Coro/Coro/AnyEvent.pm

=head1 NAME

Coro::AnyEvent - integrate threads into AnyEvent

=head1 SYNOPSIS

 use Coro;
 use AnyEvent;
 # using both Coro and AnyEvent will automatically load Coro::AnyEvent

 # or load it manually for it's utility functions:
 use Coro::AnyEvent;

 Coro::AnyEvent::sleep 5;     # block current thread for 5s
 Coro::AnyEvent::poll;        # poll for new events once
 Coro::AnyEvent::idle;        # block until process no longer busy
 Coro::AnyEvent::idle_upto 5; # same, but only up to 5 seconds

 Coro::AnyEvent::readable $fh, 60
    or die "fh didn't become readable within 60 seconds\n";

=head1 DESCRIPTION

When one naively starts to use threads in Perl, one will quickly run
into the problem that threads that block on a syscall (sleeping,
reading from a socket etc.) will block all threads.

If one then uses an event loop, the problem is that the event loop has
no knowledge of threads and will not run them before it polls for new
events, again blocking the whole process.

This module integrates threads into any event loop supported by
AnyEvent, combining event-based programming with coroutine-based
programming in a natural way.

As of Coro 5.21 and newer, this module gets loaded automatically when
AnyEvent initialises itself and Coro is used in the same process, thus
there is no need to load it manually if you just want your threads to
coexist with AnyEvent.

If you want to use any functions from this module, you of course still
need to C<use Coro::AnyEvent>, just as with other perl modules.

Also, this module autodetects the event loop used (by relying on
L<AnyEvent>) and will either automatically defer to the high-performance
L<Coro::EV> or L<Coro::Event> modules, or will use a generic integration
method that should work with any event loop supported by L<AnyEvent>.

=head1 USAGE

=head2 RUN AN EVENT LOOP - OR NOT?

For performance reasons, it is recommended that the main program or
something else runs the event loop of the event model you use, i.e.

   use Gtk2; # <- the event model
   use AnyEvent;
   use Coro:

   # initialise stuff
   async { ... };

   # now run mainloop of Gtk2
   main Gtk2;

You can move the event loop into a thread as well, although this tends to
get confusing:

   use Gtk2;
   use AnyEvent;
   use Coro:

   async { main Gtk2 };

   # do other things...
   while () {
      use Coro::AnyEvent;
      Coro::AnyEvent::sleep 1;
      print "ping...\n";
   }

You can also do nothing, in which case Coro:AnyEvent will invoke the event
loop as needed, which is less efficient, but sometimes very convenient.

What you I<MUST NOT DO EVER> is to block inside an event loop
callback. The reason is that most event loops are not reentrant and this
can cause a deadlock at best and corrupt memory at worst.

Coro will try to catch you when you block in the event loop
("FATAL:$Coro::IDLE blocked itself"), but this is just best effort and
only works when you do not run your own event loop.

To avoid this problem, simply do not block inside an event callback
- start a new thread (e.g. with C<Coro:async_pool>) or use
C<Coro::unblock_sub>.

=head2 INVERSION OF CONTROL

If you need to wait for a single event, the rouse functions will come in
handy (see the Coro manpage for details):

   # wait for single SIGINT
   {
      my $int_w = AnyEvent->signal (signal => "INT", cb => Coro::rouse_cb);
      Coro::rouse_wait;
   }

=head2 EVENT MODULES OTHER THEN ANYEVENT

Keep in mind that, as shipped, Coro and Coro::AnyEvent only work with
AnyEvent, and only when AnyEvent is actually used (i.e. initialised), so
this will not work:

   # does not work: EV without AnyEvent is not recognised
   use EV;
   use Coro;

   EV::loop;

And neither does this, unless you actually I<use> AnyEvent for something:

   # does not work: AnyEvent must be initialised (e.g. by creating watchers)
   use EV;
   use AnyEvent;
   use Coro;

   EV::loop;

This does work, however, because you create a watcher (condvars work,
too), thus forcing AnyEvent to initialise itself:

   # does work: AnyEvent is actually used
   use EV;
   use AnyEvent;
   use Coro;

   my $timer = AE::timer 1, 1, sub { };

   EV::loop;

And if you want to use AnyEvent just to bridge between Coro and your event
model of choice, you can simply force it to initialise itself, like this:

   # does work: AnyEvent is initialised manually
   use POE;
   use AnyEvent;
   use Coro;

   AnyEvent::detect; # force AnyEvent to integrate Coro into POE
   POE::Kernel->run;

=head1 FUNCTIONS

Coro::AnyEvent also offers a few functions that might be useful.

