| 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 |
|
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=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 |
|
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Or L<EV::Loop::Async> uses an interrupt object to wake up perl when new |
| 59 |
events have arrived. |
| 60 |
|
| 61 |
L<IO::AIO> and L<BDB> could also use this to speed up result reporting. |
| 62 |
|
| 63 |
=item Speedy event loop invocation |
| 64 |
|
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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 |
|
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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 |
|
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=back |
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|
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=head2 HOW TO USE |
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|
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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 |
|
| 81 |
You can signal the C<Async::Interrupt> object either by calling it's C<< |
| 82 |
->signal >> method, or, more commonly, by calling a C function. There is |
| 83 |
also the built-in (POSIX) signal source. |
| 84 |
|
| 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 |
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 |
|
| 96 |
=head1 THE Async::Interrupt CLASS |
| 97 |
|
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=over 4 |
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|
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=cut |
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|
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package Async::Interrupt; |
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|
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use common::sense; |
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|
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BEGIN { |
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# the next line forces initialisation of internal |
| 108 |
# signal handling # variables |
| 109 |
$SIG{KILL} = sub { }; |
| 110 |
|
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our $VERSION = '0.6'; |
| 112 |
|
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require XSLoader; |
| 114 |
XSLoader::load ("Async::Interrupt", $VERSION); |
| 115 |
} |
| 116 |
|
| 117 |
our $DIED = sub { warn "$@" }; |
| 118 |
|
| 119 |
=item $async = new Async::Interrupt key => value... |
| 120 |
|
| 121 |
Creates a new Async::Interrupt object. You may only use async |
| 122 |
notifications on this object while it exists, so you need to keep a |
| 123 |
reference to it at all times while it is used. |
| 124 |
|
| 125 |
Optional constructor arguments include (normally you would specify at |
| 126 |
least one of C<cb> or C<c_cb>). |
| 127 |
|
| 128 |
=over 4 |
| 129 |
|
| 130 |
=item cb => $coderef->($value) |
| 131 |
|
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Registers a perl callback to be invoked whenever the async interrupt is |
| 133 |
signalled. |
| 134 |
|
| 135 |
Note that, since this callback can be invoked at basically any time, it |
| 136 |
must not modify any well-known global variables such as C<$/> without |
| 137 |
restoring them again before returning. |
| 138 |
|
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The exceptions are C<$!> and C<$@>, which are saved and restored by |
| 140 |
Async::Interrupt. |
| 141 |
|
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If the callback should throw an exception, then it will be caught, |
| 143 |
and C<$Async::Interrupt::DIED> will be called with C<$@> containing |
| 144 |
the exception. The default will simply C<warn> about the message and |
| 145 |
continue. |
| 146 |
|
| 147 |
=item c_cb => [$c_func, $c_arg] |
| 148 |
|
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Registers a C callback the be invoked whenever the async interrupt is |
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signalled. |
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|
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The C callback must have the following prototype: |
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|
| 154 |
void c_func (pTHX_ void *c_arg, int value); |
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|
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Both C<$c_func> and C<$c_arg> must be specified as integers/IVs, and |
| 157 |
C<$value> is the C<value> passed to some earlier call to either C<$signal> |
| 158 |
or the C<signal_func> function. |
