Coro/Coro.pm

=head1 NAME

Coro - coroutine process abstraction

=head1 SYNOPSIS

 use Coro;

 async {
    # some asynchronous thread of execution
 };

 # alternatively create an async coroutine like this:

 sub some_func : Coro {
    # some more async code
 }

 cede;

=head1 DESCRIPTION

This module collection manages coroutines. Coroutines are similar
to threads but don't run in parallel at the same time even on SMP
machines. The specific flavor of coroutine use din this module also
guarentees you that it will not switch between coroutines unless
necessary, at easily-identified points in your program, so locking and
parallel access are rarely an issue, making coroutine programming much
safer than threads programming.

(Perl, however, does not natively support real threads but instead does a
very slow and memory-intensive emulation of processes using threads. This
is a performance win on Windows machines, and a loss everywhere else).

In this module, coroutines are defined as "callchain + lexical variables +
@_ + $_ + $@ + $/ + C stack), that is, a coroutine has its own callchain,
its own set of lexicals and its own set of perls most important global
variables.

=cut

package Coro;

use strict;
no warnings "uninitialized";

use Coro::State;

use base qw(Coro::State Exporter);

our $idle;    # idle handler
our $main;    # main coroutine
our $current; # current coroutine

our $VERSION = '3.3';

our @EXPORT = qw(async async_pool cede schedule terminate current unblock_sub);
our %EXPORT_TAGS = (
      prio => [qw(PRIO_MAX PRIO_HIGH PRIO_NORMAL PRIO_LOW PRIO_IDLE PRIO_MIN)],
);
our @EXPORT_OK = (@{$EXPORT_TAGS{prio}}, qw(nready));

{
   my @async;
   my $init;

   # this way of handling attributes simply is NOT scalable ;()
   sub import {
      no strict 'refs';

      Coro->export_to_level (1, @_);

      my $old = *{(caller)[0]."::MODIFY_CODE_ATTRIBUTES"}{CODE};
      *{(caller)[0]."::MODIFY_CODE_ATTRIBUTES"} = sub {
         my ($package, $ref) = (shift, shift);
         my @attrs;
         for (@_) {
            if ($_ eq "Coro") {
               push @async, $ref;
               unless ($init++) {
                  eval q{
                     sub INIT {
                        &async(pop @async) while @async;
                     }
                  };
               }
            } else {
               push @attrs, $_;
            }
         }
         return $old ? $old->($package, $ref, @attrs) : @attrs;
      };
   }

}

=over 4

=item $main

This coroutine represents the main program.

=cut

$main = new Coro;

=item $current (or as function: current)

The current coroutine (the last coroutine switched to). The initial value
is C<$main> (of course).

This variable is B<strictly> I<read-only>. It is provided for performance
reasons. If performance is not essentiel you are encouraged to use the
C<Coro::current> function instead.

=cut

# maybe some other module used Coro::Specific before...
$main->{specific} = $current->{specific}
   if $current;

_set_current $main;

sub current() { $current }

=item $idle

A callback that is called whenever the scheduler finds no ready coroutines
to run. The default implementation prints "FATAL: deadlock detected" and
exits, because the program has no other way to continue.

This hook is overwritten by modules such as C<Coro::Timer> and
C<Coro::Event> to wait on an external event that hopefully wake up a
coroutine so the scheduler can run it.

Please note that if your callback recursively invokes perl (e.g. for event
handlers), then it must be prepared to be called recursively.

=cut

$idle = sub {
   require Carp;
   Carp::croak ("FATAL: deadlock detected");
};

sub _cancel {
   my ($self) = @_;

   # free coroutine data and mark as destructed
   $self->_destroy
      or return;

   # call all destruction callbacks
   $_->(@{$self->{status}})
      for @{(delete $self->{destroy_cb}) || []};
}

# this coroutine is necessary because a coroutine
# cannot destroy itself.
my @destroy;
my $manager;

$manager = new Coro sub {
   while () {
      (shift @destroy)->_cancel
         while @destroy;

      &schedule;
   }
};

$manager->prio (PRIO_MAX);

# static methods. not really.

=back

=head2 STATIC METHODS

Static methods are actually functions that operate on the current coroutine only.

