Provided by: libasync-interrupt-perl_1.10-1build1_amd64 
NAME
Async::Interrupt - allow C/XS libraries to interrupt perl asynchronously
SYNOPSIS
use Async::Interrupt;
DESCRIPTION
This module implements a single feature only of interest to advanced perl modules, namely
asynchronous interruptions (think "UNIX signals", which are very similar).
Sometimes, modules wish to run code asynchronously (in another thread, or from a signal
handler), and then signal the perl interpreter on certain events. One common way is to
write some data to a pipe and use an event handling toolkit to watch for I/O events.
Another way is to send a signal. Those methods are slow, and in the case of a pipe, also
not asynchronous - it won't interrupt a running perl interpreter.
This module implements asynchronous notifications that enable you to signal running perl
code from another thread, asynchronously, and sometimes even without using a single
syscall.
USAGE SCENARIOS
Race-free signal handling
There seems to be no way to do race-free signal handling in perl: to catch a signal,
you have to execute Perl code, and between entering the interpreter "select" function
(or other blocking functions) and executing the select syscall is a small but relevant
timespan during which signals will be queued, but perl signal handlers will not be
executed and the blocking syscall will not be interrupted.
You can use this module to bind a signal to a callback while at the same time
activating an event pipe that you can "select" on, fixing the race completely.
This can be used to implement the signal hadling in event loops, e.g. AnyEvent, POE,
IO::Async::Loop and so on.
Background threads want speedy reporting
Assume you want very exact timing, and you can spare an extra cpu core for that. Then
you can run an extra thread that signals your perl interpreter. This means you can get
a very exact timing source while your perl code is number crunching, without even
using a syscall to communicate between your threads.
For example the deliantra game server uses a variant of this technique to interrupt
background processes regularly to send map updates to game clients.
Or EV::Loop::Async uses an interrupt object to wake up perl when new events have
arrived.
IO::AIO and BDB could also use this to speed up result reporting.
Speedy event loop invocation
One could use this module e.g. in Coro to interrupt a running coro-thread and cause it
to enter the event loop.
Or one could bind to "SIGIO" and tell some important sockets to send this signal,
causing the event loop to be entered to reduce network latency.
HOW TO USE
You can use this module by creating an "Async::Interrupt" object for each such event
source. This object stores a perl and/or a C-level callback that is invoked when the
"Async::Interrupt" object gets signalled. It is executed at the next time the perl
interpreter is running (i.e. it will interrupt a computation, but not an XS function or a
syscall).
You can signal the "Async::Interrupt" object either by calling it's "->signal" method, or,
more commonly, by calling a C function. There is also the built-in (POSIX) signal source.
The "->signal_func" returns the address of the C function that is to be called (plus an
argument to be used during the call). The signalling function also takes an integer
argument in the range SIG_ATOMIC_MIN to SIG_ATOMIC_MAX (guaranteed to allow at least
0..127).
Since this kind of interruption is fast, but can only interrupt a running interpreter,
there is optional support for signalling a pipe - that means you can also wait for the
pipe to become readable (e.g. via EV or AnyEvent). This, of course, incurs the overhead of
a "read" and "write" syscall.
USAGE EXAMPLES
Implementing race-free signal handling
This example uses a single event pipe for all signals, and one Async::Interrupt per
signal. This code is actually what the AnyEvent module uses itself when Async::Interrupt
is available.
First, create the event pipe and hook it into the event loop
$SIGPIPE = new Async::Interrupt::EventPipe;
$SIGPIPE_W = AnyEvent->io (
fh => $SIGPIPE->fileno,
poll => "r",
cb => \&_signal_check, # defined later
);
Then, for each signal to hook, create an Async::Interrupt object. The callback just sets a
global variable, as we are only interested in synchronous signals (i.e. when the event
loop polls), which is why the pipe draining is not done automatically.
