libanyevent-perl 6.120-1 / usr / lib / perl5 / AnyEvent / Impl / POE.pm

=head1 NAME

AnyEvent::Impl::POE - AnyEvent adaptor for POE

=encoding utf-8

=head1 SYNOPSIS

   use AnyEvent;
   use POE;
  
   # this module gets loaded automatically as required

=head1 DESCRIPTION

This module provides transparent support for AnyEvent. You don't have to
do anything to make POE work with AnyEvent except by loading POE before
creating the first AnyEvent watcher. There are some cases where POE will
issue spurious (and non-suppressible) warnings. These can be avoided by
loading AnyEvent::Impl::POE before loading any other modules using POE and
AnyEvent, i.e. in your main program.

AnyEvent::Impl::POE will output some spurious message how to work around
POE's spurious messages when it detects these cases.

Unfortunately, POE isn't generic enough to implement a fully working
AnyEvent backend: POE is too badly designed, too badly documented and too
badly implemented.

Here are the details, and what it means to you if you want to be
interoperable with POE:

=over 4

=item Weird messages

If you only use C<run_one_timeslice> (as AnyEvent has to for its
condition variables), POE will print an ugly, unsuppressible, message at
program exit:

   Sessions were started, but POE::Kernel's run() method was never...

The message is correct, the question is why POE prints it in the first
place in a correct program (this is not a singular case though).

AnyEvent consequently patches the POE kernel so it thinks it already
ran. Other workarounds, even the one cited in the POE documentation
itself, have serious side effects, such as throwing away events.

The author of POE verified that this is indeed true, and has no plans to
change this.

POE has other weird messages, and sometimes weird behaviour, for example,
it doesn't support overloaded code references as callbacks for no apparent
reason.

=item One POE session per Event

AnyEvent has to create one POE::Session per event watcher, which is
immensely slow and makes watchers very large. The reason for this is
lacking lifetime management (mostly undocumented, too). Without one
session/watcher it is not possible to easily keep the kernel from running
endlessly.

This is not just a problem with the way AnyEvent has to interact with
POE, but is a principal issue with POEs lifetime management (namely
that stopping the kernel stops sessions, but AnyEvent has no control
over who and when the kernel starts or stops w.r.t. AnyEvent watcher
creation/destruction).

From benchmark data it is not clear that session creation is that costly,
though - the real inefficiencies with POE seem to come from other sources,
such as event handling.

=item One watcher per fd/event combo

POE, of course, suffers from the same bug as Tk and some other badly
designed event models in that it doesn't support multiple watchers per
fd/poll combo. The workaround is the same as with Tk: AnyEvent::Impl::POE
creates a separate file descriptor to hand to POE, which isn't fast and
certainly not nice to your resources.

Of course, without the workaround, POE also prints ugly messages again
that say the program *might* be buggy.

While this is not good to performance, at least regarding speed, with a
modern Linux kernel, the overhead is actually quite small.

=item Timing Deficiencies

POE manages to not have a function that returns the current time. This is
extremely problematic, as POE can use different time functions, which can
differ by more than a second - and user code is left guessing which one is
used.

In addition, most timer functions in POE want an absolute timestamp, which
is hard to create if all you have is a relative time and no function to
return the "current time".

And of course POE doesn't handle time jumps at all (not even when using
an event loop that happens to do that, such as L<EV>, as it does its own
unoptimised timer management).

AnyEvent works around the unavailability of the current time using
relative timers exclusively, in the hope that POE gets it right at least
internally.

=item Event Non-Ordering

POE cannot guarantee the order of callback invocation for timers, and
usually gets it wrong. That is, if you have two timers, one timing out
after another (all else being equal), the callbacks might be called in
reverse order.

How one manages to even implement stuff that way escapes me.

=item Child Watchers

POE offers child watchers - which is a laudable thing, as few event loops
do. Unfortunately, they cannot even implement AnyEvent's simple child
watchers: they are not generic enough (the POE implementation isn't even
generic enough to let properly designed back-end use their native child
watcher instead - it insist on doing it itself the broken way).

Unfortunately, POE's child handling is inherently racy: if the child
exits before the handler is created (which is impossible to avoid in
general, imagine the forked program to exit immediately because of a bug,
or imagine the POE kernel being busy for a second), one has to wait for
another event to occur, which can take an indefinite amount of time.