=over 4

=item async { ... } [@args...]

Create a new asynchronous coroutine and return it's coroutine object
(usually unused). When the sub returns the new coroutine is automatically
terminated.

Calling C<exit> in a coroutine will not work correctly, so do not do that.

When the coroutine dies, the program will exit, just as in the main
program.

   # create a new coroutine that just prints its arguments
   async {
      print "@_\n";
   } 1,2,3,4;

=cut

sub async(&@) {
   my $coro = new Coro @_;
   $coro->ready;
   $coro
}

=item async_pool { ... } [@args...]

Similar to C<async>, but uses a coroutine pool, so you should not call
terminate or join (although you are allowed to), and you get a coroutine
that might have executed other code already (which can be good or bad :).

Also, the block is executed in an C<eval> context and a warning will be
issued in case of an exception instead of terminating the program, as C<async> does.

The priority will be reset to C<0> after each job, otherwise the coroutine
will be re-used "as-is".

The pool size is limited to 8 idle coroutines (this can be adjusted by
changing $Coro::POOL_SIZE), and there can be as many non-idle coros as
required.

If you are concerned about pooled coroutines growing a lot because a
single C<async_pool> used a lot of stackspace you can e.g. C<async_pool {
terminate }> once per second or so to slowly replenish the pool.

=cut

our $POOL_SIZE = 8;
our @pool;

sub pool_handler {
   while () {
      my ($cb, @arg) = @{ delete $current->{_invoke} };

      eval {
         $cb->(@arg);
      };
      warn $@ if $@;

      last if @pool >= $POOL_SIZE;
      push @pool, $current;

      $current->prio (0);
      schedule;
   }
}

sub async_pool(&@) {
   # this is also inlined into the unlock_scheduler
   my $coro = (pop @pool or new Coro \&pool_handler);

   $coro->{_invoke} = [@_];
   $coro->ready;

   $coro
}

=item schedule

Calls the scheduler. Please note that the current coroutine will not be put
into the ready queue, so calling this function usually means you will
never be called again unless something else (e.g. an event handler) calls
ready.

The canonical way to wait on external events is this:

   {
      # remember current coroutine
      my $current = $Coro::current;

      # register a hypothetical event handler
      on_event_invoke sub {
         # wake up sleeping coroutine
         $current->ready;
         undef $current;
      };

      # call schedule until event occured.
      # in case we are woken up for other reasons
      # (current still defined), loop.
      Coro::schedule while $current;
   }

=item cede

"Cede" to other coroutines. This function puts the current coroutine into the
ready queue and calls C<schedule>, which has the effect of giving up the
current "timeslice" to other coroutines of the same or higher priority.

Returns true if at least one coroutine switch has happened.

=item Coro::cede_notself

Works like cede, but is not exported by default and will cede to any
coroutine, regardless of priority, once.

Returns true if at least one coroutine switch has happened.

=item terminate [arg...]

Terminates the current coroutine with the given status values (see L<cancel>).

=cut

sub terminate {
   $current->cancel (@_);
}

=back

# dynamic methods

=head2 COROUTINE METHODS

These are the methods you can call on coroutine objects.

=over 4

=item new Coro \&sub [, @args...]

Create a new coroutine and return it. When the sub returns the coroutine
automatically terminates as if C<terminate> with the returned values were
called. To make the coroutine run you must first put it into the ready queue
by calling the ready method.

Calling C<exit> in a coroutine will not work correctly, so do not do that.

=cut

sub _run_coro {
   terminate &{+shift};
}

sub new {
   my $class = shift;

   $class->SUPER::new (\&_run_coro, @_)
}

=item $success = $coroutine->ready

Put the given coroutine into the ready queue (according to it's priority)
and return true. If the coroutine is already in the ready queue, do nothing
and return false.