=over 4

=cut

package Coro::AnyEvent;

use common::sense;

use Coro;
use AnyEvent ();

our $VERSION = 6.03;

#############################################################################
# idle handler

our $IDLE;

#############################################################################
# 0-timeout idle emulation watcher

our $ACTIVITY;

sub _activity {
   $ACTIVITY ||= AE::timer 0, 0, \&_schedule;
}

Coro::_set_readyhook (\&AnyEvent::detect);

AnyEvent::post_detect {
   my $model = $AnyEvent::MODEL;

   if ($model eq "AnyEvent::Impl::EV" and eval { require Coro::EV }) {
      # provide faster versions of some functions
      Coro::EV::_set_readyhook ();

      eval '
         *sleep = \&Coro::EV::timer_once;
         *poll  = \&Coro::EV::_poll;
         *idle  = sub() {
            my $w = EV::idle Coro::rouse_cb;
            Coro::rouse_wait;
         };
         *idle_upto = sub($) {
            my $cb = Coro::rouse_cb;
            my $t = EV::timer $_[0], 0, $cb;
            my $w = EV::idle $cb;
            Coro::rouse_wait;
         };
         *readable = sub($;$) {
            EV::READ  & Coro::EV::timed_io_once $_[0], EV::READ , $_[1]
         };
         *writable = sub($;$) {
            EV::WRITE & Coro::EV::timed_io_once $_[0], EV::WRITE, $_[1]
         };
      ';
      die if $@;

   } elsif ($model eq "AnyEvent::Impl::Event" and eval { require Coro::Event }) {
      Coro::_set_readyhook undef;
      # let Coro::Event do its thing
   } else {
      # do the inefficient thing ourselves
      Coro::_set_readyhook \&_activity;

      $IDLE = new Coro sub {
         my $_poll = AnyEvent->can ("_poll")
                  || AnyEvent->can ("one_event"); # AnyEvent < 6.0

         while () {
            $_poll->();
            Coro::schedule if Coro::nready;
         }
      };
      $IDLE->{desc} = "[AnyEvent idle process]";

      $Coro::idle = $IDLE;

      # call the readyhook, in case coroutines were already readied
      _activity;
   }

   # augment condvars

   *AnyEvent::CondVar::_send = sub {
      (delete $_[0]{_ae_coro})->ready if $_[0]{_ae_coro};
   };

   *AnyEvent::CondVar::_wait = sub {
      do {
         local $_[0]{_ae_coro} = $Coro::current;
         Coro::schedule;
      } until $_[0]{_ae_sent};
   };
};

=item Coro::AnyEvent::poll

This call will block the current thread until the event loop has polled
for potential new events and instructs the event loop to poll for new
events once, without blocking.

Note that this call will not actually execute the poll, nor will it wait
until there are some events, just block until the event loop has polled
for new events, so other threads will have a chance to run.

This is useful when you have a thread that does some computations, but you
still want to poll for new events from time to time. Simply call C<poll>
from time to time:

   my $long_calc = async {
      for (1..10000) {
         Coro::AnyEvent::poll:
         # do some stuff, make sure it takes at least 0.001s or so
      }
   }

Although you should also consider C<idle> or C<idle_upto> in such cases.

=item Coro::AnyEvent::sleep $seconds

This blocks the current thread for at least the given number of seconds.

=item Coro::AnyEvent::idle

This call is similar to C<poll> in that it will also poll for
events. Unlike C<poll>, it will only resume the thread once there are no
events to handle anymore, i.e. when the process is otherwise idle.

This is good for background threads that shouldn't use CPU time when
foreground jobs are ready to run.

