Async-Interrupt/Interrupt.pm

=head1 NAME

Async::Interrupt - allow C/XS libraries to interrupt perl asynchronously

=head1 SYNOPSIS

 use Async::Interrupt;

=head1 DESCRIPTION

This module implements a single feature only of interest to advanced perl
modules, namely asynchronous interruptions (think "UNIX signals", which
are very similar).

Sometimes, modules wish to run code asynchronously (in another thread,
or from a signal handler), and then signal the perl interpreter on
certain events. One common way is to write some data to a pipe and use an
event handling toolkit to watch for I/O events. Another way is to send
a signal. Those methods are slow, and in the case of a pipe, also not
asynchronous - it won't interrupt a running perl interpreter.

This module implements asynchronous notifications that enable you to
signal running perl code from another thread, asynchronously, and
sometimes even without using a single syscall.

=head2 USAGE SCENARIOS

=over 4

=item Race-free signal handling

There seems to be no way to do race-free signal handling in perl: to
catch a signal, you have to execute Perl code, and between entering the
interpreter C<select> function (or other blocking functions) and executing
the select syscall is a small but relevant timespan during which signals
will be queued, but perl signal handlers will not be executed and the
blocking syscall will not be interrupted.

You can use this module to bind a signal to a callback while at the same
time activating an event pipe that you can C<select> on, fixing the race
completely.

This can be used to implement the signal hadling in event loops,
e.g. L<AnyEvent>, L<POE>, L<IO::Async::Loop> and so on.

=item Background threads want speedy reporting

Assume you want very exact timing, and you can spare an extra cpu core
for that. Then you can run an extra thread that signals your perl
interpreter. This means you can get a very exact timing source while your
perl code is number crunching, without even using a syscall to communicate
between your threads.

For example the deliantra game server uses a variant of this technique
to interrupt background processes regularly to send map updates to game
clients.

Or L<EV::Loop::Async> uses an interrupt object to wake up perl when new
events have arrived.

L<IO::AIO> and L<BDB> could also use this to speed up result reporting.

=item Speedy event loop invocation

One could use this module e.g. in L<Coro> to interrupt a running coro-thread
and cause it to enter the event loop.

Or one could bind to C<SIGIO> and tell some important sockets to send this
signal, causing the event loop to be entered to reduce network latency.

=back

=head2 HOW TO USE

You can use this module by creating an C<Async::Interrupt> object for each
such event source. This object stores a perl and/or a C-level callback
that is invoked when the C<Async::Interrupt> object gets signalled. It is
executed at the next time the perl interpreter is running (i.e. it will
interrupt a computation, but not an XS function or a syscall).

You can signal the C<Async::Interrupt> object either by calling it's C<<
->signal >> method, or, more commonly, by calling a C function. There is
also the built-in (POSIX) signal source.

The C<< ->signal_func >> returns the address of the C function that is to
be called (plus an argument to be used during the call). The signalling
function also takes an integer argument in the range SIG_ATOMIC_MIN to
SIG_ATOMIC_MAX (guaranteed to allow at least 0..127).

Since this kind of interruption is fast, but can only interrupt a
I<running> interpreter, there is optional support for signalling a pipe
- that means you can also wait for the pipe to become readable (e.g. via
L<EV> or L<AnyEvent>). This, of course, incurs the overhead of a C<read>
and C<write> syscall.

=head1 USAGE EXAMPLES

=head2 Async::Interrupt to implement race-free signal handling

This example uses a single event pipe for all signals, and one
Async::Interrupt per signal.

First, create the event pipe and hook it into the event loop (this code is
actually what L<AnyEvent> uses itself):

   $SIGPIPE = new Async::Interrupt::EventPipe;
   $SIGPIPE_W = AnyEvent->io (
      fh   => $SIGPIPE->fileno,
      poll => "r",
      cb   => \&_signal_check,
   );

Then, for each signal to hook, create an Async::Interrupt object. The
callback just sets a global variable, as we are only interested in
synchronous signals (i.e. when the event loop polls), which is why the
pipe draining is not done automatically.

   my $interrupt = new Async::Interrupt
      cb             => sub { undef $SIGNAL_RECEIVED{$signum} }
      signal         => $signal,
      pipe           => [$SIGPIPE_R->filenos],
      pipe_autodrain => 0,
   ;

Finally, the I/O callback for the event pipe handles the signals:

   sub _signal_check {
      # drain the pipe first
      $SIGPIPE->drain;