=item $is_ready = $coroutine->is_ready

Return wether the coroutine is currently the ready queue or not,

=item $coroutine->cancel (arg...)

Terminates the given coroutine and makes it return the given arguments as
status (default: the empty list). Never returns if the coroutine is the
current coroutine.

=cut

sub cancel {
   my $self = shift;
   $self->{status} = [@_];

   if ($current == $self) {
      push @destroy, $self;
      $manager->ready;
      &schedule while 1;
   } else {
      $self->_cancel;
   }
}

=item $coroutine->join

Wait until the coroutine terminates and return any values given to the
C<terminate> or C<cancel> functions. C<join> can be called multiple times
from multiple coroutine.

=cut

sub join {
   my $self = shift;

   unless ($self->{status}) {
      my $current = $current;

      push @{$self->{destroy_cb}}, sub {
         $current->ready;
         undef $current;
      };

      &schedule while $current;
   }

   wantarray ? @{$self->{status}} : $self->{status}[0];
}

=item $coroutine->on_destroy (\&cb)

Registers a callback that is called when this coroutine gets destroyed,
but before it is joined. The callback gets passed the terminate arguments,
if any.

=cut

sub on_destroy {
   my ($self, $cb) = @_;

   push @{ $self->{destroy_cb} }, $cb;
}

=item $oldprio = $coroutine->prio ($newprio)

Sets (or gets, if the argument is missing) the priority of the
coroutine. Higher priority coroutines get run before lower priority
coroutines. Priorities are small signed integers (currently -4 .. +3),
that you can refer to using PRIO_xxx constants (use the import tag :prio
to get then):

   PRIO_MAX > PRIO_HIGH > PRIO_NORMAL > PRIO_LOW > PRIO_IDLE > PRIO_MIN
       3    >     1     >      0      >    -1    >    -3     >    -4

   # set priority to HIGH
   current->prio(PRIO_HIGH);

The idle coroutine ($Coro::idle) always has a lower priority than any
existing coroutine.

Changing the priority of the current coroutine will take effect immediately,
but changing the priority of coroutines in the ready queue (but not
running) will only take effect after the next schedule (of that
coroutine). This is a bug that will be fixed in some future version.

=item $newprio = $coroutine->nice ($change)

Similar to C<prio>, but subtract the given value from the priority (i.e.
higher values mean lower priority, just as in unix).

=item $olddesc = $coroutine->desc ($newdesc)

Sets (or gets in case the argument is missing) the description for this
coroutine. This is just a free-form string you can associate with a coroutine.

=cut

sub desc {
   my $old = $_[0]{desc};
   $_[0]{desc} = $_[1] if @_ > 1;
   $old;
}

=back

=head2 GLOBAL FUNCTIONS

=over 4

=item Coro::nready

Returns the number of coroutines that are currently in the ready state,
i.e. that can be swicthed to. The value C<0> means that the only runnable
coroutine is the currently running one, so C<cede> would have no effect,
and C<schedule> would cause a deadlock unless there is an idle handler
that wakes up some coroutines.

=item my $guard = Coro::guard { ... }

This creates and returns a guard object. Nothing happens until the objetc
gets destroyed, in which case the codeblock given as argument will be
executed. This is useful to free locks or other resources in case of a
runtime error or when the coroutine gets canceled, as in both cases the
guard block will be executed. The guard object supports only one method,
C<< ->cancel >>, which will keep the codeblock from being executed.

Example: set some flag and clear it again when the coroutine gets canceled
or the function returns:

   sub do_something {
      my $guard = Coro::guard { $busy = 0 };
      $busy = 1;

      # do something that requires $busy to be true
   }

=cut

sub guard(&) {
   bless \(my $cb = $_[0]), "Coro::guard"
}

sub Coro::guard::cancel {
   ${$_[0]} = sub { };
}

sub Coro::guard::DESTROY {
   ${$_[0]}->();
}


=item unblock_sub { ... }

This utility function takes a BLOCK or code reference and "unblocks" it,
returning the new coderef. This means that the new coderef will return
immediately without blocking, returning nothing, while the original code
ref will be called (with parameters) from within its own coroutine.