=item Coro::AnyEvent::idle_upto $seconds

Like C<idle>, but with a maximum waiting time.

If your process is busy handling events, calling C<idle> can mean that
your thread will never be resumed. To avoid this, you can use C<idle_upto>
and specify a timeout, after which your thread will be resumed even if the
process is completely busy.

=item Coro::AnyEvent::readable $fh_or_fileno[, $timeout]

=item Coro::AnyEvent::writable $fh_or_fileno[, $timeout]

Blocks the current thread until the given file handle (or file descriptor)
becomes readable (or writable), or the given timeout has elapsed,
whichever happens first. No timeout counts as infinite timeout.

Returns true when the file handle became ready, false when a timeout
occured.

Note that these functions are quite inefficient as compared to using a
single watcher (they recreate watchers on every invocation) or compared to
using Coro::Handle.

Note also that they only work for sources that have reasonable
non-blocking behaviour (e.g. not files).

Example: wait until STDIN becomes readable, then quit the program.

   use Coro::AnyEvent;
   print "press enter to quit...\n";
   Coro::AnyEvent::readable *STDIN;
   exit 0;

=cut

sub poll() {
   my $w = AE::timer 0, 0, Coro::rouse_cb;
   Coro::rouse_wait;
}

sub sleep($) {
   my $w = AE::timer $_[0], 0, Coro::rouse_cb;
   Coro::rouse_wait;
}

sub idle() {
   my $w = AE::idle Coro::rouse_cb;
   Coro::rouse_wait;
}

sub idle_upto($) {
   my $cb = Coro::rouse_cb;
   my $t = AE::timer shift, 0, $cb;
   my $w = AE::idle $cb;
   Coro::rouse_wait;
}

sub readable($;$) {
   my $cb = Coro::rouse_cb;
   my $w = AE::io $_[0], 0, sub { $cb->(1) };
   my $t = defined $_[1] && AE::timer $_[1], 0, sub { $cb->(0) };
   Coro::rouse_wait
}

sub writable($;$) {
   my $cb = Coro::rouse_cb;
   my $w = AE::io $_[0], 1, sub { $cb->(1) };
   my $t = defined $_[1] && AE::timer $_[1], 0, sub { $cb->(0) };
   Coro::rouse_wait
}

1;

=back

=head1 IMPLEMENTATION DETAILS

Unfortunately, few event loops (basically only L<EV> and L<Event>)
support the kind of integration required for smooth operations well, and
consequently, AnyEvent cannot completely offer the functionality required
by this module, so we need to improvise.

Here is what this module does when it has to work with other event loops:

=over 4

=item * run ready threads before blocking the process

Each time a thread is put into the ready queue (and there are no other
threads in the ready queue), a timer with an C<after> value of C<0> is
registered with AnyEvent.

This creates something similar to an I<idle> watcher, i.e. a watcher
that keeps the event loop from blocking but still polls for new
events. (Unfortunately, some badly designed event loops (e.g. Event::Lib)
don't support a timeout of C<0> and will always block for a bit).

The callback for that timer will C<cede> to other threads of the same or
higher priority for as long as such threads exists. This has the effect of
running all threads that have work to do until all threads block to wait
for external events.

If no threads of equal or higher priority are ready, it will cede to any
thread, but only once. This has the effect of running lower-priority
threads as well, but it will not keep higher priority threads from
receiving new events.

The priority used is simply the priority of the thread that runs the event
loop, usually the main program, which usually has a priority of C<0>. Note
that Coro::AnyEvent does I<not> run an event loop for you, so unless the
main program runs one, there will simply be no event loop to C<cede> to
(event handling will still work, somewhat inefficiently, but any thread
will have a higher priority than event handling in that case).

=item * provide a suitable idle callback.

In addition to hooking into C<ready>, this module will also provide a
C<$Coro::idle> handler that runs the event loop. It is best not to take
advantage of this too often, as this is rather inefficient, but it should
work perfectly fine.

=item * provide overrides for AnyEvent's condvars

This module installs overrides for AnyEvent's condvars. That is, when
the module is loaded it will provide its own condition variables. This
makes them coroutine-safe, i.e. you can safely block on them from within a
coroutine.