Of course, whenever POE reaps an unrelated child it will also output a
message for it that you cannot suppress (which shouldn't be too surprising
at this point). Very professional.

As a workaround, AnyEvent::Impl::POE will take advantage of undocumented
behaviour in POE::Kernel to catch the status of all child processes, but
it cannot guarantee delivery.

How one manages to have such a glaring bug in an event loop after ten
years of development escapes me.

(There are more annoying bugs, for example, POE runs C<waitpid>
unconditionally on finalizing, so your program will hang until all child
processes have exited.)

=item Documentation Quality

At the time of this writing, POE was in its tenth year. Still, its
documentation is extremely lacking, making it impossible to implement
stuff as trivial as AnyEvent watchers without having to resort to
undocumented behaviour or features.

For example, the POE::Kernel manpage has nine occurrences of the word TODO
with an explanation of whats missing. In general, the POE man pages are
littered with comments like "section not yet written".

Some other gems:

   This allows many object methods to also be package methods.

This is nice, but since it doesn't document I<which> methods these are,
this is utterly useless information.

   Terminal signals will kill sessions if they are not handled by a
   "sig_handled"() call. The OS signals that usually kill or dump a
   process are considered terminal in POE, but they never trigger a
   coredump. These are: HUP, INT, QUIT and TERM.

Although AnyEvent calls C<sig_handled>, removing it has no apparent
effects on POE handling SIGINT.

   refcount_increment SESSION_ID, COUNTER_NAME

Nowhere is explained which COUNTER_NAMEs are valid and which aren't - not
all scalars (or even strings) are valid counter names. Take your guess,
failure is of course completely silent. I found this out the hard way, as
the first name I came up with was silently ignored.

   get_next_event_time() returns the time the next event is due, in a form
   compatible with the UNIX time() function.

And surely, one would hope that POE supports sub-second accuracy as
documented elsewhere, unlike the explanation above implies. Yet:

   POE::Kernel timers support subsecond accuracy, but don’t expect too
   much here. Perl is not the right language for realtime programming.

... of course, Perl is not the right language to expect sub-second
accuracy - the manpage author must hate Perl to spread so much FUD in
so little space. The Deliantra game server logs with 100µs-accuracy
because Perl is fast enough to require this, and is still able to deliver
map updates with little jitter at exactly the right time. It does not,
however, use POE.

   Furthermore, since the Kernel keeps track of everything sessions do, it
   knows when a session has run out of tasks to perform.

This is impossible - how does the kernel know that a session is no longer
watching for some (external) event (e.g. by some other session)? It
cannot, and therefore this is wrong - but you would be hard pressed to
find out how to work around this and tell the kernel manually about such
events.

It gets worse, though - the notion of "task" or "resource", although used
throughout the documentation, is not defined in a usable way. For example,
waiting for a timeout is considered to be a task, waiting for a signal is
not (a session that only waits for a signal is considered finished and
gets removed). The user is left guessing when waiting for an event counts
as task and when not (in fact, the issue with signals is mentioned in
passing in a section about child watchers and directly contradicts earlier
parts in that document).

One could go on endlessly - ten years, no usable documentation.

It is likely that differences between documentation, or the one or two
things I had to guess, cause unanticipated problems with this adaptor.

=item Fragile and inconsistent API

The POE API is extremely inconsistent - sometimes you have to pass a
session argument, sometimes it gets ignored, sometimes a session-specific
method must not use a session argument.

Error handling is sub-standard as well: even for programming mistakes,
POE does not C<croak> but, in most cases, just sets C<$!> or simply does
nothing at all, leading to fragile programs.

Sometimes registering a handler uses the "eventname, parameter" ordering
(timeouts), sometimes it is "parameter, eventname" (signals). There is
little consistency overall.

=item Lack of knowledge

   The IO::Poll event loop provides an alternative that theoretically
   scales better than select().

The IO::Poll "event loop" (who in his right mind would call that an event
loop) of course scales about identically (sometimes it is a bit faster,
sometimes a bit slower) to select in theory, and also in practise, of
course, as both are O(n) in the number of file descriptors, which is
rather bad.

This is just one place where it gets obvious how little the author of the
POE manpage understands.

=item No idle events

The POE-recommended workaround to this is apparently to use
C<fork>. Consequently, idle watchers will have to be emulated by AnyEvent.

=back

On the good side, AnyEvent allows you to write your modules in a 100%
POE-compatible way (bug-for-bug compatible even), without forcing your
module to use POE - it is still open to better event models, of which
there are plenty.

