=head1 NAME

AnyEvent::Fork::Pool - simple process pool manager on top of AnyEvent::Fork

=head1 SYNOPSIS

   use AnyEvent;
   use AnyEvent::Fork::Pool;
   # use AnyEvent::Fork is not needed

   # all parameters with default values
   my $pool = new AnyEvent::Fork::Pool
      "MyWorker::run",

      # pool management
      min        => 0, # minimum # of processes
      max        => 8, # maximum # of processes
      busy_time  => 0, # wait this before starting a new process
      max_idle   => 1, # wait this before killing an idle process
      idle_time  => 1, # at most this many idle processes

      # template process
      template   => AnyEvent::Fork->new, # the template process to use
      require    => [MyWorker::], # module(s) to load
      eval       => "# perl code to execute in template",
      on_destroy => (my $finish = AE::cv),

      # parameters passed to AnyEvent::Fork::RPC
      async      => 0,
      on_error   => sub { die "FATAL: $_[0]\n" },
      on_event   => sub { my @ev = @_ },
      init       => "MyWorker::init",
      serialiser => $AnyEvent::Fork::RPC::STRING_SERIALISER,
   ;

   for (1..10) {
      $pool->call (doit => $_, sub {
         print "MyWorker::run returned @_\n";
      });
   }

   undef $pool;

   $finish->recv;

=head1 DESCRIPTION

This module uses processes created via L<AnyEvent::Fork> and the RPC
protocol implement in L<AnyEvent::Fork::RPC> to create a load-balanced
pool of processes that handles jobs.

Understanding of L<AnyEvent::Fork> is helpful but not critical to be able
to use this module, but a thorough understanding of L<AnyEvent::Fork::RPC>
is, as it defines the actual API that needs to be implemented in the
children.

=head1 EXAMPLES

=head1 API

=over 4

=cut

package AnyEvent::Fork::Pool;

use common::sense;

use Guard ();

use AnyEvent;
use AnyEvent::Fork; # we don't actually depend on it, this is for convenience
use AnyEvent::Fork::RPC;

our $VERSION = 0.1;

=item my $rpc = new AnyEvent::Fork::RPC::pool $function, [key => value...]

=over 4

=item on_error => $cb->($msg)

Called on (fatal) errors, with a descriptive (hopefully) message. If
this callback is not provided, but C<on_event> is, then the C<on_event>
callback is called with the first argument being the string C<error>,
followed by the error message.

If neither handler is provided it prints the error to STDERR and will
start failing badly.

=item on_event => $cb->(...)

Called for every call to the C<AnyEvent::Fork::RPC::event> function in the
child, with the arguments of that function passed to the callback.

Also called on errors when no C<on_error> handler is provided.

=item on_destroy => $cb->()

Called when the C<$rpc> object has been destroyed and all requests have
been successfully handled. This is useful when you queue some requests and
want the child to go away after it has handled them. The problem is that
the parent must not exit either until all requests have been handled, and
this can be accomplished by waiting for this callback.

=item init => $function (default none)

When specified (by name), this function is called in the child as the very
first thing when taking over the process, with all the arguments normally
passed to the C<AnyEvent::Fork::run> function, except the communications
socket.

It can be used to do one-time things in the child such as storing passed
parameters or opening database connections.

It is called very early - before the serialisers are created or the
C<$function> name is resolved into a function reference, so it could be
used to load any modules that provide the serialiser or function. It can
not, however, create events.

=item async => $boolean (default: 0)

The default server used in the child does all I/O blockingly, and only
allows a single RPC call to execute concurrently.

Setting C<async> to a true value switches to another implementation that
uses L<AnyEvent> in the child and allows multiple concurrent RPC calls.

The actual API in the child is documented in the section that describes
the calling semantics of the returned C<$rpc> function.

If you want to pre-load the actual back-end modules to enable memory
sharing, then you should load C<AnyEvent::Fork::RPC::Sync> for
synchronous, and C<AnyEvent::Fork::RPC::Async> for asynchronous mode.

If you use a template process and want to fork both sync and async
children, then it is permissible to load both modules.

=item serialiser => $string (default: '(sub { pack "(w/a*)*", @_ }, sub { unpack "(w/a*)*", shift })')

All arguments, result data and event data have to be serialised to be
transferred between the processes. For this, they have to be frozen and
thawed in both parent and child processes.

By default, only octet strings can be passed between the processes, which
is reasonably fast and efficient.