=item * lead to data corruption or worse

As C<unblock_sub> cannot be used by this module (as it is the module
that implements it, basically), you must not call into the event
loop recursively from any coroutine. This is not usually a difficult
restriction to live with, just use condvars, C<unblock_sub> or other means
of inter-coroutine-communications.

If you use a module that supports AnyEvent (or uses the same event loop
as AnyEvent, making the compatible), and it offers callbacks of any kind,
then you must not block in them, either (or use e.g. C<unblock_sub>), see
the description of C<unblock_sub> in the L<Coro> module.

This also means that you should load the module as early as possible,
as only condvars created after this module has been loaded will work
correctly.

=back

=head1 SEE ALSO

L<AnyEvent>, to see which event loops are supported, L<Coro::EV> and
L<Coro::Event> for more efficient and more correct solutions (they will be
used automatically if applicable).

=head1 AUTHOR

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

=cut

Revision:	1.72
Committed:	Wed Aug 3 09:43:03 2011 UTC (12 years, 10 months ago) by root
Branch:	MAIN
CVS Tags:	rel-6_03
Changes since 1.71:	+17 -21 lines
Log Message:	* empty log message *
#	Content
1	=head1 NAME
2
3	Coro::AnyEvent - integrate threads into AnyEvent
4
5	=head1 SYNOPSIS
6
7	use Coro;
8	use AnyEvent;
9	# using both Coro and AnyEvent will automatically load Coro::AnyEvent
10
11	# or load it manually for it's utility functions:
12	use Coro::AnyEvent;
13
14	Coro::AnyEvent::sleep 5; # block current thread for 5s
15	Coro::AnyEvent::poll; # poll for new events once
16	Coro::AnyEvent::idle; # block until process no longer busy
17	Coro::AnyEvent::idle_upto 5; # same, but only up to 5 seconds
18
19	Coro::AnyEvent::readable $fh, 60
20	or die "fh didn't become readable within 60 seconds\n";
21
22	=head1 DESCRIPTION
23
24	When one naively starts to use threads in Perl, one will quickly run
25	into the problem that threads that block on a syscall (sleeping,
26	reading from a socket etc.) will block all threads.
27
28	If one then uses an event loop, the problem is that the event loop has
29	no knowledge of threads and will not run them before it polls for new
30	events, again blocking the whole process.
31
32	This module integrates threads into any event loop supported by
33	AnyEvent, combining event-based programming with coroutine-based
34	programming in a natural way.
35
36	As of Coro 5.21 and newer, this module gets loaded automatically when
37	AnyEvent initialises itself and Coro is used in the same process, thus
38	there is no need to load it manually if you just want your threads to
39	coexist with AnyEvent.
40
41	If you want to use any functions from this module, you of course still
42	need to C<use Coro::AnyEvent>, just as with other perl modules.
43
44	Also, this module autodetects the event loop used (by relying on
45	L<AnyEvent>) and will either automatically defer to the high-performance
46	L<Coro::EV> or L<Coro::Event> modules, or will use a generic integration
47	method that should work with any event loop supported by L<AnyEvent>.
48
49	=head1 USAGE
50
51	=head2 RUN AN EVENT LOOP - OR NOT?
52
53	For performance reasons, it is recommended that the main program or
54	something else runs the event loop of the event model you use, i.e.
55
56	use Gtk2; # <- the event model
57	use AnyEvent;
58	use Coro:
59
60	# initialise stuff
61	async { ... };
62
63	# now run mainloop of Gtk2
64	main Gtk2;
65
66	You can move the event loop into a thread as well, although this tends to
67	get confusing:
68
69	use Gtk2;
70	use AnyEvent;
71	use Coro:
72
73	async { main Gtk2 };
74
75	# do other things...
76	while () {
77	use Coro::AnyEvent;
78	Coro::AnyEvent::sleep 1;
79	print "ping...\n";
80	}
81
82	You can also do nothing, in which case Coro:AnyEvent will invoke the event