my $interrupt = new Async::Interrupt
cb => sub { undef $SIGNAL_RECEIVED{$signum} }
signal => $signum,
pipe => [$SIGPIPE->filenos],
pipe_autodrain => 0,
;
Finally, the I/O callback for the event pipe handles the signals:
sub _signal_check {
# drain the pipe first
$SIGPIPE->drain;
# two loops, just to be sure
while (%SIGNAL_RECEIVED) {
for (keys %SIGNAL_RECEIVED) {
delete $SIGNAL_RECEIVED{$_};
warn "signal $_ received\n";
}
}
}
Interrupt perl from another thread
This example interrupts the Perl interpreter from another thread, via the XS API. This is
used by e.g. the EV::Loop::Async module.
On the Perl level, a new loop object (which contains the thread) is created, by first
calling some XS constructor, querying the C-level callback function and feeding that as
the "c_cb" into the Async::Interrupt constructor:
my $self = XS_thread_constructor;
my ($c_func, $c_arg) = _c_func $self; # return the c callback
my $asy = new Async::Interrupt c_cb => [$c_func, $c_arg];
Then the newly created Interrupt object is queried for the signaling function that the
newly created thread should call, and this is in turn told to the thread object:
_attach $self, $asy->signal_func;
So to repeat: first the XS object is created, then it is queried for the callback that
should be called when the Interrupt object gets signalled.
Then the interrupt object is queried for the callback fucntion that the thread should call
to signal the Interrupt object, and this callback is then attached to the thread.
You have to be careful that your new thread is not signalling before the signal function
was configured, for example by starting the background thread only within "_attach".
That concludes the Perl part.
The XS part consists of the actual constructor which creates a thread, which is not
relevant for this example, and two functions, "_c_func", which returns the Perl-side
callback, and "_attach", which configures the signalling functioon that is safe toc all
from another thread. For simplicity, we will use global variables to store the functions,
normally you would somehow attach them to $self.
The "c_func" simply returns the address of a static function and arranges for the object
pointed to by $self to be passed to it, as an integer:
void
_c_func (SV *loop)
PPCODE:
EXTEND (SP, 2);
PUSHs (sv_2mortal (newSViv (PTR2IV (c_func))));
PUSHs (sv_2mortal (newSViv (SvRV (loop))));
This would be the callback (since it runs in a normal Perl context, it is permissible to
manipulate Perl values):
static void
c_func (pTHX_ void *loop_, int value)
{
SV *loop_object = (SV *)loop_;
...
}
And this attaches the signalling callback:
static void (*my_sig_func) (void *signal_arg, int value);
static void *my_sig_arg;
void
_attach (SV *loop_, IV sig_func, void *sig_arg)
CODE:
{
my_sig_func = sig_func;
my_sig_arg = sig_arg;
/* now run the thread */
thread_create (&u->tid, l_run, 0);
}
And "l_run" (the background thread) would eventually call the signaling function:
my_sig_func (my_sig_arg, 0);
You can have a look at EV::Loop::Async for an actual example using intra-thread
communication, locking and so on.
THE Async::Interrupt CLASS
$async = new Async::Interrupt key => value...
Creates a new Async::Interrupt object. You may only use async notifications on this
object while it exists, so you need to keep a reference to it at all times while it is
used.
Optional constructor arguments include (normally you would specify at least one of
"cb" or "c_cb").
cb => $coderef->($value)
Registers a perl callback to be invoked whenever the async interrupt is signalled.
Note that, since this callback can be invoked at basically any time, it must not
modify any well-known global variables such as $/ without restoring them again
before returning.
The exceptions are $! and $@, which are saved and restored by Async::Interrupt.
If the callback should throw an exception, then it will be caught, and
$Async::Interrupt::DIED will be called with $@ containing the exception. The
default will simply "warn" about the message and continue.
c_cb => [$c_func, $c_arg]
Registers a C callback the be invoked whenever the async interrupt is signalled.
The C callback must have the following prototype:
void c_func (pTHX_ void *c_arg, int value);
Both $c_func and $c_arg must be specified as integers/IVs, and $value is the
"value" passed to some earlier call to either $signal or the "signal_func"
function.
