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1.1 |
=head1 NAME |
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Async::Interrupt - allow C/XS libraries to interrupt perl asynchronously |
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=head1 SYNOPSIS |
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use Async::Interrupt; |
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=head1 DESCRIPTION |
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This module implements a single feature only of interest to advanced perl |
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1.4 |
modules, namely asynchronous interruptions (think "UNIX signals", which |
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1.1 |
are very similar). |
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1.8 |
Sometimes, modules wish to run code asynchronously (in another thread, |
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or from a signal handler), and then signal the perl interpreter on |
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certain events. One common way is to write some data to a pipe and use an |
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event handling toolkit to watch for I/O events. Another way is to send |
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a signal. Those methods are slow, and in the case of a pipe, also not |
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asynchronous - it won't interrupt a running perl interpreter. |
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1.1 |
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This module implements asynchronous notifications that enable you to |
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1.8 |
signal running perl code from another thread, asynchronously, and |
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sometimes even without using a single syscall. |
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1.1 |
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1.8 |
=head2 USAGE SCENARIOS |
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=over 4 |
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=item Race-free signal handling |
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There seems to be no way to do race-free signal handling in perl: to |
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catch a signal, you have to execute Perl code, and between entering the |
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interpreter C<select> function (or other blocking functions) and executing |
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the select syscall is a small but relevant timespan during which signals |
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will be queued, but perl signal handlers will not be executed and the |
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blocking syscall will not be interrupted. |
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You can use this module to bind a signal to a callback while at the same |
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time activating an event pipe that you can C<select> on, fixing the race |
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completely. |
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This can be used to implement the signal hadling in event loops, |
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e.g. L<AnyEvent>, L<POE>, L<IO::Async::Loop> and so on. |
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=item Background threads want speedy reporting |
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Assume you want very exact timing, and you can spare an extra cpu core |
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for that. Then you can run an extra thread that signals your perl |
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interpreter. This means you can get a very exact timing source while your |
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perl code is number crunching, without even using a syscall to communicate |
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between your threads. |
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For example the deliantra game server uses a variant of this technique |
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to interrupt background processes regularly to send map updates to game |
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clients. |
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1.17 |
Or L<EV::Loop::Async> uses an interrupt object to wake up perl when new |
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events have arrived. |
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1.8 |
L<IO::AIO> and L<BDB> could also use this to speed up result reporting. |
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=item Speedy event loop invocation |
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One could use this module e.g. in L<Coro> to interrupt a running coro-thread |
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and cause it to enter the event loop. |
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Or one could bind to C<SIGIO> and tell some important sockets to send this |
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signal, causing the event loop to be entered to reduce network latency. |
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=back |
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=head2 HOW TO USE |
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You can use this module by creating an C<Async::Interrupt> object for each |
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such event source. This object stores a perl and/or a C-level callback |
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that is invoked when the C<Async::Interrupt> object gets signalled. It is |
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executed at the next time the perl interpreter is running (i.e. it will |
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interrupt a computation, but not an XS function or a syscall). |
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1.2 |
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You can signal the C<Async::Interrupt> object either by calling it's C<< |
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1.8 |
