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.

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.

=over 4

=cut

package Async::Interrupt;

no warnings;

BEGIN {
   $VERSION = '0.041';

   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. 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> and C<int> are both async-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:	1.9
Committed:	Sun Jul 12 16:33:26 2009 UTC (14 years, 11 months ago) by root
Branch:	MAIN
CVS Tags:	rel-0_041
Changes since 1.8:	+1 -1 lines
Log Message:	0.041
#	Content
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	modules, namely asynchronous interruptions (think "UNIX signals", which
13	are very similar).
14
15	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
22	This module implements asynchronous notifications that enable you to
23	signal running perl code from another thread, asynchronously, and
24	sometimes even without using a single syscall.
25
26	=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	L<IO::AIO> and L<BDB> could also use this to speed up result reporting.
59
60	=item Speedy event loop invocation
61
62	One could use this module e.g. in L<Coro> to interrupt a running coro-thread
63	and cause it to enter the event loop.
64
65	Or one could bind to C<SIGIO> and tell some important sockets to send this
66	signal, causing the event loop to be entered to reduce network latency.
67
68	=back
69
70	=head2 HOW TO USE
71
72	You can use this module by creating an C<Async::Interrupt> object for each
73	such event source. This object stores a perl and/or a C-level callback
74	that is invoked when the C<Async::Interrupt> object gets signalled. It is
75	executed at the next time the perl interpreter is running (i.e. it will
76	interrupt a computation, but not an XS function or a syscall).
77
78	You can signal the C<Async::Interrupt> object either by calling it's C<<
79	->signal >> method, or, more commonly, by calling a C function. There is
80	also the built-in (POSIX) signal source.
81
82	The C<< ->signal_func >> returns the address of the C function that is to
83	be called (plus an argument to be used during the call). The signalling
84	function also takes an integer argument in the range SIG_ATOMIC_MIN to
85	SIG_ATOMIC_MAX (guaranteed to allow at least 0..127).
86
87	Since this kind of interruption is fast, but can only interrupt a
88	I<running> interpreter, there is optional support for signalling a pipe
89	- that means you can also wait for the pipe to become readable (e.g. via
90	L<EV> or L<AnyEvent>). This, of course, incurs the overhead of a C<read>
91	and C<write> syscall.
92
93	=over 4
94
95	=cut
96
97	package Async::Interrupt;
98
99	no warnings;
100
101	BEGIN {
102	$VERSION = '0.041';
103
104	require XSLoader;
105	XSLoader::load Async::Interrupt::, $VERSION;
106	}
107
108	our $DIED = sub { warn "$@" };
109
110	=item $async = new Async::Interrupt key => value...
111
112	Creates a new Async::Interrupt object. You may only use async
113	notifications on this object while it exists, so you need to keep a
114	reference to it at all times while it is used.
115
116	Optional constructor arguments include (normally you would specify at
117	least one of C<cb> or C<c_cb>).
118
119	=over 4
120
121	=item cb => $coderef->($value)
122
123	Registers a perl callback to be invoked whenever the async interrupt is
124	signalled.
125
126	Note that, since this callback can be invoked at basically any time, it
127	must not modify any well-known global variables such as C<$/> without
128	restoring them again before returning.
129
130	The exceptions are C<$!> and C<$@>, which are saved and restored by
131	Async::Interrupt.
132
133	If the callback should throw an exception, then it will be caught,
134	and C<$Async::Interrupt::DIED> will be called with C<$@> containing
135	the exception. The default will simply C<warn> about the message and
136	continue.
137
138	=item c_cb => [$c_func, $c_arg]
139
140	Registers a C callback the be invoked whenever the async interrupt is
141	signalled.
142
143	The C callback must have the following prototype:
144
145	void c_func (pTHX_ void *c_arg, int value);
146
147	Both C<$c_func> and C<$c_arg> must be specified as integers/IVs, and
148	C<$value> is the C<value> passed to some earlier call to either C<$signal>
149	or the C<signal_func> function.
150
151	Note that, because the callback can be invoked at almost any time, you
152	have to be careful at saving and restoring global variables that Perl
153	might use (the exception is C<errno>, which is saved and restored by
154	Async::Interrupt). The callback itself runs as part of the perl context,
155	so you can call any perl functions and modify any perl data structures (in
156	which case the requirements set out for C<cb> apply as well).
157
158	=item signal => $signame_or_value
159
160	When this parameter is specified, then the Async::Interrupt will hook the