| 159 |
|
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Note that, because the callback can be invoked at almost any time, you |
| 161 |
have to be careful at saving and restoring global variables that Perl |
| 162 |
might use (the exception is C<errno>, which is saved and restored by |
| 163 |
Async::Interrupt). The callback itself runs as part of the perl context, |
| 164 |
so you can call any perl functions and modify any perl data structures (in |
| 165 |
which case the requirements set out for C<cb> apply as well). |
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|
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=item var => $scalar_ref |
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|
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When specified, then the given argument must be a reference to a |
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scalar. The scalar will be set to C<0> initially. Signalling the interrupt |
| 171 |
object will set it to the passed value, handling the interrupt will reset |
| 172 |
it to C<0> again. |
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|
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Note that the only thing you are legally allowed to do is to is to check |
| 175 |
the variable in a boolean or integer context (e.g. comparing it with a |
| 176 |
string, or printing it, will I<destroy> it and might cause your program to |
| 177 |
crash or worse). |
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|
| 179 |
=item signal => $signame_or_value |
| 180 |
|
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When this parameter is specified, then the Async::Interrupt will hook the |
| 182 |
given signal, that is, it will effectively call C<< ->signal (0) >> each time |
| 183 |
the given signal is caught by the process. |
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|
| 185 |
Only one async can hook a given signal, and the signal will be restored to |
| 186 |
defaults when the Async::Interrupt object gets destroyed. |
| 187 |
|
| 188 |
=item pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing] |
| 189 |
|
| 190 |
Specifies two file descriptors (or file handles) that should be signalled |
| 191 |
whenever the async interrupt is signalled. This means a single octet will |
| 192 |
be written to it, and before the callback is being invoked, it will be |
| 193 |
read again. Due to races, it is unlikely but possible that multiple octets |
| 194 |
are written. It is required that the file handles are both in nonblocking |
| 195 |
mode. |
| 196 |
|
| 197 |
The object will keep a reference to the file handles. |
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|
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This can be used to ensure that async notifications will interrupt event |
| 200 |
frameworks as well. |
| 201 |
|
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Note that C<Async::Interrupt> will create a suitable signal fd |
| 203 |
automatically when your program requests one, so you don't have to specify |
| 204 |
this argument when all you want is an extra file descriptor to watch. |
| 205 |
|
| 206 |
If you want to share a single event pipe between multiple Async::Interrupt |
| 207 |
objects, you can use the C<Async::Interrupt::EventPipe> class to manage |
| 208 |
those. |
| 209 |
|
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=back |
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|
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=cut |
| 213 |
|
| 214 |
sub new { |
| 215 |
my ($class, %arg) = @_; |
| 216 |
|
| 217 |
bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}, $arg{var}), $class |
| 218 |
} |
| 219 |
|
| 220 |
=item ($signal_func, $signal_arg) = $async->signal_func |
| 221 |
|
| 222 |
Returns the address of a function to call asynchronously. The function |
| 223 |
has the following prototype and needs to be passed the specified |
| 224 |
C<$signal_arg>, which is a C<void *> cast to C<IV>: |
| 225 |
|
| 226 |
void (*signal_func) (void *signal_arg, int value) |
| 227 |
|
| 228 |
An example call would look like: |
| 229 |
|
| 230 |
signal_func (signal_arg, 0); |
| 231 |
|
| 232 |
The function is safe to call from within signal and thread contexts, at |
| 233 |
any time. The specified C<value> is passed to both C and Perl callback. |
| 234 |
|
| 235 |
C<$value> must be in the valid range for a C<sig_atomic_t>, except C<0> |
| 236 |
(1..127 is portable). |
| 237 |
|
| 238 |
If the function is called while the Async::Interrupt object is already |
| 239 |
signaled but before the callbacks are being executed, then the stored |