      # two loops, just to be sure
      while (%SIGNAL_RECEIVED) {
         for (keys %SIGNAL_RECEIVED) {
            delete $SIGNAL_RECEIVED{$_};
            warn "signal $_ received\n";
         }
      }
   }


=head1 THE Async::Interrupt CLASS

=over 4

=cut

package Async::Interrupt;

use common::sense;

BEGIN {
   # the next line forces initialisation of internal
   # signal handling # variables
   $SIG{KILL} = sub { };

   our $VERSION = '0.6';

   require XSLoader;
   XSLoader::load ("Async::Interrupt", $VERSION);
}

our $DIED = sub { warn "$@" };

=item $async = new Async::Interrupt key => value...

Creates a new Async::Interrupt object. You may only use async
notifications on this object while it exists, so you need to keep a
reference to it at all times while it is used.

Optional constructor arguments include (normally you would specify at
least one of C<cb> or C<c_cb>).

=over 4

=item cb => $coderef->($value)

Registers a perl callback to be invoked whenever the async interrupt is
signalled.

Note that, since this callback can be invoked at basically any time, it
must not modify any well-known global variables such as C<$/> without
restoring them again before returning.

The exceptions are C<$!> and C<$@>, which are saved and restored by
Async::Interrupt.

If the callback should throw an exception, then it will be caught,
and C<$Async::Interrupt::DIED> will be called with C<$@> containing
the exception.  The default will simply C<warn> about the message and
continue.

=item c_cb => [$c_func, $c_arg]

Registers a C callback the be invoked whenever the async interrupt is
signalled.

The C callback must have the following prototype:

   void c_func (pTHX_ void *c_arg, int value);

Both C<$c_func> and C<$c_arg> must be specified as integers/IVs, and
C<$value> is the C<value> passed to some earlier call to either C<$signal>
or the C<signal_func> function.

Note that, because the callback can be invoked at almost any time, you
have to be careful at saving and restoring global variables that Perl
might use (the exception is C<errno>, which is saved and restored by
Async::Interrupt). The callback itself runs as part of the perl context,
so you can call any perl functions and modify any perl data structures (in
which case the requirements set out for C<cb> apply as well).

=item var => $scalar_ref

When specified, then the given argument must be a reference to a
scalar. The scalar will be set to C<0> initially. Signalling the interrupt
object will set it to the passed value, handling the interrupt will reset
it to C<0> again.

Note that the only thing you are legally allowed to do is to is to check
the variable in a boolean or integer context (e.g. comparing it with a
string, or printing it, will I<destroy> it and might cause your program to
crash or worse).

=item signal => $signame_or_value

When this parameter is specified, then the Async::Interrupt will hook the
given signal, that is, it will effectively call C<< ->signal (0) >> each time
the given signal is caught by the process.

Only one async can hook a given signal, and the signal will be restored to
defaults when the Async::Interrupt object gets destroyed.

=item pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing]

Specifies two file descriptors (or file handles) that should be signalled
whenever the async interrupt is signalled. This means a single octet will
be written to it, and before the callback is being invoked, it will be
read again. Due to races, it is unlikely but possible that multiple octets
are written. It is required that the file handles are both in nonblocking
mode.

The object will keep a reference to the file handles.

This can be used to ensure that async notifications will interrupt event
frameworks as well.

Note that C<Async::Interrupt> will create a suitable signal fd
automatically when your program requests one, so you don't have to specify
this argument when all you want is an extra file descriptor to watch.

If you want to share a single event pipe between multiple Async::Interrupt
objects, you can use the C<Async::Interrupt::EventPipe> class to manage
those.

=back

=cut

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

   bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}, $arg{var}), $class
}

=item ($signal_func, $signal_arg) = $async->signal_func

Returns the address of a function to call asynchronously. The function
has the following prototype and needs to be passed the specified
C<$signal_arg>, which is a C<void *> cast to C<IV>:

   void (*signal_func) (void *signal_arg, int value)

An example call would look like:

   signal_func (signal_arg, 0);

The function is safe to call from within signal and thread contexts, at
any time. The specified C<value> is passed to both C and Perl callback.

C<$value> must be in the valid range for a C<sig_atomic_t>, except C<0>
(1..127 is portable).

If the function is called while the Async::Interrupt object is already
signaled but before the callbacks are being executed, then the stored
C<value> is either the old or the new one. Due to the asynchronous
nature of the code, the C<value> can even be passed to two consecutive
invocations of the callback.