83	loop as needed, which is less efficient, but sometimes very convenient.
84
85	What you I<MUST NOT DO EVER> is to block inside an event loop
86	callback. The reason is that most event loops are not reentrant and this
87	can cause a deadlock at best and corrupt memory at worst.
88
89	Coro will try to catch you when you block in the event loop
90	("FATAL:$Coro::IDLE blocked itself"), but this is just best effort and
91	only works when you do not run your own event loop.
92
93	To avoid this problem, simply do not block inside an event callback
94	- start a new thread (e.g. with C<Coro:async_pool>) or use
95	C<Coro::unblock_sub>.
96
97	=head2 INVERSION OF CONTROL
98
99	If you need to wait for a single event, the rouse functions will come in
100	handy (see the Coro manpage for details):
101
102	# wait for single SIGINT
103	{
104	my $int_w = AnyEvent->signal (signal => "INT", cb => Coro::rouse_cb);
105	Coro::rouse_wait;
106	}
107
108	=head2 EVENT MODULES OTHER THEN ANYEVENT
109
110	Keep in mind that, as shipped, Coro and Coro::AnyEvent only work with
111	AnyEvent, and only when AnyEvent is actually used (i.e. initialised), so
112	this will not work:
113
114	# does not work: EV without AnyEvent is not recognised
115	use EV;
116	use Coro;
117
118	EV::loop;
119
120	And neither does this, unless you actually I<use> AnyEvent for something:
121
122	# does not work: AnyEvent must be initialised (e.g. by creating watchers)
123	use EV;
124	use AnyEvent;
125	use Coro;
126
127	EV::loop;
128
129	This does work, however, because you create a watcher (condvars work,
130	too), thus forcing AnyEvent to initialise itself:
131
132	# does work: AnyEvent is actually used
133	use EV;
134	use AnyEvent;
135	use Coro;
136
137	my $timer = AE::timer 1, 1, sub { };
138
139	EV::loop;
140
141	And if you want to use AnyEvent just to bridge between Coro and your event
142	model of choice, you can simply force it to initialise itself, like this:
143
144	# does work: AnyEvent is initialised manually
145	use POE;
146	use AnyEvent;
147	use Coro;
148
149	AnyEvent::detect; # force AnyEvent to integrate Coro into POE
150	POE::Kernel->run;
151
152	=head1 FUNCTIONS
153
154	Coro::AnyEvent also offers a few functions that might be useful.
155
156	=over 4
157
158	=cut
159
160	package Coro::AnyEvent;
161
162	use common::sense;
163
164	use Coro;
165	use AnyEvent ();
166
167	our $VERSION = 6.03;
168
169	#############################################################################
170	# idle handler
171
172	our $IDLE;
173
174	#############################################################################
175	# 0-timeout idle emulation watcher
176
177	our $ACTIVITY;
178
179	sub _activity {
180	$ACTIVITY \|\|= AE::timer 0, 0, \&_schedule;
181	}
182
183	Coro::_set_readyhook (\&AnyEvent::detect);
184
185	AnyEvent::post_detect {
186	my $model = $AnyEvent::MODEL;
187
188	if ($model eq "AnyEvent::Impl::EV" and eval { require Coro::EV }) {
189	# provide faster versions of some functions
190	Coro::EV::_set_readyhook ();
191
192	eval '
193	*sleep = \&Coro::EV::timer_once;
194	*poll = \&Coro::EV::_poll;
195	*idle = sub() {
196	my $w = EV::idle Coro::rouse_cb;
197	Coro::rouse_wait;
198	};
199	*idle_upto = sub($) {
200	my $cb = Coro::rouse_cb;
201	my $t = EV::timer $_[0], 0, $cb;
202	my $w = EV::idle $cb;
203	Coro::rouse_wait;
204	};
205	*readable = sub($;$) {
206	EV::READ & Coro::EV::timed_io_once $_[0], EV::READ , $_[1]
207	};
208	*writable = sub($;$) {
209	EV::WRITE & Coro::EV::timed_io_once $_[0], EV::WRITE, $_[1]
210	};
211	';
212	die if $@;
213
214	} elsif ($model eq "AnyEvent::Impl::Event" and eval { require Coro::Event }) {
215	Coro::_set_readyhook undef;
216	# let Coro::Event do its thing
217	} else {
218	# do the inefficient thing ourselves
219	Coro::_set_readyhook \&_activity;
220
221	$IDLE = new Coro sub {
222	my $_poll = AnyEvent->can ("_poll")
223	\|\| AnyEvent->can ("one_event"); # AnyEvent < 6.0