->signal >> method, or, more commonly, by calling a C function. There is |
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also the built-in (POSIX) signal source. |
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1.2 |
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The C<< ->signal_func >> returns the address of the C function that is to |
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be called (plus an argument to be used during the call). The signalling |
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function also takes an integer argument in the range SIG_ATOMIC_MIN to |
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SIG_ATOMIC_MAX (guaranteed to allow at least 0..127). |
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Since this kind of interruption is fast, but can only interrupt a |
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1.8 |
I<running> interpreter, there is optional support for signalling a pipe |
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- that means you can also wait for the pipe to become readable (e.g. via |
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L<EV> or L<AnyEvent>). This, of course, incurs the overhead of a C<read> |
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and C<write> syscall. |
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1.2 |
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1.18 |
=head1 USAGE EXAMPLES |
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1.19 |
=head2 Implementing race-free signal handling |
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1.18 |
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This example uses a single event pipe for all signals, and one |
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1.19 |
Async::Interrupt per signal. This code is actually what the L<AnyEvent> |
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module uses itself when Async::Interrupt is available. |
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1.18 |
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1.19 |
First, create the event pipe and hook it into the event loop |
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1.18 |
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$SIGPIPE = new Async::Interrupt::EventPipe; |
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$SIGPIPE_W = AnyEvent->io ( |
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fh => $SIGPIPE->fileno, |
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poll => "r", |
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1.19 |
cb => \&_signal_check, # defined later |
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1.18 |
); |
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Then, for each signal to hook, create an Async::Interrupt object. The |
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callback just sets a global variable, as we are only interested in |
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synchronous signals (i.e. when the event loop polls), which is why the |
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pipe draining is not done automatically. |
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my $interrupt = new Async::Interrupt |
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cb => sub { undef $SIGNAL_RECEIVED{$signum} } |
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1.19 |
signal => $signum, |
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pipe => [$SIGPIPE->filenos], |
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1.18 |
pipe_autodrain => 0, |
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; |
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Finally, the I/O callback for the event pipe handles the signals: |
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sub _signal_check { |
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# drain the pipe first |
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$SIGPIPE->drain; |
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# two loops, just to be sure |
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while (%SIGNAL_RECEIVED) { |
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for (keys %SIGNAL_RECEIVED) { |
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delete $SIGNAL_RECEIVED{$_}; |
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warn "signal $_ received\n"; |
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} |
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} |
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} |
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1.19 |
=head2 Interrupt perl from another thread |
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1.18 |
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1.19 |
This example interrupts the Perl interpreter from another thread, via the |
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XS API. This is used by e.g. the L<EV::Loop::Async> module. |
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#TODO# |
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1.18 |
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1.16 |
=head1 THE Async::Interrupt CLASS |
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1.1 |
=over 4 |
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=cut |
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package Async::Interrupt; |
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1.10 |
use common::sense; |
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1.2 |
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1.1 |
BEGIN { |
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1.11 |
# the next line forces initialisation of internal |
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1.19 |
# signal handling variables, otherwise, PL_sig_pending |
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# etc. will be null pointers. |
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1.11 |
$SIG{KILL} = sub { }; |
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1.19 |
our $VERSION = '1.0'; |
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1.1 |
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require XSLoader; |
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1.11 |
XSLoader::load ("Async::Interrupt", $VERSION); |
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1.1 |
} |
