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=head1 NAME |
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|
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Async::Interrupt - allow C/XS libraries to interrupt perl asynchronously |
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|
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=head1 SYNOPSIS |
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|
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use Async::Interrupt; |
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|
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=head1 DESCRIPTION |
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|
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This module implements a single feature only of interest to advanced perl |
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modules, namely asynchronous interruptions (think "UNIX signals", which |
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are very similar). |
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|
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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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|
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This module implements asynchronous notifications that enable you to |
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signal running perl code from another thread, asynchronously, and |
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sometimes even without using a single syscall. |
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|
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=head2 USAGE SCENARIOS |
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|
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=over 4 |
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|
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=item Race-free signal handling |
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|
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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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|
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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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|
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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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|
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=item Background threads want speedy reporting |
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|
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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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|
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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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|
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L<IO::AIO> and L<BDB> could also use this to speed up result reporting. |
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|
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=item Speedy event loop invocation |
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|
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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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|
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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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|
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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 |
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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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|
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You can signal the C<Async::Interrupt> object either by calling it's C<< |
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->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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|
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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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|
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Since this kind of interruption is fast, but can only interrupt a |
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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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|
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=head1 THE Async::Interrupt CLASS |
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|
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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 |
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# signal handling # variables |
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$SIG{KILL} = sub { }; |
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|
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our $VERSION = '0.6'; |
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|
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require XSLoader; |
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XSLoader::load ("Async::Interrupt", $VERSION); |
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} |
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|
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our $DIED = sub { warn "$@" }; |
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|
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=item $async = new Async::Interrupt key => value... |
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|
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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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|
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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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|
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=over 4 |
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|
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=item cb => $coderef->($value) |
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|
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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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|
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Note that, since this callback can be invoked at basically any time, it |
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must not modify any well-known global variables such as C<$/> without |
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restoring them again before returning. |
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|
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The exceptions are C<$!> and C<$@>, which are saved and restored by |
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Async::Interrupt. |
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|
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If the callback should throw an exception, then it will be caught, |
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and C<$Async::Interrupt::DIED> will be called with C<$@> containing |
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the exception. The default will simply C<warn> about the message and |
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continue. |
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|
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=item c_cb => [$c_func, $c_arg] |
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|
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Registers a C callback the be invoked whenever the async interrupt is |
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signalled. |
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|
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The C callback must have the following prototype: |
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|
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void c_func (pTHX_ void *c_arg, int value); |
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|
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Both C<$c_func> and C<$c_arg> must be specified as integers/IVs, and |
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C<$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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|
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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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might use (the exception is C<errno>, which is saved and restored by |
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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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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> intiially. Signalling the interrupt |
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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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|
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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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|
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=item signal => $signame_or_value |
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|
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When this parameter is specified, then the Async::Interrupt will hook the |
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given signal, that is, it will effectively call C<< ->signal (0) >> each time |
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the given signal is caught by the process. |
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|
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Only one async can hook a given signal, and the signal will be restored to |
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defaults when the Async::Interrupt object gets destroyed. |
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|
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=item pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing] |
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|
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Specifies two file descriptors (or file handles) that should be signalled |
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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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are written. It is required that the file handles are both in nonblocking |
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mode. |
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|
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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 |
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frameworks as well. |
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|
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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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this argument when all you want is an extra file descriptor to watch. |
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|
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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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|
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=back |
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|
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=cut |
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|
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sub new { |
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my ($class, %arg) = @_; |
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|
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bless \(_alloc $arg{cb}, @{$arg{c_cb}}[0,1], @{$arg{pipe}}[0,1], $arg{signal}, $arg{var}), $class |
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} |
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|
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=item ($signal_func, $signal_arg) = $async->signal_func |
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|
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Returns the address of a function to call asynchronously. The function has |
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the following prototype and needs to be passed the specified C<$c_arg>, |
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which is a C<void *> cast to C<IV>: |
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|
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void (*signal_func) (void *signal_arg, int value) |
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|
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An example call would look like: |
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|
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signal_func (signal_arg, 0); |
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|
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The function is safe to call from within signal and thread contexts, at |
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any time. The specified C<value> is passed to both C and Perl callback. |
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|
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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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|