224
225	while () {
226	$_poll->();
227	Coro::schedule if Coro::nready;
228	}
229	};
230	$IDLE->{desc} = "[AnyEvent idle process]";
231
232	$Coro::idle = $IDLE;
233
234	# call the readyhook, in case coroutines were already readied
235	_activity;
236	}
237
238	# augment condvars
239
240	*AnyEvent::CondVar::_send = sub {
241	(delete $_[0]{_ae_coro})->ready if $_[0]{_ae_coro};
242	};
243
244	*AnyEvent::CondVar::_wait = sub {
245	do {
246	local $_[0]{_ae_coro} = $Coro::current;
247	Coro::schedule;
248	} until $_[0]{_ae_sent};
249	};
250	};
251
252	=item Coro::AnyEvent::poll
253
254	This call will block the current thread until the event loop has polled
255	for potential new events and instructs the event loop to poll for new
256	events once, without blocking.
257
258	Note that this call will not actually execute the poll, nor will it wait
259	until there are some events, just block until the event loop has polled
260	for new events, so other threads will have a chance to run.
261
262	This is useful when you have a thread that does some computations, but you
263	still want to poll for new events from time to time. Simply call C<poll>
264	from time to time:
265
266	my $long_calc = async {
267	for (1..10000) {
268	Coro::AnyEvent::poll:
269	# do some stuff, make sure it takes at least 0.001s or so
270	}
271	}
272
273	Although you should also consider C<idle> or C<idle_upto> in such cases.
274
275	=item Coro::AnyEvent::sleep $seconds
276
277	This blocks the current thread for at least the given number of seconds.
278
279	=item Coro::AnyEvent::idle
280
281	This call is similar to C<poll> in that it will also poll for
282	events. Unlike C<poll>, it will only resume the thread once there are no
283	events to handle anymore, i.e. when the process is otherwise idle.
284
285	This is good for background threads that shouldn't use CPU time when
286	foreground jobs are ready to run.
287
288	=item Coro::AnyEvent::idle_upto $seconds
289
290	Like C<idle>, but with a maximum waiting time.
291
292	If your process is busy handling events, calling C<idle> can mean that
293	your thread will never be resumed. To avoid this, you can use C<idle_upto>
294	and specify a timeout, after which your thread will be resumed even if the
295	process is completely busy.
296
297	=item Coro::AnyEvent::readable $fh_or_fileno[, $timeout]
298
299	=item Coro::AnyEvent::writable $fh_or_fileno[, $timeout]
300
301	Blocks the current thread until the given file handle (or file descriptor)
302	becomes readable (or writable), or the given timeout has elapsed,
303	whichever happens first. No timeout counts as infinite timeout.
304
305	Returns true when the file handle became ready, false when a timeout
306	occured.
307
308	Note that these functions are quite inefficient as compared to using a
309	single watcher (they recreate watchers on every invocation) or compared to
310	using Coro::Handle.
311
312	Note also that they only work for sources that have reasonable
313	non-blocking behaviour (e.g. not files).
314
315	Example: wait until STDIN becomes readable, then quit the program.
316
317	use Coro::AnyEvent;
318	print "press enter to quit...\n";
319	Coro::AnyEvent::readable *STDIN;
320	exit 0;
321
322	=cut
323
324	sub poll() {
325	my $w = AE::timer 0, 0, Coro::rouse_cb;
326	Coro::rouse_wait;
327	}
328
329	sub sleep($) {
330	my $w = AE::timer $_[0], 0, Coro::rouse_cb;
331	Coro::rouse_wait;
332	}
333
334	sub idle() {
335	my $w = AE::idle Coro::rouse_cb;
336	Coro::rouse_wait;
337	}
338
339	sub idle_upto($) {
340	my $cb = Coro::rouse_cb;
341	my $t = AE::timer shift, 0, $cb;
342	my $w = AE::idle $cb;
343	Coro::rouse_wait;
344	}
345
346	sub readable($;$) {
347	my $cb = Coro::rouse_cb;
348	my $w = AE::io $_[0], 0, sub { $cb->(1) };
349	my $t = defined $_[1] && AE::timer $_[1], 0, sub { $cb->(0) };
350	Coro::rouse_wait
351	}
352
353	sub writable($;$) {
354	my $cb = Coro::rouse_cb;