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1.2 |
our $DIED = sub { warn "$@" }; |
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1.1 |
=item $async = new Async::Interrupt key => value... |
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Creates a new Async::Interrupt object. You may only use async |
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notifications on this object while it exists, so you need to keep a |
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reference to it at all times while it is used. |
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Optional constructor arguments include (normally you would specify at |
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least one of C<cb> or C<c_cb>). |
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=over 4 |
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=item cb => $coderef->($value) |
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Registers a perl callback to be invoked whenever the async interrupt is |
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signalled. |
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Note that, since this callback can be invoked at basically any time, it |
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1.2 |
must not modify any well-known global variables such as C<$/> without |
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restoring them again before returning. |
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The exceptions are C<$!> and C<$@>, which are saved and restored by |
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Async::Interrupt. |
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1.1 |
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1.2 |
If the callback should throw an exception, then it will be caught, |
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and C<$Async::Interrupt::DIED> will be called with C<$@> containing |
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1.19 |
the exception. The default will simply C<warn> about the message and |
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1.2 |
continue. |
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=item c_cb => [$c_func, $c_arg] |
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1.1 |
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Registers a C callback the be invoked whenever the async interrupt is |
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signalled. |
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The C callback must have the following prototype: |
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1.2 |
void c_func (pTHX_ void *c_arg, int value); |
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1.1 |
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1.2 |
Both C<$c_func> and C<$c_arg> must be specified as integers/IVs, and |
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C<$value> is the C<value> passed to some earlier call to either C<$signal> |
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or the C<signal_func> function. |
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1.1 |
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Note that, because the callback can be invoked at almost any time, you |
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have to be careful at saving and restoring global variables that Perl |
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1.4 |
might use (the exception is C<errno>, which is saved and restored by |
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1.2 |
Async::Interrupt). The callback itself runs as part of the perl context, |
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so you can call any perl functions and modify any perl data structures (in |
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1.4 |
which case the requirements set out for C<cb> apply as well). |
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1.1 |
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1.13 |
=item var => $scalar_ref |
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When specified, then the given argument must be a reference to a |
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1.17 |
scalar. The scalar will be set to C<0> initially. Signalling the interrupt |
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1.13 |
object will set it to the passed value, handling the interrupt will reset |
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it to C<0> again. |
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Note that the only thing you are legally allowed to do is to is to check |
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the variable in a boolean or integer context (e.g. comparing it with a |
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string, or printing it, will I<destroy> it and might cause your program to |
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crash or worse). |
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1.6 |
=item signal => $signame_or_value |
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When this parameter is specified, then the Async::Interrupt will hook the |
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given signal, that is, it will effectively call C<< ->signal (0) >> each time |
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the given signal is caught by the process. |
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Only one async can hook a given signal, and the signal will be restored to |
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defaults when the Async::Interrupt object gets destroyed. |
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1.19 |
=item signal_hysteresis => $boolean |
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Sets the initial signal hysteresis state, see the C<signal_hysteresis> |
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method, below. |
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1.2 |
=item pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing] |
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1.1 |
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1.2 |
Specifies two file descriptors (or file handles) that should be signalled |
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1.1 |
whenever the async interrupt is signalled. This means a single octet will |