| 240 |
C<value> is either the old or the new one. Due to the asynchronous |
| 241 |
nature of the code, the C<value> can even be passed to two consecutive |
| 242 |
invocations of the callback. |
| 243 |
|
| 244 |
=item $address = $async->c_var |
| 245 |
|
| 246 |
Returns the address (cast to IV) of an C<IV> variable. The variable is set |
| 247 |
to C<0> initially and gets set to the passed value whenever the object |
| 248 |
gets signalled, and reset to C<0> once the interrupt has been handled. |
| 249 |
|
| 250 |
Note that it is often beneficial to just call C<PERL_ASYNC_CHECK ()> to |
| 251 |
handle any interrupts. |
| 252 |
|
| 253 |
Example: call some XS function to store the address, then show C code |
| 254 |
waiting for it. |
| 255 |
|
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my_xs_func $async->c_var; |
| 257 |
|
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static IV *valuep; |
| 259 |
|
| 260 |
void |
| 261 |
my_xs_func (void *addr) |
| 262 |
CODE: |
| 263 |
valuep = (IV *)addr; |
| 264 |
|
| 265 |
// code in a loop, waiting |
| 266 |
while (!*valuep) |
| 267 |
; // do something |
| 268 |
|
| 269 |
=item $async->signal ($value=1) |
| 270 |
|
| 271 |
This signals the given async object from Perl code. Semi-obviously, this |
| 272 |
will instantly trigger the callback invocation (it does not, as the name |
| 273 |
might imply, do anything with POSIX signals). |
| 274 |
|
| 275 |
C<$value> must be in the valid range for a C<sig_atomic_t>, except C<0> |
| 276 |
(1..127 is portable). |
| 277 |
|
| 278 |
=item $async->signal_hysteresis ($enable) |
| 279 |
|
| 280 |
Enables or disables signal hysteresis (default: disabled). If a POSIX |
| 281 |
signal is used as a signal source for the interrupt object, then enabling |
| 282 |
signal hysteresis causes Async::Interrupt to reset the signal action to |
| 283 |
C<SIG_IGN> in the signal handler and restore it just before handling the |
| 284 |
interruption. |
| 285 |
|
| 286 |
When you expect a lot of signals (e.g. when using SIGIO), then enabling |
| 287 |
signal hysteresis can reduce the number of handler invocations |
| 288 |
considerably, at the cost of two extra syscalls. |
| 289 |
|
| 290 |
Note that setting the signal to C<SIG_IGN> can have unintended side |
| 291 |
effects when you fork and exec other programs, as often they do nto expect |
| 292 |
signals to be ignored by default. |
| 293 |
|
| 294 |
=item $async->block |
| 295 |
|
| 296 |
=item $async->unblock |
| 297 |
|
| 298 |
Sometimes you need a "critical section" of code that will not be |
| 299 |
interrupted by an Async::Interrupt. This can be implemented by calling C<< |
| 300 |
$async->block >> before the critical section, and C<< $async->unblock >> |
| 301 |
afterwards. |
| 302 |
|
| 303 |
Note that there must be exactly one call of C<unblock> for every previous |
| 304 |
call to C<block> (i.e. calls can nest). |
| 305 |
|
| 306 |
Since ensuring this in the presence of exceptions and threads is |
| 307 |
usually more difficult than you imagine, I recommend using C<< |
| 308 |
$async->scoped_block >> instead. |
| 309 |
|
| 310 |
=item $async->scope_block |
| 311 |
|
| 312 |
This call C<< $async->block >> and installs a handler that is called when |
| 313 |
the current scope is exited (via an exception, by canceling the Coro |
| 314 |
thread, by calling last/goto etc.). |
| 315 |
|
| 316 |
This is the recommended (and fastest) way to implement critical sections. |
| 317 |
|
| 318 |
=item ($block_func, $block_arg) = $async->scope_block_func |
| 319 |
|
| 320 |
Returns the address of a function that implements the C<scope_block> |
| 321 |
functionality. |
| 322 |
|
| 323 |
It has the following prototype and needs to be passed the specified |
| 324 |
C<$block_arg>, which is a C<void *> cast to C<IV>: |
| 325 |
|
| 326 |
void (*block_func) (void *block_arg) |
| 327 |
|
| 328 |
An example call would look like: |
| 329 |
|
| 330 |
block_func (block_arg); |
| 331 |
|
| 332 |
The function is safe to call only from within the toplevel of a perl XS |
| 333 |
function and will call C<LEAVE> and C<ENTER> (in this order!). |
| 334 |
|
| 335 |
=item $async->pipe_enable |
| 336 |
|
| 337 |
=item $async->pipe_disable |
| 338 |
|
| 339 |
Enable/disable signalling the pipe when the interrupt occurs (default is |
| 340 |
enabled). Writing to a pipe is relatively expensive, so it can be disabled |
| 341 |
when you know you are not waiting for it (for example, with L<EV> you |
| 342 |
could disable the pipe in a check watcher, and enable it in a prepare |
| 343 |
watcher). |
| 344 |
|
| 345 |
Note that currently, while C<pipe_disable> is in effect, no attempt to |