=item $address = $async->c_var

Returns the address (cast to IV) of an C<IV> variable. The variable is set
to C<0> initially and gets set to the passed value whenever the object
gets signalled, and reset to C<0> once the interrupt has been handled.

Note that it is often beneficial to just call C<PERL_ASYNC_CHECK ()> to
handle any interrupts.

Example: call some XS function to store the address, then show C code
waiting for it.

   my_xs_func $async->c_var;

   static IV *valuep;

   void
   my_xs_func (void *addr)
           CODE:
           valuep = (IV *)addr;

   // code in a loop, waiting
   while (!*valuep)
     ; // do something

=item $async->signal ($value=1)

This signals the given async object from Perl code. Semi-obviously, this
will instantly trigger the callback invocation (it does not, as the name
might imply, do anything with POSIX signals).

C<$value> must be in the valid range for a C<sig_atomic_t>, except C<0>
(1..127 is portable).

=item $async->signal_hysteresis ($enable)

Enables or disables signal hysteresis (default: disabled). If a POSIX
signal is used as a signal source for the interrupt object, then enabling
signal hysteresis causes Async::Interrupt to reset the signal action to
C<SIG_IGN> in the signal handler and restore it just before handling the
interruption.

When you expect a lot of signals (e.g. when using SIGIO), then enabling
signal hysteresis can reduce the number of handler invocations
considerably, at the cost of two extra syscalls.

Note that setting the signal to C<SIG_IGN> can have unintended side
effects when you fork and exec other programs, as often they do nto expect
signals to be ignored by default.

=item $async->block

=item $async->unblock

Sometimes you need a "critical section" of code that will not be
interrupted by an Async::Interrupt. This can be implemented by calling C<<
$async->block >> before the critical section, and C<< $async->unblock >>
afterwards.

Note that there must be exactly one call of C<unblock> for every previous
call to C<block> (i.e. calls can nest).

Since ensuring this in the presence of exceptions and threads is
usually more difficult than you imagine, I recommend using C<<
$async->scoped_block >> instead.

=item $async->scope_block

This call C<< $async->block >> and installs a handler that is called when
the current scope is exited (via an exception, by canceling the Coro
thread, by calling last/goto etc.).

This is the recommended (and fastest) way to implement critical sections.

=item ($block_func, $block_arg) = $async->scope_block_func

Returns the address of a function that implements the C<scope_block>
functionality.

It has the following prototype and needs to be passed the specified
C<$block_arg>, which is a C<void *> cast to C<IV>:

   void (*block_func) (void *block_arg)

An example call would look like:

   block_func (block_arg);

The function is safe to call only from within the toplevel of a perl XS
function and will call C<LEAVE> and C<ENTER> (in this order!).

=item $async->pipe_enable

=item $async->pipe_disable

Enable/disable signalling the pipe when the interrupt occurs (default is
enabled). Writing to a pipe is relatively expensive, so it can be disabled
when you know you are not waiting for it (for example, with L<EV> you
could disable the pipe in a check watcher, and enable it in a prepare
watcher).

Note that currently, while C<pipe_disable> is in effect, no attempt to
read from the pipe will be done when handling events. This might change as
soon as I realize why this is a mistake.

=item $fileno = $async->pipe_fileno

Returns the reading side of the signalling pipe. If no signalling pipe is
currently attached to the object, it will dynamically create one.

Note that the only valid oepration on this file descriptor is to wait
until it is readable. The fd might belong currently to a pipe, a tcp
socket, or an eventfd, depending on the platform, and is guaranteed to be
C<select>able.

=item $async->pipe_autodrain ($enable)

Enables (C<1>) or disables (C<0>) automatic draining of the pipe (default:
enabled). When automatic draining is enabled, then Async::Interrupt will
automatically clear the pipe. Otherwise the user is responsible for this
draining.

This is useful when you want to share one pipe among many Async::Interrupt
objects.

=item $async->post_fork

The object will not normally be usable after a fork (as the pipe fd is
shared between processes). Calling this method after a fork in the child
ensures that the object will work as expected again. It only needs to be
called when the async object is used in the child.

This only works when the pipe was created by Async::Interrupt.

Async::Interrupt ensures that the reading file descriptor does not change
it's value.

=item $signum = Async::Interrupt::sig2num $signame_or_number

=item $signame = Async::Interrupt::sig2name $signame_or_number

These two convenience functions simply convert a signal name or number to
the corresponding name or number. They are not used by this module and
exist just because perl doesn't have a nice way to do this on its own.