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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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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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|
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=item $address = $async->c_var |
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|
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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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|
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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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|
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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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|
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my_xs_func $async->c_var; |
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|
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static IV *valuep; |
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|
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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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|
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// code in a loop, waiting |
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while (!*valuep) |
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; // do something |
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|
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=item $async->signal ($value=1) |
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|
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This signals the given async object from Perl code. Semi-obviously, this |
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will instantly trigger the callback invocation. |
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|
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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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|
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=item $async->block |
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|
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=item $async->unblock |
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|
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Sometimes you need a "critical section" of code that will not be |
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interrupted by an Async::Interrupt. This can be implemented by calling C<< |
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$async->block >> before the critical section, and C<< $async->unblock >> |
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afterwards. |
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|
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Note that there must be exactly one call of C<unblock> for every previous |
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call to C<block> (i.e. calls can nest). |
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|
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Since ensuring this in the presence of exceptions and threads is |
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usually more difficult than you imagine, I recommend using C<< |
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$async->scoped_block >> instead. |
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|
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=item $async->scope_block |
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|
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This call C<< $async->block >> and installs a handler that is called when |
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the current scope is exited (via an exception, by canceling the Coro |
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thread, by calling last/goto etc.). |
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|
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This is the recommended (and fastest) way to implement critical sections. |
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|
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=item $async->pipe_enable |
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|
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=item $async->pipe_disable |
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|
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Enable/disable signalling the pipe when the interrupt occurs (default is |
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enabled). Writing to a pipe is relatively expensive, so it can be disabled |
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when you know you are not waiting for it (for example, with L<EV> you |
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could disable the pipe in a check watcher, and enable it in a prepare |
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watcher). |
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|
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Note that currently, while C<pipe_disable> is in effect, no attempt to |
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read from the pipe will be done when handling events. This might change as |
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soon as I realize why this is a mistake. |
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|
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=item $fileno = $async->pipe_fileno |
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|
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Returns the reading side of the signalling pipe. If no signalling pipe is |
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currently attached to the object, it will dynamically create one. |
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|
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Note that the only valid oepration on this file descriptor is to wait |
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until it is readable. The fd might belong currently to a pipe, a tcp |
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socket, or an eventfd, depending on the platform, and is guaranteed to be |
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C<select>able. |
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|
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=item $async->pipe_autodrain ($enable) |
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|
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Enables (C<1>) or disables (C<0>) automatic draining of the pipe (default: |
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enabled). When automatic draining is enabled, then Async::Interrupt will |
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automatically clear the pipe. Otherwise the user is responsible for this |
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draining. |
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|
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This is useful when you want to share one pipe among many Async::Interrupt |
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objects. |
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|
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=item $async->post_fork |
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|
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The object will not normally be usable after a fork (as the pipe fd is |
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shared between processes). Calling this method after a fork in the child |
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ensures that the object will work as expected again. It only needs to be |
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called when the async object is used in the child. |
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|
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This only works when the pipe was created by Async::Interrupt. |
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|
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Async::Interrupt ensures that the reading file descriptor does not change |
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it's value. |
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|
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=back |
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|
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=head1 THE Async::Interrupt::EventPipe CLASS |
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|
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Pipes are the predominent utility to make asynchronous signals |
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synchronous. However, pipes are hard to come by: they don't exist on the |
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broken windows platform, and on GNU/Linux systems, you might want to use |
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an C<eventfd> instead. |
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|
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This class creates selectable event pipes in a portable fashion: on |
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windows, it will try to create a tcp socket pair, on GNU/Linux, it will |
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try to create an eventfd and everywhere else it will try to use a normal |
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pipe. |
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|
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=over 4 |
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|
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=item $epipe = new Async::Interrupt::EventPipe |
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|
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This creates and returns an eventpipe object. This object is simply a |
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blessed array reference: |
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|
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=item ($r_fd, $w_fd) = $epipe->filenos |
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|
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Returns the read-side file descriptor and the write-side file descriptor. |
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|
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Example: pass an eventpipe object as pipe to the Async::Interrupt |
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constructor, and create an AnyEvent watcher for the read side. |
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|
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my $epipe = new Async::Interrupt::EventPipe; |
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my $asy = new Async::Interrupt pipe => [$epipe->filenos]; |
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my $iow = AnyEvent->io (fh => $epipe->fileno, poll => 'r', cb => sub { }); |
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|
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=item $r_fd = $epipe->fileno |
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|
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Return only the reading/listening side. |
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|
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=item $epipe->signal |
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|
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Write something to the pipe, in a portable fashion. |
| 383 |
|
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=item $epipe->drain |
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|
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Drain (empty) the pipe. |
| 387 |
|
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=item $epipe->renew |
| 389 |
|
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Recreates the pipe (useful after a fork). The reading side will not change |
| 391 |
it's file descriptor number, but the writing side might. |
| 392 |
|
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=back |
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|
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=cut |
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|
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1; |
| 398 |
|
| 399 |
=head1 EXAMPLE |
| 400 |
|
| 401 |
There really should be a complete C/XS example. Bug me about it. Better |
| 402 |
yet, create one. |
| 403 |
|
| 404 |
=head1 IMPLEMENTATION DETAILS AND LIMITATIONS |
| 405 |
|
| 406 |
This module works by "hijacking" SIGKILL, which is guaranteed to always |
| 407 |
exist, but also cannot be caught, so is always available. |
| 408 |
|
| 409 |
Basically, this module fakes the occurance of a SIGKILL signal and |
| 410 |
then intercepts the interpreter handling it. This makes normal signal |
| 411 |
handling slower (probably unmeasurably, though), but has the advantage |
| 412 |
of not requiring a special runops function, nor slowing down normal perl |
| 413 |
execution a bit. |
| 414 |
|
| 415 |
It assumes that C<sig_atomic_t>, C<int> and C<IV> are all async-safe to |
| 416 |
modify. |
| 417 |
|
| 418 |
=head1 AUTHOR |
| 419 |
|
| 420 |
Marc Lehmann <schmorp@schmorp.de> |
| 421 |
http://home.schmorp.de/ |
| 422 |
|
| 423 |
=cut |
| 424 |
|