355	my $w = AE::io $_[0], 1, sub { $cb->(1) };
356	my $t = defined $_[1] && AE::timer $_[1], 0, sub { $cb->(0) };
357	Coro::rouse_wait
358	}
359
360	1;
361
362	=back
363
364	=head1 IMPLEMENTATION DETAILS
365
366	Unfortunately, few event loops (basically only L<EV> and L<Event>)
367	support the kind of integration required for smooth operations well, and
368	consequently, AnyEvent cannot completely offer the functionality required
369	by this module, so we need to improvise.
370
371	Here is what this module does when it has to work with other event loops:
372
373	=over 4
374
375	=item * run ready threads before blocking the process
376
377	Each time a thread is put into the ready queue (and there are no other
378	threads in the ready queue), a timer with an C<after> value of C<0> is
379	registered with AnyEvent.
380
381	This creates something similar to an I<idle> watcher, i.e. a watcher
382	that keeps the event loop from blocking but still polls for new
383	events. (Unfortunately, some badly designed event loops (e.g. Event::Lib)
384	don't support a timeout of C<0> and will always block for a bit).
385
386	The callback for that timer will C<cede> to other threads of the same or
387	higher priority for as long as such threads exists. This has the effect of
388	running all threads that have work to do until all threads block to wait
389	for external events.
390
391	If no threads of equal or higher priority are ready, it will cede to any
392	thread, but only once. This has the effect of running lower-priority
393	threads as well, but it will not keep higher priority threads from
394	receiving new events.
395
396	The priority used is simply the priority of the thread that runs the event
397	loop, usually the main program, which usually has a priority of C<0>. Note
398	that Coro::AnyEvent does I<not> run an event loop for you, so unless the
399	main program runs one, there will simply be no event loop to C<cede> to
400	(event handling will still work, somewhat inefficiently, but any thread
401	will have a higher priority than event handling in that case).
402
403	=item * provide a suitable idle callback.
404
405	In addition to hooking into C<ready>, this module will also provide a
406	C<$Coro::idle> handler that runs the event loop. It is best not to take
407	advantage of this too often, as this is rather inefficient, but it should
408	work perfectly fine.
409
410	=item * provide overrides for AnyEvent's condvars
411
412	This module installs overrides for AnyEvent's condvars. That is, when
413	the module is loaded it will provide its own condition variables. This
414	makes them coroutine-safe, i.e. you can safely block on them from within a
415	coroutine.
416
417	=item * lead to data corruption or worse
418
419	As C<unblock_sub> cannot be used by this module (as it is the module
420	that implements it, basically), you must not call into the event
421	loop recursively from any coroutine. This is not usually a difficult
422	restriction to live with, just use condvars, C<unblock_sub> or other means
423	of inter-coroutine-communications.
424
425	If you use a module that supports AnyEvent (or uses the same event loop
426	as AnyEvent, making the compatible), and it offers callbacks of any kind,
427	then you must not block in them, either (or use e.g. C<unblock_sub>), see
428	the description of C<unblock_sub> in the L<Coro> module.
429
430	This also means that you should load the module as early as possible,
431	as only condvars created after this module has been loaded will work
432	correctly.
433
434	=back
435
436	=head1 SEE ALSO
437
438	L<AnyEvent>, to see which event loops are supported, L<Coro::EV> and
439	L<Coro::Event> for more efficient and more correct solutions (they will be
440	used automatically if applicable).
441
442	=head1 AUTHOR
443
444	Marc Lehmann <schmorp@schmorp.de>
445	http://home.schmorp.de/
446
447	=cut
448