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be written to it, and before the callback is being invoked, it will be |
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read again. Due to races, it is unlikely but possible that multiple octets |
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1.2 |
are written. It is required that the file handles are both in nonblocking |
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mode. |
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1.1 |
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1.2 |
The object will keep a reference to the file handles. |
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1.1 |
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This can be used to ensure that async notifications will interrupt event |
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frameworks as well. |
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1.13 |
Note that C<Async::Interrupt> will create a suitable signal fd |
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automatically when your program requests one, so you don't have to specify |
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1.16 |
this argument when all you want is an extra file descriptor to watch. |
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If you want to share a single event pipe between multiple Async::Interrupt |
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objects, you can use the C<Async::Interrupt::EventPipe> class to manage |
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those. |
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1.13 |
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1.19 |
=item pipe_autodrain => $boolean |
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Sets the initial autodrain state, see the C<pipe_autodrain> method, below. |
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1.1 |
=back |
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=cut |
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sub new { |
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my ($class, %arg) = @_; |
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1.19 |
my $self = bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}, $arg{var}), $class; |
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# urgs, reminds me of Event |
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for my $attr (qw(pipe_autodrain signal_hysteresis)) { |
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$self->$attr ($arg{$attr}) if exists $arg{$attr}; |
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} |
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$self |
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1.1 |
} |
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1.2 |
=item ($signal_func, $signal_arg) = $async->signal_func |
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1.1 |
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1.17 |
Returns the address of a function to call asynchronously. The function |
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has the following prototype and needs to be passed the specified |
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C<$signal_arg>, which is a C<void *> cast to C<IV>: |
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1.1 |
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void (*signal_func) (void *signal_arg, int value) |
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An example call would look like: |
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signal_func (signal_arg, 0); |
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1.2 |
The function is safe to call from within signal and thread contexts, at |
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1.1 |
any time. The specified C<value> is passed to both C and Perl callback. |
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1.13 |
C<$value> must be in the valid range for a C<sig_atomic_t>, except C<0> |
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(1..127 is portable). |
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1.2 |
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1.1 |
If the function is called while the Async::Interrupt object is already |
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signaled but before the callbacks are being executed, then the stored |
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1.2 |
C<value> is either the old or the new one. Due to the asynchronous |
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nature of the code, the C<value> can even be passed to two consecutive |
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invocations of the callback. |
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1.1 |
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1.13 |
=item $address = $async->c_var |
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Returns the address (cast to IV) of an C<IV> variable. The variable is set |
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to C<0> initially and gets set to the passed value whenever the object |
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gets signalled, and reset to C<0> once the interrupt has been handled. |
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Note that it is often beneficial to just call C<PERL_ASYNC_CHECK ()> to |
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handle any interrupts. |
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Example: call some XS function to store the address, then show C code |
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waiting for it. |
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my_xs_func $async->c_var; |
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static IV *valuep; |
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void |
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my_xs_func (void *addr) |
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CODE: |
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valuep = (IV *)addr; |
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// code in a loop, waiting |
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while (!*valuep) |
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1.16 |
; // do something |
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1.13 |
