}

=item $resolver->max_outstanding ($nrequests)

Sets the maximum number of outstanding requests to C<$nrequests>. See the
C<max_outstanding> constructor argument.

=cut

sub max_outstanding {
   my ($self, $max) = @_;

   $self->{max_outstanding} = $max;
   $self->_scheduler;
}

sub _compile {
   my $self = shift;

   my %search; $self->{search} = [grep 0 < length, grep !$search{$_}++, @{ $self->{search} }];
   my %server; $self->{server} = [grep 0 < length, grep !$server{$_}++, @{ $self->{server} }];

   unless (@{ $self->{server} }) {
      # use 127.0.0.1 by default, and one opendns nameserver as fallback
      $self->{server} = [v127.0.0.1, $DNS_FALLBACK[rand @DNS_FALLBACK]];
   }

   my @retry;

   for my $timeout (@{ $self->{timeout} }) {
      for my $server (@{ $self->{server} }) {
         push @retry, [$server, $timeout];
      }
   }

   $self->{retry} = \@retry;
}

sub _feed {
   my ($self, $res) = @_;

   $res = dns_unpack $res
      or return;

   my $id = $self->{id}{$res->{id}};

   return unless ref $id;

   $NOW = time;
   $id->[1]->($res);
}

sub _recv {
   my ($self, $pkt, $peer) = @_;

   # we ignore errors (often one gets port unreachable, but there is
   # no good way to take advantage of that.

   my ($port, $host) = AnyEvent::Socket::unpack_sockaddr ($peer);

   return unless $port == 53 && grep $_ eq $host, @{ $self->{server} };

   $self->_feed ($pkt);
}

sub _free_id {
   my ($self, $id, $timeout) = @_;

   if ($timeout) {
      # we need to block the id for a while
      $self->{id}{$id} = 1;
      push @{ $self->{reuse_q} }, [$NOW + $self->{reuse}, $id];
   } else {
      # we can quickly recycle the id
      delete $self->{id}{$id};
   }

   --$self->{outstanding};
   $self->_scheduler;
}

# execute a single request, involves sending it with timeouts to multiple servers
sub _exec {
   my ($self, $req) = @_;

   my $retry; # of retries
   my $do_retry;

   $do_retry = sub {
      my $retry_cfg = $self->{retry}[$retry++]
         or do {
            # failure
            $self->_free_id ($req->[2], $retry > 1);
            undef $do_retry; return $req->[1]->();
         };

      my ($server, $timeout) = @$retry_cfg;
      
      $self->{id}{$req->[2]} = [AnyEvent->timer (after => $timeout, cb => sub {
         $NOW = time;