The reason this fucntion exists is that many event libraries (such as the
venerable L<Event|Event> module) are not coroutine-safe (a weaker form
of thread-safety). This means you must not block within event callbacks,
otherwise you might suffer from crashes or worse.

This function allows your callbacks to block by executing them in another
coroutine where it is safe to block. One example where blocking is handy
is when you use the L<Coro::AIO|Coro::AIO> functions to save results to
disk.

In short: simply use C<unblock_sub { ... }> instead of C<sub { ... }> when
creating event callbacks that want to block.

=cut

our @unblock_queue;

# we create a special coro because we want to cede,
# to reduce pressure on the coro pool (because most callbacks
# return immediately and can be reused) and because we cannot cede
# inside an event callback.
our $unblock_scheduler = async {
   while () {
      while (my $cb = pop @unblock_queue) {
         # this is an inlined copy of async_pool
         my $coro = (pop @pool or new Coro \&pool_handler);

         $coro->{_invoke} = $cb;
         $coro->ready;
         cede; # for short-lived callbacks, this reduces pressure on the coro pool
      }
      schedule; # sleep well
   }
};

sub unblock_sub(&) {
   my $cb = shift;

   sub {
      unshift @unblock_queue, [$cb, @_];
      $unblock_scheduler->ready;
   }
}

=back

=cut

1;

=head1 BUGS/LIMITATIONS

 - you must make very sure that no coro is still active on global
   destruction. very bad things might happen otherwise (usually segfaults).

 - this module is not thread-safe. You should only ever use this module
   from the same thread (this requirement might be losened in the future
   to allow per-thread schedulers, but Coro::State does not yet allow
   this).

=head1 SEE ALSO

Support/Utility: L<Coro::Cont>, L<Coro::Specific>, L<Coro::State>, L<Coro::Util>.

Locking/IPC: L<Coro::Signal>, L<Coro::Channel>, L<Coro::Semaphore>, L<Coro::SemaphoreSet>, L<Coro::RWLock>.

Event/IO: L<Coro::Timer>, L<Coro::Event>, L<Coro::Handle>, L<Coro::Socket>, L<Coro::Select>.

