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),
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 form another thread, asynchronously, without
issuing syscalls.

It works by creating an C<Async::Interrupt> object for each such use. 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.

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 also 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.

=over 4

=cut

package Async::Interrupt;

no warnings;

BEGIN {
   $VERSION = '0.03';

   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 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.

You can get a portable pipe and set non-blocking mode portably by using
e.g. L<AnyEvent::Util> from the L<AnyEvent> distribution.

It is also possible to pass in a linux eventfd as both read and write
handle (which is faster than a pipe).

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.

=back

=cut

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

   bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}), $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<$c_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> (0..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 $async->signal ($value=0)

This signals the given async object from Perl code. Semi-obviously, this
will instantly trigger the callback invocation.

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

=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 $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 when C<fd_disable> is in effect, no attempt to read from the
pipe will be done.

=cut

1;

=back

=head1 EXAMPLE

There really should be a complete C/XS example. Bug me about it.

=head1 IMPLEMENTATION DETAILS AND LIMITATIONS

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

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

It assumes that C<sig_atomic_t> and C<int> are both exception-safe to
modify (C<sig_atomic_> is used by this module, and perl itself uses
C<int>, so we can assume that this is quite portable, at least w.r.t.
signals).

=head1 AUTHOR

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

=cut

Revision:	1.6
Committed:	Sat Jul 11 22:16:50 2009 UTC (14 years, 10 months ago) by root
Branch:	MAIN
Changes since 1.5:	+28 -3 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			Sometimes, modules wish to run code asynchronously (in another thread),
16			and then signal the perl interpreter on certain events. One common way is
17			to write some data to a pipe and use an event handling toolkit to watch
18			for I/O events. Another way is to send a signal. Those methods are slow,
19			and in the case of a pipe, also not asynchronous - it won't interrupt a
20			running perl interpreter.
21
22			This module implements asynchronous notifications that enable you to
23			signal running perl code form another thread, asynchronously, without
24			issuing syscalls.
25
26	root	1.2	It works by creating an C<Async::Interrupt> object for each such use. This
27			object stores a perl and/or a C-level callback that is invoked when the
28			C<Async::Interrupt> object gets signalled. It is executed at the next time
29			the perl interpreter is running (i.e. it will interrupt a computation, but
30			not an XS function or a syscall).
31
32			You can signal the C<Async::Interrupt> object either by calling it's C<<
33			->signal >> method, or, more commonly, by calling a C function.
34
35			The C<< ->signal_func >> returns the address of the C function that is to
36			be called (plus an argument to be used during the call). The signalling
37			function also takes an integer argument in the range SIG_ATOMIC_MIN to
38			SIG_ATOMIC_MAX (guaranteed to allow at least 0..127).
39
40			Since this kind of interruption is fast, but can only interrupt a
41			I<running> interpreter, there is optional support for also signalling a
42	root	1.4	pipe - that means you can also wait for the pipe to become readable (e.g.
43			via L<EV> or L<AnyEvent>). This, of course, incurs the overhead of a
44			C<read> and C<write> syscall.
45	root	1.2
46	root	1.1	=over 4
47
48			=cut
49
50			package Async::Interrupt;
51
52	root	1.2	no warnings;
53
54	root	1.1	BEGIN {
55	root	1.5	$VERSION = '0.03';
56	root	1.1
57			require XSLoader;
58			XSLoader::load Async::Interrupt::, $VERSION;
59			}
60
61	root	1.2	our $DIED = sub { warn "$@" };
62
63	root	1.1	=item $async = new Async::Interrupt key => value...
64
65			Creates a new Async::Interrupt object. You may only use async
66			notifications on this object while it exists, so you need to keep a
67			reference to it at all times while it is used.
68
69			Optional constructor arguments include (normally you would specify at
70			least one of C<cb> or C<c_cb>).
71
72			=over 4
73
74			=item cb => $coderef->($value)
75
76			Registers a perl callback to be invoked whenever the async interrupt is
77			signalled.
78
79			Note that, since this callback can be invoked at basically any time, it
80	root	1.2	must not modify any well-known global variables such as C<$/> without
81			restoring them again before returning.
82
83			The exceptions are C<$!> and C<$@>, which are saved and restored by
84			Async::Interrupt.
85	root	1.1
86	root	1.2	If the callback should throw an exception, then it will be caught,
87			and C<$Async::Interrupt::DIED> will be called with C<$@> containing
88			the exception. The default will simply C<warn> about the message and
89			continue.
90
91			=item c_cb => [$c_func, $c_arg]
92	root	1.1
93			Registers a C callback the be invoked whenever the async interrupt is
94			signalled.
95
96			The C callback must have the following prototype:
97
98	root	1.2	void c_func (pTHX_ void *c_arg, int value);
99	root	1.1
100	root	1.2	Both C<$c_func> and C<$c_arg> must be specified as integers/IVs, and
101			C<$value> is the C<value> passed to some earlier call to either C<$signal>
102			or the C<signal_func> function.
103	root	1.1
104			Note that, because the callback can be invoked at almost any time, you