161	given signal, that is, it will effectively call C<< ->signal (0) >> each time
162	the given signal is caught by the process.
163
164	Only one async can hook a given signal, and the signal will be restored to
165	defaults when the Async::Interrupt object gets destroyed.
166
167	=item pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing]
168
169	Specifies two file descriptors (or file handles) that should be signalled
170	whenever the async interrupt is signalled. This means a single octet will
171	be written to it, and before the callback is being invoked, it will be
172	read again. Due to races, it is unlikely but possible that multiple octets
173	are written. It is required that the file handles are both in nonblocking
174	mode.
175
176	You can get a portable pipe and set non-blocking mode portably by using
177	e.g. L<AnyEvent::Util> from the L<AnyEvent> distribution.
178
179	It is also possible to pass in a linux eventfd as both read and write
180	handle (which is faster than a pipe).
181
182	The object will keep a reference to the file handles.
183
184	This can be used to ensure that async notifications will interrupt event
185	frameworks as well.
186
187	=back
188
189	=cut
190
191	sub new {
192	my ($class, %arg) = @_;
193
194	bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}), $class
195	}
196
197	=item ($signal_func, $signal_arg) = $async->signal_func
198
199	Returns the address of a function to call asynchronously. The function has
200	the following prototype and needs to be passed the specified C<$c_arg>,
201	which is a C<void *> cast to C<IV>:
202
203	void (signal_func) (void signal_arg, int value)
204
205	An example call would look like:
206
207	signal_func (signal_arg, 0);
208
209	The function is safe to call from within signal and thread contexts, at
210	any time. The specified C<value> is passed to both C and Perl callback.
211
212	C<$value> must be in the valid range for a C<sig_atomic_t> (0..127 is
213	portable).
214
215	If the function is called while the Async::Interrupt object is already
216	signaled but before the callbacks are being executed, then the stored
217	C<value> is either the old or the new one. Due to the asynchronous
218	nature of the code, the C<value> can even be passed to two consecutive
219	invocations of the callback.
220
221	=item $async->signal ($value=0)
222
223	This signals the given async object from Perl code. Semi-obviously, this
224	will instantly trigger the callback invocation.
225
226	C<$value> must be in the valid range for a C<sig_atomic_t> (0..127 is
227	portable).
228
229	=item $async->block
230
231	=item $async->unblock
232
233	Sometimes you need a "critical section" of code that will not be
234	interrupted by an Async::Interrupt. This can be implemented by calling C<<
235	$async->block >> before the critical section, and C<< $async->unblock >>
236	afterwards.
237
238	Note that there must be exactly one call of C<unblock> for every previous
239	call to C<block> (i.e. calls can nest).
240
241	Since ensuring this in the presence of exceptions and threads is
242	usually more difficult than you imagine, I recommend using C<<
243	$async->scoped_block >> instead.
244
245	=item $async->scope_block
246
247	This call C<< $async->block >> and installs a handler that is called when
248	the current scope is exited (via an exception, by canceling the Coro
249	thread, by calling last/goto etc.).
250
251	This is the recommended (and fastest) way to implement critical sections.
252
253	=item $async->pipe_enable
254
255	=item $async->pipe_disable
256
257	Enable/disable signalling the pipe when the interrupt occurs (default is
258	enabled). Writing to a pipe is relatively expensive, so it can be disabled
259	when you know you are not waiting for it (for example, with L<EV> you
260	could disable the pipe in a check watcher, and enable it in a prepare
261	watcher).
262
263	Note that when C<fd_disable> is in effect, no attempt to read from the
264	pipe will be done.
265
266	=cut
267
268	1;
269
270	=back
271
272	=head1 EXAMPLE
273
274	There really should be a complete C/XS example. Bug me about it. Better
275	yet, create one.
276
277	=head1 IMPLEMENTATION DETAILS AND LIMITATIONS
278
279	This module works by "hijacking" SIGKILL, which is guaranteed to always
280	exist, but also cannot be caught, so is always available.
281
282	Basically, this module fakes the occurance of a SIGKILL signal and
283	then intercepts the interpreter handling it. This makes normal signal
284	handling slower (probably unmeasurably, though), but has the advantage
285	of not requiring a special runops function, nor slowing down normal perl
286	execution a bit.
287
288	It assumes that C<sig_atomic_t> and C<int> are both async-safe to modify
289	(C<sig_atomic_> is used by this module, and perl itself uses C<int>, so we
290	can assume that this is quite portable, at least w.r.t. signals).
291
292	=head1 AUTHOR
293
294	Marc Lehmann <schmorp@schmorp.de>
295	http://home.schmorp.de/
296
297	=cut
298