They will return C<undef> on illegal names or numbers.

=back

=head1 THE Async::Interrupt::EventPipe CLASS

Pipes are the predominent utility to make asynchronous signals
synchronous. However, pipes are hard to come by: they don't exist on the
broken windows platform, and on GNU/Linux systems, you might want to use
an C<eventfd> instead.

This class creates selectable event pipes in a portable fashion: on
windows, it will try to create a tcp socket pair, on GNU/Linux, it will
try to create an eventfd and everywhere else it will try to use a normal
pipe.

=over 4

=item $epipe = new Async::Interrupt::EventPipe

This creates and returns an eventpipe object. This object is simply a
blessed array reference:

=item ($r_fd, $w_fd) = $epipe->filenos

Returns the read-side file descriptor and the write-side file descriptor.

Example: pass an eventpipe object as pipe to the Async::Interrupt
constructor, and create an AnyEvent watcher for the read side.

   my $epipe = new Async::Interrupt::EventPipe;
   my $asy = new Async::Interrupt pipe => [$epipe->filenos];
   my $iow = AnyEvent->io (fh => $epipe->fileno, poll => 'r', cb => sub { });

=item $r_fd = $epipe->fileno

Return only the reading/listening side.

=item $epipe->signal

Write something to the pipe, in a portable fashion.

=item $epipe->drain

Drain (empty) the pipe.

=item $epipe->renew

Recreates the pipe (useful after a fork). The reading side will not change
it's file descriptor number, but the writing side might.

=back

=cut

1;

=head1 EXAMPLE

There really should be a complete C/XS example. Bug me about it. Better
yet, create one.

=head1 IMPLEMENTATION DETAILS AND LIMITATIONS

This module works by "hijacking" SIGKILL, which is guaranteed to always
exist, but also cannot be caught, so is always available.

Basically, this module fakes the occurance of a SIGKILL signal and
then intercepts the interpreter handling it. This makes normal signal
handling slower (probably unmeasurably, though), but has the advantage
of not requiring a special runops function, nor slowing down normal perl
execution a bit.

It assumes that C<sig_atomic_t>, C<int> and C<IV> are all async-safe to
modify.