Embedding: L<Coro:MakeMaker>

=head1 AUTHOR

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

=cut

Revision:	1.107
Committed:	Fri Jan 5 18:25:51 2007 UTC (17 years, 5 months ago) by root
Branch:	MAIN
Changes since 1.106:	+4 -0 lines
Log Message:	* empty log message *
#	Content
1	=head1 NAME
2
3	Coro - coroutine process abstraction
4
5	=head1 SYNOPSIS
6
7	use Coro;
8
9	async {
10	# some asynchronous thread of execution
11	};
12
13	# alternatively create an async coroutine like this:
14
15	sub some_func : Coro {
16	# some more async code
17	}
18
19	cede;
20
21	=head1 DESCRIPTION
22
23	This module collection manages coroutines. Coroutines are similar
24	to threads but don't run in parallel at the same time even on SMP
25	machines. The specific flavor of coroutine use din this module also
26	guarentees you that it will not switch between coroutines unless
27	necessary, at easily-identified points in your program, so locking and
28	parallel access are rarely an issue, making coroutine programming much
29	safer than threads programming.
30
31	(Perl, however, does not natively support real threads but instead does a
32	very slow and memory-intensive emulation of processes using threads. This
33	is a performance win on Windows machines, and a loss everywhere else).
34
35	In this module, coroutines are defined as "callchain + lexical variables +
36	@_ + $_ + $@ + $/ + C stack), that is, a coroutine has its own callchain,
37	its own set of lexicals and its own set of perls most important global
38	variables.
39
40	=cut
41
42	package Coro;
43
44	use strict;
45	no warnings "uninitialized";
46
47	use Coro::State;
48
49	use base qw(Coro::State Exporter);
50
51	our $idle; # idle handler
52	our $main; # main coroutine
53	our $current; # current coroutine
54
55	our $VERSION = '3.3';
56
57	our @EXPORT = qw(async async_pool cede schedule terminate current unblock_sub);
58	our %EXPORT_TAGS = (
59	prio => [qw(PRIO_MAX PRIO_HIGH PRIO_NORMAL PRIO_LOW PRIO_IDLE PRIO_MIN)],
60	);
61	our @EXPORT_OK = (@{$EXPORT_TAGS{prio}}, qw(nready));
62
63	{
64	my @async;
65	my $init;
66
67	# this way of handling attributes simply is NOT scalable ;()
68	sub import {
69	no strict 'refs';
70
71	Coro->export_to_level (1, @_);
72
73	my $old = *{(caller)[0]."::MODIFY_CODE_ATTRIBUTES"}{CODE};
74	*{(caller)[0]."::MODIFY_CODE_ATTRIBUTES"} = sub {
75	my ($package, $ref) = (shift, shift);
76	my @attrs;
77	for (@_) {
78	if ($_ eq "Coro") {
79	push @async, $ref;
80	unless ($init++) {
81	eval q{
82	sub INIT {
83	&async(pop @async) while @async;
84	}
85	};
86	}
87	} else {
88	push @attrs, $_;
89	}
90	}
91	return $old ? $old->($package, $ref, @attrs) : @attrs;
92	};
93	}
94
95	}
96
97	=over 4
98
99	=item $main
100
101	This coroutine represents the main program.
102
103	=cut
104
105	$main = new Coro;
106
107	=item $current (or as function: current)
108
109	The current coroutine (the last coroutine switched to). The initial value
110	is C<$main> (of course).
111
112	This variable is B<strictly> I<read-only>. It is provided for performance
113	reasons. If performance is not essentiel you are encouraged to use the
114	C<Coro::current> function instead.
115
116	=cut
117
118	# maybe some other module used Coro::Specific before...
119	$main->{specific} = $current->{specific}
120	if $current;
121
122	_set_current $main;
123
124	sub current() { $current }
125
126	=item $idle
127
128	A callback that is called whenever the scheduler finds no ready coroutines
129	to run. The default implementation prints "FATAL: deadlock detected" and
130	exits, because the program has no other way to continue.
131
132	This hook is overwritten by modules such as C<Coro::Timer> and
133	C<Coro::Event> to wait on an external event that hopefully wake up a
134	coroutine so the scheduler can run it.
135
136	Please note that if your callback recursively invokes perl (e.g. for event
137	handlers), then it must be prepared to be called recursively.
138
139	=cut
140
141	$idle = sub {
142	require Carp;
143	Carp::croak ("FATAL: deadlock detected");
144	};
145
146	sub _cancel {