For more complicated use cases, you can provide your own freeze and thaw
functions, by specifying a string with perl source code. It's supposed to
return two code references when evaluated: the first receives a list of
perl values and must return an octet string. The second receives the octet
string and must return the original list of values.

If you need an external module for serialisation, then you can either
pre-load it into your L<AnyEvent::Fork> process, or you can add a C<use>
or C<require> statement into the serialiser string. Or both.

=back

See the examples section earlier in this document for some actual
examples.

=cut

sub new {
   my ($self, $function, %arg) = @_;

   my $serialiser = delete $arg{serialiser} || $STRING_SERIALISER;
   my $on_event   = delete $arg{on_event};
   my $on_error   = delete $arg{on_error};
   my $on_destroy = delete $arg{on_destroy};
   
   # default for on_error is to on_event, if specified
   $on_error ||= $on_event
               ? sub { $on_event->(error => shift) }
               : sub { die "AnyEvent::Fork::RPC: uncaught error: $_[0].\n" };

   # default for on_event is to raise an error
   $on_event ||= sub { $on_error->("event received, but no on_event handler") };

   my ($f, $t) = eval $serialiser; die $@ if $@;

   my (@rcb, %rcb, $fh, $shutdown, $wbuf, $ww);
   my ($rlen, $rbuf, $rw) = 512 - 16;

   my $wcb = sub {
      my $len = syswrite $fh, $wbuf;

      unless (defined $len) {
         if ($! != Errno::EAGAIN && $! != Errno::EWOULDBLOCK) {
            undef $rw; undef $ww; # it ends here
            $on_error->("$!");
         }
      }

      substr $wbuf, 0, $len, "";

      unless (length $wbuf) {
         undef $ww;
         $shutdown and shutdown $fh, 1;
      }
   };

   my $module = "AnyEvent::Fork::RPC::" . ($arg{async} ? "Async" : "Sync");

   $self->require ($module)
        ->send_arg ($function, $arg{init}, $serialiser)
        ->run ("$module\::run", sub {
      $fh = shift;

      my ($id, $len);
      $rw = AE::io $fh, 0, sub {
         $rlen = $rlen * 2 + 16 if $rlen - 128 < length $rbuf;
         $len = sysread $fh, $rbuf, $rlen - length $rbuf, length $rbuf;

         if ($len) {
            while (8 <= length $rbuf) {
               ($id, $len) = unpack "LL", $rbuf;
               8 + $len <= length $rbuf
                  or last;

               my @r = $t->(substr $rbuf, 8, $len);
               substr $rbuf, 0, 8 + $len, "";

               if ($id) {
                  if (@rcb) {
                     (shift @rcb)->(@r);
                  } elsif (my $cb = delete $rcb{$id}) {
                     $cb->(@r);
                  } else {
                     undef $rw; undef $ww;
                     $on_error->("unexpected data from child");
                  }
               } else {
                  $on_event->(@r);
               }
            }
         } elsif (defined $len) {
            undef $rw; undef $ww; # it ends here

            if (@rcb || %rcb) {
               $on_error->("unexpected eof");
            } else {
               $on_destroy->();
            }
         } elsif ($! != Errno::EAGAIN && $! != Errno::EWOULDBLOCK) {
            undef $rw; undef $ww; # it ends here
            $on_error->("read: $!");
         }
      };

      $ww ||= AE::io $fh, 1, $wcb;
   });

   my $guard = Guard::guard {
      $shutdown = 1;
      $ww ||= $fh && AE::io $fh, 1, $wcb;
   };

   my $id;

   $arg{async}
      ? sub {
           $id = ($id == 0xffffffff ? 0 : $id) + 1;
           $id = ($id == 0xffffffff ? 0 : $id) + 1 while exists $rcb{$id}; # rarely loops

           $rcb{$id} = pop;

           $guard; # keep it alive

           $wbuf .= pack "LL/a*", $id, &$f;
           $ww ||= $fh && AE::io $fh, 1, $wcb;
        }
      : sub {
           push @rcb, pop;

           $guard; # keep it alive

           $wbuf .= pack "L/a*", &$f;
           $ww ||= $fh && AE::io $fh, 1, $wcb;
        }
}

=item $pool->call (..., $cb->(...))

=back

=head1 SEE ALSO

L<AnyEvent::Fork>, to create the processes in the first place.

L<AnyEvent::Fork::RPC>, which implements the RPC protocol and API.

=head1 AUTHOR AND CONTACT INFORMATION

 Marc Lehmann <schmorp@schmorp.de>
 http://software.schmorp.de/pkg/AnyEvent-Fork-Pool

=cut

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