=head1 AUTHOR

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

=cut

Revision:	1.18
Committed:	Tue Jul 28 12:50:16 2009 UTC (14 years, 10 months ago) by root
Branch:	MAIN
Changes since 1.17:	+56 -0 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	=head1 NAME
2
3			Async::Interrupt - allow C/XS libraries to interrupt perl asynchronously
4
5			=head1 SYNOPSIS
6
7			use Async::Interrupt;
8
9			=head1 DESCRIPTION
10
11			This module implements a single feature only of interest to advanced perl
12	root	1.4	modules, namely asynchronous interruptions (think "UNIX signals", which
13	root	1.1	are very similar).
14
15	root	1.8	Sometimes, modules wish to run code asynchronously (in another thread,
16			or from a signal handler), and then signal the perl interpreter on
17			certain events. One common way is to write some data to a pipe and use an
18			event handling toolkit to watch for I/O events. Another way is to send
19			a signal. Those methods are slow, and in the case of a pipe, also not
20			asynchronous - it won't interrupt a running perl interpreter.
21	root	1.1
22			This module implements asynchronous notifications that enable you to
23	root	1.8	signal running perl code from another thread, asynchronously, and
24			sometimes even without using a single syscall.
25	root	1.1
26	root	1.8	=head2 USAGE SCENARIOS
27
28			=over 4
29
30			=item Race-free signal handling
31
32			There seems to be no way to do race-free signal handling in perl: to
33			catch a signal, you have to execute Perl code, and between entering the
34			interpreter C<select> function (or other blocking functions) and executing
35			the select syscall is a small but relevant timespan during which signals
36			will be queued, but perl signal handlers will not be executed and the
37			blocking syscall will not be interrupted.
38
39			You can use this module to bind a signal to a callback while at the same
40			time activating an event pipe that you can C<select> on, fixing the race
41			completely.
42
43			This can be used to implement the signal hadling in event loops,
44			e.g. L<AnyEvent>, L<POE>, L<IO::Async::Loop> and so on.
45
46			=item Background threads want speedy reporting
47
48			Assume you want very exact timing, and you can spare an extra cpu core
49			for that. Then you can run an extra thread that signals your perl
50			interpreter. This means you can get a very exact timing source while your
51			perl code is number crunching, without even using a syscall to communicate
52			between your threads.
53
54			For example the deliantra game server uses a variant of this technique
55			to interrupt background processes regularly to send map updates to game
56			clients.
57
58	root	1.17	Or L<EV::Loop::Async> uses an interrupt object to wake up perl when new
59			events have arrived.
60
61	root	1.8	L<IO::AIO> and L<BDB> could also use this to speed up result reporting.
62
63			=item Speedy event loop invocation
64
65			One could use this module e.g. in L<Coro> to interrupt a running coro-thread
66			and cause it to enter the event loop.
67
68			Or one could bind to C<SIGIO> and tell some important sockets to send this
69			signal, causing the event loop to be entered to reduce network latency.
70
71			=back
72
73			=head2 HOW TO USE
74
75			You can use this module by creating an C<Async::Interrupt> object for each
76			such event source. This object stores a perl and/or a C-level callback
77			that is invoked when the C<Async::Interrupt> object gets signalled. It is
78			executed at the next time the perl interpreter is running (i.e. it will
79			interrupt a computation, but not an XS function or a syscall).
80	root	1.2
81			You can signal the C<Async::Interrupt> object either by calling it's C<<
82	root	1.8	->signal >> method, or, more commonly, by calling a C function. There is
83			also the built-in (POSIX) signal source.
84	root	1.2
85			The C<< ->signal_func >> returns the address of the C function that is to
86			be called (plus an argument to be used during the call). The signalling
87			function also takes an integer argument in the range SIG_ATOMIC_MIN to
88			SIG_ATOMIC_MAX (guaranteed to allow at least 0..127).
89
90			Since this kind of interruption is fast, but can only interrupt a
91	root	1.8	I<running> interpreter, there is optional support for signalling a pipe
92			- that means you can also wait for the pipe to become readable (e.g. via
93			L<EV> or L<AnyEvent>). This, of course, incurs the overhead of a C<read>
94			and C<write> syscall.
95	root	1.2
96	root	1.18	=head1 USAGE EXAMPLES
97
98			=head2 Async::Interrupt to implement race-free signal handling
99
100			This example uses a single event pipe for all signals, and one
101			Async::Interrupt per signal.
102
103			First, create the event pipe and hook it into the event loop (this code is
104			actually what L<AnyEvent> uses itself):
105
106			$SIGPIPE = new Async::Interrupt::EventPipe;
107			$SIGPIPE_W = AnyEvent->io (
108			fh => $SIGPIPE->fileno,
109			poll => "r",
110			cb => \&_signal_check,
111			);
112
113			Then, for each signal to hook, create an Async::Interrupt object. The
114			callback just sets a global variable, as we are only interested in
