	             $L variable $self => \$M                           
# 	                           $$M => \%Promises::Deferred::J       
#
# 200
# 200