| 346 |
read from the pipe will be done when handling events. This might change as |
| 347 |
soon as I realize why this is a mistake. |
| 348 |
|
| 349 |
=item $fileno = $async->pipe_fileno |
| 350 |
|
| 351 |
Returns the reading side of the signalling pipe. If no signalling pipe is |
| 352 |
currently attached to the object, it will dynamically create one. |
| 353 |
|
| 354 |
Note that the only valid oepration on this file descriptor is to wait |
| 355 |
until it is readable. The fd might belong currently to a pipe, a tcp |
| 356 |
socket, or an eventfd, depending on the platform, and is guaranteed to be |
| 357 |
C<select>able. |
| 358 |
|
| 359 |
=item $async->pipe_autodrain ($enable) |
| 360 |
|
| 361 |
Enables (C<1>) or disables (C<0>) automatic draining of the pipe (default: |
| 362 |
enabled). When automatic draining is enabled, then Async::Interrupt will |
| 363 |
automatically clear the pipe. Otherwise the user is responsible for this |
| 364 |
draining. |
| 365 |
|
| 366 |
This is useful when you want to share one pipe among many Async::Interrupt |
| 367 |
objects. |
| 368 |
|
| 369 |
=item $async->post_fork |
| 370 |
|
| 371 |
The object will not normally be usable after a fork (as the pipe fd is |
| 372 |
shared between processes). Calling this method after a fork in the child |
| 373 |
ensures that the object will work as expected again. It only needs to be |
| 374 |
called when the async object is used in the child. |
| 375 |
|
| 376 |
This only works when the pipe was created by Async::Interrupt. |
| 377 |
|
| 378 |
Async::Interrupt ensures that the reading file descriptor does not change |
| 379 |
it's value. |
| 380 |
|
| 381 |
=back |
| 382 |
|
| 383 |
=head1 THE Async::Interrupt::EventPipe CLASS |
| 384 |
|
| 385 |
Pipes are the predominent utility to make asynchronous signals |
| 386 |
synchronous. However, pipes are hard to come by: they don't exist on the |
| 387 |
broken windows platform, and on GNU/Linux systems, you might want to use |
| 388 |
an C<eventfd> instead. |
| 389 |
|
| 390 |
This class creates selectable event pipes in a portable fashion: on |
| 391 |
windows, it will try to create a tcp socket pair, on GNU/Linux, it will |
| 392 |
try to create an eventfd and everywhere else it will try to use a normal |
| 393 |
pipe. |
| 394 |
|
| 395 |
=over 4 |
| 396 |
|
| 397 |
=item $epipe = new Async::Interrupt::EventPipe |
| 398 |
|
| 399 |
This creates and returns an eventpipe object. This object is simply a |
| 400 |
blessed array reference: |
| 401 |
|
| 402 |
=item ($r_fd, $w_fd) = $epipe->filenos |
| 403 |
|
| 404 |
Returns the read-side file descriptor and the write-side file descriptor. |
| 405 |
|
| 406 |
Example: pass an eventpipe object as pipe to the Async::Interrupt |
| 407 |
constructor, and create an AnyEvent watcher for the read side. |
| 408 |
|
| 409 |
my $epipe = new Async::Interrupt::EventPipe; |
| 410 |
my $asy = new Async::Interrupt pipe => [$epipe->filenos]; |
| 411 |
my $iow = AnyEvent->io (fh => $epipe->fileno, poll => 'r', cb => sub { }); |
| 412 |
|
| 413 |
=item $r_fd = $epipe->fileno |
| 414 |
|
| 415 |
Return only the reading/listening side. |
| 416 |
|
| 417 |
=item $epipe->signal |
| 418 |
|
| 419 |
Write something to the pipe, in a portable fashion. |
| 420 |
|
| 421 |
=item $epipe->drain |
| 422 |
|
| 423 |
Drain (empty) the pipe. |
| 424 |
|
| 425 |
=item $epipe->renew |
| 426 |
|
| 427 |
Recreates the pipe (useful after a fork). The reading side will not change |
| 428 |
it's file descriptor number, but the writing side might. |
| 429 |
|
| 430 |
=back |
| 431 |
|
| 432 |
=cut |
| 433 |
|
| 434 |
1; |
| 435 |
|
| 436 |
=head1 EXAMPLE |
| 437 |
|
| 438 |
There really should be a complete C/XS example. Bug me about it. Better |
| 439 |
yet, create one. |
| 440 |
|
| 441 |
=head1 IMPLEMENTATION DETAILS AND LIMITATIONS |
| 442 |
|
| 443 |
This module works by "hijacking" SIGKILL, which is guaranteed to always |
| 444 |
exist, but also cannot be caught, so is always available. |
| 445 |
|
| 446 |
Basically, this module fakes the occurance of a SIGKILL signal and |
| 447 |
then intercepts the interpreter handling it. This makes normal signal |
| 448 |
handling slower (probably unmeasurably, though), but has the advantage |
| 449 |
of not requiring a special runops function, nor slowing down normal perl |
| 450 |
execution a bit. |
| 451 |
|
| 452 |
It assumes that C<sig_atomic_t>, C<int> and C<IV> are all async-safe to |
| 453 |
modify. |
| 454 |
|
| 455 |
=head1 AUTHOR |
| 456 |
|
| 457 |
Marc Lehmann <schmorp@schmorp.de> |
| 458 |
http://home.schmorp.de/ |
| 459 |
|
| 460 |
=cut |
| 461 |
|