115			synchronous signals (i.e. when the event loop polls), which is why the
116			pipe draining is not done automatically.
117
118			my $interrupt = new Async::Interrupt
119			cb => sub { undef $SIGNAL_RECEIVED{$signum} }
120			signal => $signal,
121			pipe => [$SIGPIPE_R->filenos],
122			pipe_autodrain => 0,
123			;
124
125			Finally, the I/O callback for the event pipe handles the signals:
126
127			sub _signal_check {
128			# drain the pipe first
129			$SIGPIPE->drain;
130
131			# two loops, just to be sure
132			while (%SIGNAL_RECEIVED) {
133			for (keys %SIGNAL_RECEIVED) {
134			delete $SIGNAL_RECEIVED{$_};
135			warn "signal $_ received\n";
136			}
137			}
138			}
139
140
141
142	root	1.16	=head1 THE Async::Interrupt CLASS
143
144	root	1.1	=over 4
145
146			=cut
147
148			package Async::Interrupt;
149
150	root	1.10	use common::sense;
151	root	1.2
152	root	1.1	BEGIN {
153	root	1.11	# the next line forces initialisation of internal
154			# signal handling # variables
155			$SIG{KILL} = sub { };
156
157	root	1.16	our $VERSION = '0.6';
158	root	1.1
159			require XSLoader;
160	root	1.11	XSLoader::load ("Async::Interrupt", $VERSION);
161	root	1.1	}
162
163	root	1.2	our $DIED = sub { warn "$@" };
164
165	root	1.1	=item $async = new Async::Interrupt key => value...
166
167			Creates a new Async::Interrupt object. You may only use async
168			notifications on this object while it exists, so you need to keep a
169			reference to it at all times while it is used.
170
171			Optional constructor arguments include (normally you would specify at
172			least one of C<cb> or C<c_cb>).
173
174			=over 4
175
176			=item cb => $coderef->($value)
177
178			Registers a perl callback to be invoked whenever the async interrupt is
179			signalled.
180
181			Note that, since this callback can be invoked at basically any time, it
182	root	1.2	must not modify any well-known global variables such as C<$/> without
183			restoring them again before returning.
184
185			The exceptions are C<$!> and C<$@>, which are saved and restored by
186			Async::Interrupt.
187	root	1.1
188	root	1.2	If the callback should throw an exception, then it will be caught,
189			and C<$Async::Interrupt::DIED> will be called with C<$@> containing
190			the exception. The default will simply C<warn> about the message and
191			continue.
192
193			=item c_cb => [$c_func, $c_arg]
194	root	1.1
195			Registers a C callback the be invoked whenever the async interrupt is
196			signalled.
197
198			The C callback must have the following prototype:
199
200	root	1.2	void c_func (pTHX_ void *c_arg, int value);
201	root	1.1
202	root	1.2	Both C<$c_func> and C<$c_arg> must be specified as integers/IVs, and
203			C<$value> is the C<value> passed to some earlier call to either C<$signal>
204			or the C<signal_func> function.
205	root	1.1
206			Note that, because the callback can be invoked at almost any time, you
207			have to be careful at saving and restoring global variables that Perl
208	root	1.4	might use (the exception is C<errno>, which is saved and restored by
209	root	1.2	Async::Interrupt). The callback itself runs as part of the perl context,
210			so you can call any perl functions and modify any perl data structures (in
211	root	1.4	which case the requirements set out for C<cb> apply as well).
212	root	1.1
213	root	1.13	=item var => $scalar_ref
214
215			When specified, then the given argument must be a reference to a
216	root	1.17	scalar. The scalar will be set to C<0> initially. Signalling the interrupt
217	root	1.13	object will set it to the passed value, handling the interrupt will reset
218			it to C<0> again.
219
220			Note that the only thing you are legally allowed to do is to is to check
221			the variable in a boolean or integer context (e.g. comparing it with a
222			string, or printing it, will I<destroy> it and might cause your program to
223			crash or worse).
224
225	root	1.6	=item signal => $signame_or_value
226
227			When this parameter is specified, then the Async::Interrupt will hook the
228			given signal, that is, it will effectively call C<< ->signal (0) >> each time
229			the given signal is caught by the process.
230
231			Only one async can hook a given signal, and the signal will be restored to
232			defaults when the Async::Interrupt object gets destroyed.
233
234	root	1.2	=item pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing]
235	root	1.1
236	root	1.2	Specifies two file descriptors (or file handles) that should be signalled
237	root	1.1	whenever the async interrupt is signalled. This means a single octet will
238			be written to it, and before the callback is being invoked, it will be
239			read again. Due to races, it is unlikely but possible that multiple octets
240	root	1.2	are written. It is required that the file handles are both in nonblocking
241			mode.
242	root	1.1
243	root	1.2	The object will keep a reference to the file handles.
244	root	1.1
245			This can be used to ensure that async notifications will interrupt event
246			frameworks as well.
247
248	root	1.13	Note that C<Async::Interrupt> will create a suitable signal fd