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=item $async->signal ($value=1) |
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1.1 |
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This signals the given async object from Perl code. Semi-obviously, this |
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1.17 |
will instantly trigger the callback invocation (it does not, as the name |
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might imply, do anything with POSIX signals). |
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1.1 |
|
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1.13 |
C<$value> must be in the valid range for a C<sig_atomic_t>, except C<0> |
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(1..127 is portable). |
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1.2 |
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1.17 |
=item $async->signal_hysteresis ($enable) |
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Enables or disables signal hysteresis (default: disabled). If a POSIX |
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signal is used as a signal source for the interrupt object, then enabling |
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signal hysteresis causes Async::Interrupt to reset the signal action to |
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C<SIG_IGN> in the signal handler and restore it just before handling the |
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interruption. |
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When you expect a lot of signals (e.g. when using SIGIO), then enabling |
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signal hysteresis can reduce the number of handler invocations |
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considerably, at the cost of two extra syscalls. |
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Note that setting the signal to C<SIG_IGN> can have unintended side |
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effects when you fork and exec other programs, as often they do nto expect |
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signals to be ignored by default. |
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1.2 |
=item $async->block |
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1.3 |
=item $async->unblock |
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|
366 |
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Sometimes you need a "critical section" of code that will not be |
367 |
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interrupted by an Async::Interrupt. This can be implemented by calling C<< |
368 |
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$async->block >> before the critical section, and C<< $async->unblock >> |
369 |
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afterwards. |
370 |
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|
371 |
root |
1.4 |
Note that there must be exactly one call of C<unblock> for every previous |
372 |
root |
1.3 |
call to C<block> (i.e. calls can nest). |
373 |
|
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|
374 |
root |
1.4 |
Since ensuring this in the presence of exceptions and threads is |
375 |
root |
1.3 |
usually more difficult than you imagine, I recommend using C<< |
376 |
|
|
$async->scoped_block >> instead. |
377 |
root |
1.2 |
|
378 |
root |
1.3 |
=item $async->scope_block |
379 |
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|
380 |
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This call C<< $async->block >> and installs a handler that is called when |
381 |
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the current scope is exited (via an exception, by canceling the Coro |
382 |
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thread, by calling last/goto etc.). |
383 |
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|
384 |
|
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This is the recommended (and fastest) way to implement critical sections. |
385 |
root |
1.2 |
|
386 |
root |
1.17 |
=item ($block_func, $block_arg) = $async->scope_block_func |
387 |
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|
388 |
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Returns the address of a function that implements the C<scope_block> |
389 |
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functionality. |
390 |
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|
391 |
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It has the following prototype and needs to be passed the specified |
392 |
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C<$block_arg>, which is a C<void *> cast to C<IV>: |
393 |
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|
394 |
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void (*block_func) (void *block_arg) |
395 |
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|
396 |
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An example call would look like: |
397 |
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|
398 |
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block_func (block_arg); |
399 |
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|
400 |
|
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The function is safe to call only from within the toplevel of a perl XS |
401 |
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function and will call C<LEAVE> and C<ENTER> (in this order!). |
402 |
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|
403 |
root |
1.6 |
=item $async->pipe_enable |
404 |
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|
405 |
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=item $async->pipe_disable |
406 |
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|
407 |
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Enable/disable signalling the pipe when the interrupt occurs (default is |
408 |
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enabled). Writing to a pipe is relatively expensive, so it can be disabled |
409 |
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when you know you are not waiting for it (for example, with L<EV> you |
410 |
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could disable the pipe in a check watcher, and enable it in a prepare |
411 |
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watcher). |