147	my ($self) = @_;
148
149	# free coroutine data and mark as destructed
150	$self->_destroy
151	or return;
152
153	# call all destruction callbacks
154	$_->(@{$self->{status}})
155	for @{(delete $self->{destroy_cb}) \|\| []};
156	}
157
158	# this coroutine is necessary because a coroutine
159	# cannot destroy itself.
160	my @destroy;
161	my $manager;
162
163	$manager = new Coro sub {
164	while () {
165	(shift @destroy)->_cancel
166	while @destroy;
167
168	&schedule;
169	}
170	};
171
172	$manager->prio (PRIO_MAX);
173
174	# static methods. not really.
175
176	=back
177
178	=head2 STATIC METHODS
179
180	Static methods are actually functions that operate on the current coroutine only.
181
182	=over 4
183
184	=item async { ... } [@args...]
185
186	Create a new asynchronous coroutine and return it's coroutine object
187	(usually unused). When the sub returns the new coroutine is automatically
188	terminated.
189
190	Calling C<exit> in a coroutine will not work correctly, so do not do that.
191
192	When the coroutine dies, the program will exit, just as in the main
193	program.
194
195	# create a new coroutine that just prints its arguments
196	async {
197	print "@_\n";
198	} 1,2,3,4;
199
200	=cut
201
202	sub async(&@) {
203	my $coro = new Coro @_;
204	$coro->ready;
205	$coro
206	}
207
208	=item async_pool { ... } [@args...]
209
210	Similar to C<async>, but uses a coroutine pool, so you should not call
211	terminate or join (although you are allowed to), and you get a coroutine
212	that might have executed other code already (which can be good or bad :).
213
214	Also, the block is executed in an C<eval> context and a warning will be
215	issued in case of an exception instead of terminating the program, as C<async> does.
216
217	The priority will be reset to C<0> after each job, otherwise the coroutine
218	will be re-used "as-is".
219
220	The pool size is limited to 8 idle coroutines (this can be adjusted by
221	changing $Coro::POOL_SIZE), and there can be as many non-idle coros as
222	required.
223
224	If you are concerned about pooled coroutines growing a lot because a
225	single C<async_pool> used a lot of stackspace you can e.g. C<async_pool {
226	terminate }> once per second or so to slowly replenish the pool.
227
228	=cut
229
230	our $POOL_SIZE = 8;
231	our @pool;
232
233	sub pool_handler {
234	while () {
235	my ($cb, @arg) = @{ delete $current->{_invoke} };
236
237	eval {
238	$cb->(@arg);
239	};
240	warn $@ if $@;
241
242	last if @pool >= $POOL_SIZE;
243	push @pool, $current;
244
245	$current->prio (0);
246	schedule;
247	}
248	}
249
250	sub async_pool(&@) {
251	# this is also inlined into the unlock_scheduler
252	my $coro = (pop @pool or new Coro \&pool_handler);
253
254	$coro->{_invoke} = [@_];
255	$coro->ready;
256
257	$coro
258	}
259
260	=item schedule
261
262	Calls the scheduler. Please note that the current coroutine will not be put
263	into the ready queue, so calling this function usually means you will
264	never be called again unless something else (e.g. an event handler) calls
265	ready.
266
267	The canonical way to wait on external events is this:
268
269	{
270	# remember current coroutine
271	my $current = $Coro::current;
272
273	# register a hypothetical event handler
274	on_event_invoke sub {
275	# wake up sleeping coroutine
276	$current->ready;
277	undef $current;
278	};
279
280	# call schedule until event occured.
281	# in case we are woken up for other reasons
282	# (current still defined), loop.
283	Coro::schedule while $current;
284	}
285
286	=item cede
287
288	"Cede" to other coroutines. This function puts the current coroutine into the
289	ready queue and calls C<schedule>, which has the effect of giving up the
290	current "timeslice" to other coroutines of the same or higher priority.
291
292	Returns true if at least one coroutine switch has happened.
293
294	=item Coro::cede_notself
295
296	Works like cede, but is not exported by default and will cede to any
297	coroutine, regardless of priority, once.
298
299	Returns true if at least one coroutine switch has happened.