249			automatically when your program requests one, so you don't have to specify
250	root	1.16	this argument when all you want is an extra file descriptor to watch.
251
252			If you want to share a single event pipe between multiple Async::Interrupt
253			objects, you can use the C<Async::Interrupt::EventPipe> class to manage
254			those.
255	root	1.13
256	root	1.1	=back
257
258			=cut
259
260			sub new {
261			my ($class, %arg) = @_;
262
263	root	1.13	bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}, $arg{var}), $class
264	root	1.1	}
265
266	root	1.2	=item ($signal_func, $signal_arg) = $async->signal_func
267	root	1.1
268	root	1.17	Returns the address of a function to call asynchronously. The function
269			has the following prototype and needs to be passed the specified
270			C<$signal_arg>, which is a C<void *> cast to C<IV>:
271	root	1.1
272			void (signal_func) (void signal_arg, int value)
273
274			An example call would look like:
275
276			signal_func (signal_arg, 0);
277
278	root	1.2	The function is safe to call from within signal and thread contexts, at
279	root	1.1	any time. The specified C<value> is passed to both C and Perl callback.
280
281	root	1.13	C<$value> must be in the valid range for a C<sig_atomic_t>, except C<0>
282			(1..127 is portable).
283	root	1.2
284	root	1.1	If the function is called while the Async::Interrupt object is already
285			signaled but before the callbacks are being executed, then the stored
286	root	1.2	C<value> is either the old or the new one. Due to the asynchronous
287			nature of the code, the C<value> can even be passed to two consecutive
288			invocations of the callback.
289	root	1.1
290	root	1.13	=item $address = $async->c_var
291
292			Returns the address (cast to IV) of an C<IV> variable. The variable is set
293			to C<0> initially and gets set to the passed value whenever the object
294			gets signalled, and reset to C<0> once the interrupt has been handled.
295
296			Note that it is often beneficial to just call C<PERL_ASYNC_CHECK ()> to
297			handle any interrupts.
298
299			Example: call some XS function to store the address, then show C code
300			waiting for it.
301
302			my_xs_func $async->c_var;
303
304			static IV *valuep;
305
306			void
307			my_xs_func (void *addr)
308			CODE:
309			valuep = (IV *)addr;
310
311			// code in a loop, waiting
312			while (!*valuep)
313	root	1.16	; // do something
314	root	1.13
315			=item $async->signal ($value=1)
316	root	1.1
317			This signals the given async object from Perl code. Semi-obviously, this
318	root	1.17	will instantly trigger the callback invocation (it does not, as the name
319			might imply, do anything with POSIX signals).
320	root	1.1
321	root	1.13	C<$value> must be in the valid range for a C<sig_atomic_t>, except C<0>
322			(1..127 is portable).
323	root	1.2
324	root	1.17	=item $async->signal_hysteresis ($enable)
325
326			Enables or disables signal hysteresis (default: disabled). If a POSIX
327			signal is used as a signal source for the interrupt object, then enabling
328			signal hysteresis causes Async::Interrupt to reset the signal action to
329			C<SIG_IGN> in the signal handler and restore it just before handling the
330			interruption.
331
332			When you expect a lot of signals (e.g. when using SIGIO), then enabling
333			signal hysteresis can reduce the number of handler invocations
334			considerably, at the cost of two extra syscalls.
335
336			Note that setting the signal to C<SIG_IGN> can have unintended side
337			effects when you fork and exec other programs, as often they do nto expect
338			signals to be ignored by default.
339
340	root	1.2	=item $async->block
341
342	root	1.3	=item $async->unblock
343
344			Sometimes you need a "critical section" of code that will not be
345			interrupted by an Async::Interrupt. This can be implemented by calling C<<
346			$async->block >> before the critical section, and C<< $async->unblock >>
347			afterwards.
348
349	root	1.4	Note that there must be exactly one call of C<unblock> for every previous
350	root	1.3	call to C<block> (i.e. calls can nest).
351
352	root	1.4	Since ensuring this in the presence of exceptions and threads is
353	root	1.3	usually more difficult than you imagine, I recommend using C<<
354			$async->scoped_block >> instead.
355	root	1.2
356	root	1.3	=item $async->scope_block
357
358			This call C<< $async->block >> and installs a handler that is called when
359			the current scope is exited (via an exception, by canceling the Coro
360			thread, by calling last/goto etc.).
361
362			This is the recommended (and fastest) way to implement critical sections.
363	root	1.2
364	root	1.17	=item ($block_func, $block_arg) = $async->scope_block_func
365
366			Returns the address of a function that implements the C<scope_block>
367			functionality.
368
369			It has the following prototype and needs to be passed the specified
370			C<$block_arg>, which is a C<void *> cast to C<IV>:
371
372			void (block_func) (void block_arg)
373
374			An example call would look like:
375
376			block_func (block_arg);
377
378			The function is safe to call only from within the toplevel of a perl XS
379			function and will call C<LEAVE> and C<ENTER> (in this order!).