105			have to be careful at saving and restoring global variables that Perl
106	root	1.4	might use (the exception is C<errno>, which is saved and restored by
107	root	1.2	Async::Interrupt). The callback itself runs as part of the perl context,
108			so you can call any perl functions and modify any perl data structures (in
109	root	1.4	which case the requirements set out for C<cb> apply as well).
110	root	1.1
111	root	1.6	=item signal => $signame_or_value
112
113			When this parameter is specified, then the Async::Interrupt will hook the
114			given signal, that is, it will effectively call C<< ->signal (0) >> each time
115			the given signal is caught by the process.
116
117			Only one async can hook a given signal, and the signal will be restored to
118			defaults when the Async::Interrupt object gets destroyed.
119
120	root	1.2	=item pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing]
121	root	1.1
122	root	1.2	Specifies two file descriptors (or file handles) that should be signalled
123	root	1.1	whenever the async interrupt is signalled. This means a single octet will
124			be written to it, and before the callback is being invoked, it will be
125			read again. Due to races, it is unlikely but possible that multiple octets
126	root	1.2	are written. It is required that the file handles are both in nonblocking
127			mode.
128	root	1.1
129	root	1.6	You can get a portable pipe and set non-blocking mode portably by using
130			e.g. L<AnyEvent::Util> from the L<AnyEvent> distribution.
131
132			It is also possible to pass in a linux eventfd as both read and write
133			handle (which is faster than a pipe).
134	root	1.1
135	root	1.2	The object will keep a reference to the file handles.
136	root	1.1
137			This can be used to ensure that async notifications will interrupt event
138			frameworks as well.
139
140			=back
141
142			=cut
143
144			sub new {
145			my ($class, %arg) = @_;
146
147	root	1.6	bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}), $class
148	root	1.1	}
149
150	root	1.2	=item ($signal_func, $signal_arg) = $async->signal_func
151	root	1.1
152			Returns the address of a function to call asynchronously. The function has
153			the following prototype and needs to be passed the specified C<$c_arg>,
154			which is a C<void *> cast to C<IV>:
155
156			void (signal_func) (void signal_arg, int value)
157
158			An example call would look like:
159
160			signal_func (signal_arg, 0);
161
162	root	1.2	The function is safe to call from within signal and thread contexts, at
163	root	1.1	any time. The specified C<value> is passed to both C and Perl callback.
164
165	root	1.2	C<$value> must be in the valid range for a C<sig_atomic_t> (0..127 is
166			portable).
167
168	root	1.1	If the function is called while the Async::Interrupt object is already
169			signaled but before the callbacks are being executed, then the stored
170	root	1.2	C<value> is either the old or the new one. Due to the asynchronous
171			nature of the code, the C<value> can even be passed to two consecutive
172			invocations of the callback.
173	root	1.1
174			=item $async->signal ($value=0)
175
176			This signals the given async object from Perl code. Semi-obviously, this
177			will instantly trigger the callback invocation.
178
179	root	1.2	C<$value> must be in the valid range for a C<sig_atomic_t> (0..127 is
180			portable).
181
182			=item $async->block
183
184	root	1.3	=item $async->unblock
185
186			Sometimes you need a "critical section" of code that will not be
187			interrupted by an Async::Interrupt. This can be implemented by calling C<<
188			$async->block >> before the critical section, and C<< $async->unblock >>
189			afterwards.
190
191	root	1.4	Note that there must be exactly one call of C<unblock> for every previous
192	root	1.3	call to C<block> (i.e. calls can nest).
193
194	root	1.4	Since ensuring this in the presence of exceptions and threads is
195	root	1.3	usually more difficult than you imagine, I recommend using C<<
196			$async->scoped_block >> instead.
197	root	1.2
198	root	1.3	=item $async->scope_block
199
200			This call C<< $async->block >> and installs a handler that is called when
201			the current scope is exited (via an exception, by canceling the Coro
202			thread, by calling last/goto etc.).
203
204			This is the recommended (and fastest) way to implement critical sections.
205	root	1.2
206	root	1.6	=item $async->pipe_enable
207
208			=item $async->pipe_disable
209
210			Enable/disable signalling the pipe when the interrupt occurs (default is
211			enabled). Writing to a pipe is relatively expensive, so it can be disabled
212			when you know you are not waiting for it (for example, with L<EV> you
213			could disable the pipe in a check watcher, and enable it in a prepare
214			watcher).
215
216			Note that when C<fd_disable> is in effect, no attempt to read from the
217			pipe will be done.
218
219	root	1.1	=cut
220
221			1;
222
223			=back
224
225	root	1.2	=head1 EXAMPLE
226
227	root	1.4	There really should be a complete C/XS example. Bug me about it.
228	root	1.2
229			=head1 IMPLEMENTATION DETAILS AND LIMITATIONS
230
231	root	1.4	This module works by "hijacking" SIGKILL, which is guaranteed to be always
232	root	1.2	available in perl, but also cannot be caught, so is always available.
233
234			Basically, this module fakes the receive of a SIGKILL signal and
235			then catches it. This makes normal signal handling slower (probably
236			unmeasurably), but has the advantage of not requiring a special runops nor
237			slowing down normal perl execution a bit.
238
239			It assumes that C<sig_atomic_t> and C<int> are both exception-safe to
240			modify (C<sig_atomic_> is used by this module, and perl itself uses
241	root	1.4	C<int>, so we can assume that this is quite portable, at least w.r.t.
242	root	1.2	signals).
243
244	root	1.1	=head1 AUTHOR
245
246			Marc Lehmann <schmorp@schmorp.de>
247			http://home.schmorp.de/
248
249			=cut
250