300
301	=item terminate [arg...]
302
303	Terminates the current coroutine with the given status values (see L<cancel>).
304
305	=cut
306
307	sub terminate {
308	$current->cancel (@_);
309	}
310
311	=back
312
313	# dynamic methods
314
315	=head2 COROUTINE METHODS
316
317	These are the methods you can call on coroutine objects.
318
319	=over 4
320
321	=item new Coro \&sub [, @args...]
322
323	Create a new coroutine and return it. When the sub returns the coroutine
324	automatically terminates as if C<terminate> with the returned values were
325	called. To make the coroutine run you must first put it into the ready queue
326	by calling the ready method.
327
328	Calling C<exit> in a coroutine will not work correctly, so do not do that.
329
330	=cut
331
332	sub _run_coro {
333	terminate &{+shift};
334	}
335
336	sub new {
337	my $class = shift;
338
339	$class->SUPER::new (\&_run_coro, @_)
340	}
341
342	=item $success = $coroutine->ready
343
344	Put the given coroutine into the ready queue (according to it's priority)
345	and return true. If the coroutine is already in the ready queue, do nothing
346	and return false.
347
348	=item $is_ready = $coroutine->is_ready
349
350	Return wether the coroutine is currently the ready queue or not,
351
352	=item $coroutine->cancel (arg...)
353
354	Terminates the given coroutine and makes it return the given arguments as
355	status (default: the empty list). Never returns if the coroutine is the
356	current coroutine.
357
358	=cut
359
360	sub cancel {
361	my $self = shift;
362	$self->{status} = [@_];
363
364	if ($current == $self) {
365	push @destroy, $self;
366	$manager->ready;
367	&schedule while 1;
368	} else {
369	$self->_cancel;
370	}
371	}
372
373	=item $coroutine->join
374
375	Wait until the coroutine terminates and return any values given to the
376	C<terminate> or C<cancel> functions. C<join> can be called multiple times
377	from multiple coroutine.
378
379	=cut
380
381	sub join {
382	my $self = shift;
383
384	unless ($self->{status}) {
385	my $current = $current;
386
387	push @{$self->{destroy_cb}}, sub {
388	$current->ready;
389	undef $current;
390	};
391
392	&schedule while $current;
393	}
394
395	wantarray ? @{$self->{status}} : $self->{status}[0];
396	}
397
398	=item $coroutine->on_destroy (\&cb)
399
400	Registers a callback that is called when this coroutine gets destroyed,
401	but before it is joined. The callback gets passed the terminate arguments,
402	if any.
403
404	=cut
405
406	sub on_destroy {
407	my ($self, $cb) = @_;
408
409	push @{ $self->{destroy_cb} }, $cb;
410	}
411
412	=item $oldprio = $coroutine->prio ($newprio)
413
414	Sets (or gets, if the argument is missing) the priority of the
415	coroutine. Higher priority coroutines get run before lower priority
416	coroutines. Priorities are small signed integers (currently -4 .. +3),
417	that you can refer to using PRIO_xxx constants (use the import tag :prio
418	to get then):
419
420	PRIO_MAX > PRIO_HIGH > PRIO_NORMAL > PRIO_LOW > PRIO_IDLE > PRIO_MIN
421	3 > 1 > 0 > -1 > -3 > -4
422
423	# set priority to HIGH
424	current->prio(PRIO_HIGH);
425
426	The idle coroutine ($Coro::idle) always has a lower priority than any
427	existing coroutine.
428
429	Changing the priority of the current coroutine will take effect immediately,
430	but changing the priority of coroutines in the ready queue (but not
431	running) will only take effect after the next schedule (of that
432	coroutine). This is a bug that will be fixed in some future version.
433
434	=item $newprio = $coroutine->nice ($change)
435
436	Similar to C<prio>, but subtract the given value from the priority (i.e.
437	higher values mean lower priority, just as in unix).
438
439	=item $olddesc = $coroutine->desc ($newdesc)
440
441	Sets (or gets in case the argument is missing) the description for this
442	coroutine. This is just a free-form string you can associate with a coroutine.
443
444	=cut
445
446	sub desc {
447	my $old = $_[0]{desc};
448	$_[0]{desc} = $_[1] if @_ > 1;
449	$old;
450	}
451
452	=back
453
454	=head2 GLOBAL FUNCTIONS
455