412 |
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|
413 |
root |
1.13 |
Note that currently, while C<pipe_disable> is in effect, no attempt to |
414 |
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|
read from the pipe will be done when handling events. This might change as |
415 |
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|
soon as I realize why this is a mistake. |
416 |
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|
417 |
|
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=item $fileno = $async->pipe_fileno |
418 |
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|
419 |
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Returns the reading side of the signalling pipe. If no signalling pipe is |
420 |
|
|
currently attached to the object, it will dynamically create one. |
421 |
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|
422 |
|
|
Note that the only valid oepration on this file descriptor is to wait |
423 |
|
|
until it is readable. The fd might belong currently to a pipe, a tcp |
424 |
|
|
socket, or an eventfd, depending on the platform, and is guaranteed to be |
425 |
|
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C<select>able. |
426 |
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|
427 |
root |
1.16 |
=item $async->pipe_autodrain ($enable) |
428 |
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|
429 |
|
|
Enables (C<1>) or disables (C<0>) automatic draining of the pipe (default: |
430 |
|
|
enabled). When automatic draining is enabled, then Async::Interrupt will |
431 |
|
|
automatically clear the pipe. Otherwise the user is responsible for this |
432 |
|
|
draining. |
433 |
|
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|
434 |
|
|
This is useful when you want to share one pipe among many Async::Interrupt |
435 |
|
|
objects. |
436 |
|
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|
437 |
root |
1.13 |
=item $async->post_fork |
438 |
|
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|
439 |
|
|
The object will not normally be usable after a fork (as the pipe fd is |
440 |
|
|
shared between processes). Calling this method after a fork in the child |
441 |
|
|
ensures that the object will work as expected again. It only needs to be |
442 |
|
|
called when the async object is used in the child. |
443 |
|
|
|
444 |
|
|
This only works when the pipe was created by Async::Interrupt. |
445 |
|
|
|
446 |
|
|
Async::Interrupt ensures that the reading file descriptor does not change |
447 |
|
|
it's value. |
448 |
root |
1.6 |
|
449 |
root |
1.18 |
=item $signum = Async::Interrupt::sig2num $signame_or_number |
450 |
|
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|
451 |
|
|
=item $signame = Async::Interrupt::sig2name $signame_or_number |
452 |
|
|
|
453 |
|
|
These two convenience functions simply convert a signal name or number to |
454 |
|
|
the corresponding name or number. They are not used by this module and |
455 |
|
|
exist just because perl doesn't have a nice way to do this on its own. |
456 |
|
|
|
457 |
|
|
They will return C<undef> on illegal names or numbers. |
458 |
|
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|
459 |
root |
1.16 |
=back |
460 |
|
|
|
461 |
|
|
=head1 THE Async::Interrupt::EventPipe CLASS |
462 |
|
|
|
463 |
|
|
Pipes are the predominent utility to make asynchronous signals |
464 |
|
|
synchronous. However, pipes are hard to come by: they don't exist on the |
465 |
|
|
broken windows platform, and on GNU/Linux systems, you might want to use |
466 |
|
|
an C<eventfd> instead. |
467 |
|
|
|
468 |
|
|
This class creates selectable event pipes in a portable fashion: on |
469 |
|
|
windows, it will try to create a tcp socket pair, on GNU/Linux, it will |
470 |
|
|
try to create an eventfd and everywhere else it will try to use a normal |
471 |
|
|
pipe. |
472 |
|
|
|
473 |
|
|
=over 4 |
474 |
|
|
|
475 |
|
|
=item $epipe = new Async::Interrupt::EventPipe |
476 |
|
|
|
477 |
|
|
This creates and returns an eventpipe object. This object is simply a |
478 |
|
|
blessed array reference: |
479 |
|
|
|
480 |
|
|
=item ($r_fd, $w_fd) = $epipe->filenos |
481 |
|
|
|
482 |
|
|
Returns the read-side file descriptor and the write-side file descriptor. |
483 |
|
|
|
484 |
|
|
Example: pass an eventpipe object as pipe to the Async::Interrupt |
485 |
|
|
constructor, and create an AnyEvent watcher for the read side. |
486 |
|
|
|
487 |
|
|
my $epipe = new Async::Interrupt::EventPipe; |
488 |
|
|
my $asy = new Async::Interrupt pipe => [$epipe->filenos]; |
489 |
|
|
my $iow = AnyEvent->io (fh => $epipe->fileno, poll => 'r', cb => sub { }); |
490 |
|
|
|
491 |
|
|
=item $r_fd = $epipe->fileno |
492 |
|
|
|
493 |
|
|
Return only the reading/listening side. |
494 |
|
|
|
495 |
|
|
=item $epipe->signal |
496 |
|
|
|
497 |
|
|
Write something to the pipe, in a portable fashion. |
498 |
|
|
|
499 |
|
|
=item $epipe->drain |
500 |
|
|
|
501 |
|
|
Drain (empty) the pipe. |
502 |
|
|
|
503 |
|
|
=item $epipe->renew |
504 |
|
|
|
505 |
|
|
Recreates the pipe (useful after a fork). The reading side will not change |
506 |
|
|
it's file descriptor number, but the writing side might. |
507 |
|
|
|
508 |
|
|
=back |
509 |
|
|
|
510 |
root |
1.1 |
=cut |
511 |
|
|
|
512 |
|
|
1; |
513 |
|
|
|
514 |
root |
1.2 |
=head1 EXAMPLE |
515 |
|
|
|
516 |
root |
1.8 |
There really should be a complete C/XS example. Bug me about it. Better |
517 |
|
|
yet, create one. |
518 |
root |
1.2 |
|
519 |
|
|
=head1 IMPLEMENTATION DETAILS AND LIMITATIONS |
520 |
|
|
|
521 |
root |
1.8 |
This module works by "hijacking" SIGKILL, which is guaranteed to always |
522 |
|
|
exist, but also cannot be caught, so is always available. |
523 |
root |
1.2 |
|
524 |
root |
1.8 |
Basically, this module fakes the occurance of a SIGKILL signal and |
525 |
|
|
then intercepts the interpreter handling it. This makes normal signal |
526 |
|
|
handling slower (probably unmeasurably, though), but has the advantage |
527 |
|
|
of not requiring a special runops function, nor slowing down normal perl |
528 |
|
|
execution a bit. |
529 |
|
|
|
530 |
root |
1.13 |
It assumes that C<sig_atomic_t>, C<int> and C<IV> are all async-safe to |
531 |
|
|
modify. |
532 |
root |
1.2 |
|
533 |
root |
1.1 |
=head1 AUTHOR |
534 |
|
|
|
535 |
|
|
Marc Lehmann <schmorp@schmorp.de> |
536 |
|
|
http://home.schmorp.de/ |
537 |
|
|
|
538 |
|
|
=cut |
539 |
|
|
|