Oh, and one other positive thing:

   RUNNING_IN_HELL

POE knows about the nature of the beast!

=cut

package AnyEvent::Impl::POE;

use AnyEvent (); BEGIN { AnyEvent::common_sense }
use POE;

# suppress an idiotic warning inside POE
${ POE::Kernel->new->[POE::Kernel::KR_RUN] } |= POE::Kernel::KR_RUN_CALLED;

sub io {
   my ($class, %arg) = @_;

   # POE itself might do the right thing, but some POE backends don't,
   # so do the safe thing, it's not as if this will slow us down
   # any further *g*
   my ($fh, $pee) = AnyEvent::_dupfh $arg{poll}, $arg{fh}, "select_read", "select_write";

   my $cb = delete $arg{cb}; my $cb = sub { &$cb }; # POE doesn't like callable objects

   my $session = POE::Session->create (
      inline_states => {
         _start => sub { $_[KERNEL]->$pee ($fh => "ready") },
         ready  => sub { $cb->() },
         stop   => sub { $_[KERNEL]->$pee ($fh) },
      },
   );
   bless \\$session, "AnyEvent::Impl::POE"
}

sub timer {
   my ($class, %arg) = @_;

   my $after = delete $arg{after};
   my $ival  = delete $arg{interval};
   my $cb    = delete $arg{cb}; my $cb = sub { &$cb }; # POE doesn't like callable objects

   my $session = POE::Session->create (
      inline_states => {
         _start => sub {
            $_[KERNEL]->delay_set (timeout => $after);
         },
         timeout => $ival ? sub { $_[KERNEL]->delay_set (timeout => $ival); $cb->() } : $cb,
         stop => sub {
            $_[KERNEL]->alarm_remove_all;
         },
      },
   );
   bless \\$session, "AnyEvent::Impl::POE"
}

sub signal {
   my ($class, %arg) = @_;
   my $signal = AnyEvent::Base::sig2name delete $arg{signal};
   my $cb     = delete $arg{cb}; my $cb = sub { &$cb }; # POE doesn't like callable objects
   my $session = POE::Session->create (
      inline_states => {
         _start => sub {
            # I suck - POE
         },
         start => sub {
            $_[KERNEL]->sig ($signal => "catch");
            $_[KERNEL]->refcount_increment ($_[SESSION]->ID => "poe");
         },
         catch => sub {
            $cb->();
            $_[KERNEL]->sig_handled;
         },
         stop => sub {
            $_[KERNEL]->refcount_decrement ($_[SESSION]->ID => "poe");
            $_[KERNEL]->sig ($signal);
         },
      },
   );
   POE::Kernel->call ($session, "start");
   bless \\$session, "AnyEvent::Impl::POE"
}

sub child {
   my ($class, %arg) = @_;
   my $pid = delete $arg{pid};
   my $cb  = delete $arg{cb}; my $cb = sub { &$cb }; # POE doesn't like callable objects
   my $session = POE::Session->create (
      inline_states => {
         _start => sub {
            # I suck - POE
         },
         start => sub {
            $_[KERNEL]->sig (CHLD => "child");
            $_[KERNEL]->refcount_increment ($_[SESSION]->ID => "poe");
         },
         child => sub {
            my ($rpid, $status) = @_[ARG1, ARG2];

            $cb->($rpid, $status) if $rpid == $pid || $pid == 0;
         },
         stop => sub {
            $_[KERNEL]->refcount_decrement ($_[SESSION]->ID => "poe");
            $_[KERNEL]->sig ("CHLD");
         },
      },
   );
   # newer POE versions lose signals unless we call ->sig early.
   POE::Kernel->call ($session, "start");
   bless \\$session, "AnyEvent::Impl::POE"
}

sub DESTROY {
   POE::Kernel->call (${${$_[0]}}, "stop");
}

#sub loop {
#   POE::Kernel->run;
#}

sub _poll {
   POE::Kernel->loop_do_timeslice;
}

sub AnyEvent::CondVar::Base::_wait {
   POE::Kernel->loop_do_timeslice until exists $_[0]{_ae_sent};
}

1;

=head1 SEE ALSO

L<AnyEvent>, L<POE>.

=head1 AUTHOR

 Marc Lehmann <schmorp@schmorp.de>
 http://home.schmorp.de/

=cut