456	=over 4
457
458	=item Coro::nready
459
460	Returns the number of coroutines that are currently in the ready state,
461	i.e. that can be swicthed to. The value C<0> means that the only runnable
462	coroutine is the currently running one, so C<cede> would have no effect,
463	and C<schedule> would cause a deadlock unless there is an idle handler
464	that wakes up some coroutines.
465
466	=item my $guard = Coro::guard { ... }
467
468	This creates and returns a guard object. Nothing happens until the objetc
469	gets destroyed, in which case the codeblock given as argument will be
470	executed. This is useful to free locks or other resources in case of a
471	runtime error or when the coroutine gets canceled, as in both cases the
472	guard block will be executed. The guard object supports only one method,
473	C<< ->cancel >>, which will keep the codeblock from being executed.
474
475	Example: set some flag and clear it again when the coroutine gets canceled
476	or the function returns:
477
478	sub do_something {
479	my $guard = Coro::guard { $busy = 0 };
480	$busy = 1;
481
482	# do something that requires $busy to be true
483	}
484
485	=cut
486
487	sub guard(&) {
488	bless \(my $cb = $_[0]), "Coro::guard"
489	}
490
491	sub Coro::guard::cancel {
492	${$_[0]} = sub { };
493	}
494
495	sub Coro::guard::DESTROY {
496	${$_[0]}->();
497	}
498
499
500	=item unblock_sub { ... }
501
502	This utility function takes a BLOCK or code reference and "unblocks" it,
503	returning the new coderef. This means that the new coderef will return
504	immediately without blocking, returning nothing, while the original code
505	ref will be called (with parameters) from within its own coroutine.
506
507	The reason this fucntion exists is that many event libraries (such as the
508	venerable L<Event\|Event> module) are not coroutine-safe (a weaker form
509	of thread-safety). This means you must not block within event callbacks,
510	otherwise you might suffer from crashes or worse.
511
512	This function allows your callbacks to block by executing them in another
513	coroutine where it is safe to block. One example where blocking is handy
514	is when you use the L<Coro::AIO\|Coro::AIO> functions to save results to
515	disk.
516
517	In short: simply use C<unblock_sub { ... }> instead of C<sub { ... }> when
518	creating event callbacks that want to block.
519
520	=cut
521
522	our @unblock_queue;
523
524	# we create a special coro because we want to cede,
525	# to reduce pressure on the coro pool (because most callbacks
526	# return immediately and can be reused) and because we cannot cede
527	# inside an event callback.
528	our $unblock_scheduler = async {
529	while () {
530	while (my $cb = pop @unblock_queue) {
531	# this is an inlined copy of async_pool
532	my $coro = (pop @pool or new Coro \&pool_handler);
533
534	$coro->{_invoke} = $cb;
535	$coro->ready;
536	cede; # for short-lived callbacks, this reduces pressure on the coro pool
537	}
538	schedule; # sleep well
539	}
540	};
541
542	sub unblock_sub(&) {
543	my $cb = shift;
544
545	sub {
546	unshift @unblock_queue, [$cb, @_];
547	$unblock_scheduler->ready;
548	}
549	}
550
551	=back
552
553	=cut
554
555	1;
556
557	=head1 BUGS/LIMITATIONS
558
559	- you must make very sure that no coro is still active on global
560	destruction. very bad things might happen otherwise (usually segfaults).
561
562	- this module is not thread-safe. You should only ever use this module
563	from the same thread (this requirement might be losened in the future
564	to allow per-thread schedulers, but Coro::State does not yet allow
565	this).
566
567	=head1 SEE ALSO
568
569	Support/Utility: L<Coro::Cont>, L<Coro::Specific>, L<Coro::State>, L<Coro::Util>.
570
571	Locking/IPC: L<Coro::Signal>, L<Coro::Channel>, L<Coro::Semaphore>, L<Coro::SemaphoreSet>, L<Coro::RWLock>.
572
573	Event/IO: L<Coro::Timer>, L<Coro::Event>, L<Coro::Handle>, L<Coro::Socket>, L<Coro::Select>.
574
575	Embedding: L<Coro:MakeMaker>
576
577	=head1 AUTHOR
578
579	Marc Lehmann <schmorp@schmorp.de>
580	http://home.schmorp.de/
581
582	=cut
583