380
381	root	1.6	=item $async->pipe_enable
382
383			=item $async->pipe_disable
384
385			Enable/disable signalling the pipe when the interrupt occurs (default is
386			enabled). Writing to a pipe is relatively expensive, so it can be disabled
387			when you know you are not waiting for it (for example, with L<EV> you
388			could disable the pipe in a check watcher, and enable it in a prepare
389			watcher).
390
391	root	1.13	Note that currently, while C<pipe_disable> is in effect, no attempt to
392			read from the pipe will be done when handling events. This might change as
393			soon as I realize why this is a mistake.
394
395			=item $fileno = $async->pipe_fileno
396
397			Returns the reading side of the signalling pipe. If no signalling pipe is
398			currently attached to the object, it will dynamically create one.
399
400			Note that the only valid oepration on this file descriptor is to wait
401			until it is readable. The fd might belong currently to a pipe, a tcp
402			socket, or an eventfd, depending on the platform, and is guaranteed to be
403			C<select>able.
404
405	root	1.16	=item $async->pipe_autodrain ($enable)
406
407			Enables (C<1>) or disables (C<0>) automatic draining of the pipe (default:
408			enabled). When automatic draining is enabled, then Async::Interrupt will
409			automatically clear the pipe. Otherwise the user is responsible for this
410			draining.
411
412			This is useful when you want to share one pipe among many Async::Interrupt
413			objects.
414
415	root	1.13	=item $async->post_fork
416
417			The object will not normally be usable after a fork (as the pipe fd is
418			shared between processes). Calling this method after a fork in the child
419			ensures that the object will work as expected again. It only needs to be
420			called when the async object is used in the child.
421
422			This only works when the pipe was created by Async::Interrupt.
423
424			Async::Interrupt ensures that the reading file descriptor does not change
425			it's value.
426	root	1.6
427	root	1.18	=item $signum = Async::Interrupt::sig2num $signame_or_number
428
429			=item $signame = Async::Interrupt::sig2name $signame_or_number
430
431			These two convenience functions simply convert a signal name or number to
432			the corresponding name or number. They are not used by this module and
433			exist just because perl doesn't have a nice way to do this on its own.
434
435			They will return C<undef> on illegal names or numbers.
436
437	root	1.16	=back
438
439			=head1 THE Async::Interrupt::EventPipe CLASS
440
441			Pipes are the predominent utility to make asynchronous signals
442			synchronous. However, pipes are hard to come by: they don't exist on the
443			broken windows platform, and on GNU/Linux systems, you might want to use
444			an C<eventfd> instead.
445
446			This class creates selectable event pipes in a portable fashion: on
447			windows, it will try to create a tcp socket pair, on GNU/Linux, it will
448			try to create an eventfd and everywhere else it will try to use a normal
449			pipe.
450
451			=over 4
452
453			=item $epipe = new Async::Interrupt::EventPipe
454
455			This creates and returns an eventpipe object. This object is simply a
456			blessed array reference:
457
458			=item ($r_fd, $w_fd) = $epipe->filenos
459
460			Returns the read-side file descriptor and the write-side file descriptor.
461
462			Example: pass an eventpipe object as pipe to the Async::Interrupt
463			constructor, and create an AnyEvent watcher for the read side.
464
465			my $epipe = new Async::Interrupt::EventPipe;
466			my $asy = new Async::Interrupt pipe => [$epipe->filenos];
467			my $iow = AnyEvent->io (fh => $epipe->fileno, poll => 'r', cb => sub { });
468
469			=item $r_fd = $epipe->fileno
470
471			Return only the reading/listening side.
472
473			=item $epipe->signal
474
475			Write something to the pipe, in a portable fashion.
476
477			=item $epipe->drain
478
479			Drain (empty) the pipe.
480
481			=item $epipe->renew
482
483			Recreates the pipe (useful after a fork). The reading side will not change
484			it's file descriptor number, but the writing side might.
485
486			=back
487
488	root	1.1	=cut
489
490			1;
491
492	root	1.2	=head1 EXAMPLE
493
494	root	1.8	There really should be a complete C/XS example. Bug me about it. Better
495			yet, create one.
496	root	1.2
497			=head1 IMPLEMENTATION DETAILS AND LIMITATIONS
498
499	root	1.8	This module works by "hijacking" SIGKILL, which is guaranteed to always
500			exist, but also cannot be caught, so is always available.
501	root	1.2
502	root	1.8	Basically, this module fakes the occurance of a SIGKILL signal and
503			then intercepts the interpreter handling it. This makes normal signal
504			handling slower (probably unmeasurably, though), but has the advantage
505			of not requiring a special runops function, nor slowing down normal perl
506			execution a bit.
507
508	root	1.13	It assumes that C<sig_atomic_t>, C<int> and C<IV> are all async-safe to
509			modify.
510	root	1.2
511	root	1.1	=head1 AUTHOR
512
513			Marc Lehmann <schmorp@schmorp.de>
514			http://home.schmorp.de/
515
516			=cut
517