Note that, because the callback can be invoked at almost any time, you have to be
careful at saving and restoring global variables that Perl might use (the
exception is "errno", which is saved and restored by Async::Interrupt). The
callback itself runs as part of the perl context, so you can call any perl
functions and modify any perl data structures (in which case the requirements set
out for "cb" apply as well).
var => $scalar_ref
When specified, then the given argument must be a reference to a scalar. The
scalar will be set to 0 initially. Signalling the interrupt object will set it to
the passed value, handling the interrupt will reset it to 0 again.
Note that the only thing you are legally allowed to do is to is to check the
variable in a boolean or integer context (e.g. comparing it with a string, or
printing it, will destroy it and might cause your program to crash or worse).
signal => $signame_or_value
When this parameter is specified, then the Async::Interrupt will hook the given
signal, that is, it will effectively call "->signal (0)" each time the given
signal is caught by the process.
Only one async can hook a given signal, and the signal will be restored to
defaults when the Async::Interrupt object gets destroyed.
signal_hysteresis => $boolean
Sets the initial signal hysteresis state, see the "signal_hysteresis" method,
below.
pipe => [$fileno_or_fh_for_reading, $fileno_or_fh_for_writing]
Specifies two file descriptors (or file handles) that should be signalled whenever
the async interrupt is signalled. This means a single octet will be written to it,
and before the callback is being invoked, it will be read again. Due to races, it
is unlikely but possible that multiple octets are written. It is required that the
file handles are both in nonblocking mode.
The object will keep a reference to the file handles.
This can be used to ensure that async notifications will interrupt event
frameworks as well.
Note that "Async::Interrupt" will create a suitable signal fd automatically when
your program requests one, so you don't have to specify this argument when all you
want is an extra file descriptor to watch.
If you want to share a single event pipe between multiple Async::Interrupt
objects, you can use the "Async::Interrupt::EventPipe" class to manage those.
pipe_autodrain => $boolean
Sets the initial autodrain state, see the "pipe_autodrain" method, below.
($signal_func, $signal_arg) = $async->signal_func
Returns the address of a function to call asynchronously. The function has the
following prototype and needs to be passed the specified $signal_arg, which is a "void
*" cast to "IV":
void (*signal_func) (void *signal_arg, int value)
An example call would look like:
signal_func (signal_arg, 0);
The function is safe to call from within signal and thread contexts, at any time. The
specified "value" is passed to both C and Perl callback.
$value must be in the valid range for a "sig_atomic_t", except 0 (1..127 is portable).
If the function is called while the Async::Interrupt object is already signaled but
before the callbacks are being executed, then the stored "value" is either the old or
the new one. Due to the asynchronous nature of the code, the "value" can even be
passed to two consecutive invocations of the callback.
$address = $async->c_var
Returns the address (cast to IV) of an "IV" variable. The variable is set to 0
initially and gets set to the passed value whenever the object gets signalled, and
reset to 0 once the interrupt has been handled.
Note that it is often beneficial to just call "PERL_ASYNC_CHECK ()" to handle any
interrupts.
Example: call some XS function to store the address, then show C code waiting for it.
my_xs_func $async->c_var;
static IV *valuep;
void
my_xs_func (void *addr)
CODE:
valuep = (IV *)addr;
// code in a loop, waiting
while (!*valuep)
; // do something
$async->signal ($value=1)
This signals the given async object from Perl code. Semi-obviously, this will
instantly trigger the callback invocation (it does not, as the name might imply, do
anything with POSIX signals).
$value must be in the valid range for a "sig_atomic_t", except 0 (1..127 is portable).
$async->handle
Calls the callback if the object is pending.
This method does not need to be called normally, as it will be invoked automatically.
However, it can be used to force handling of outstanding interrupts while the object
is blocked.
One reason why one might want to do that is when you want to switch from asynchronous
interruptions to synchronous one, using e.g. an event loop. To do that, one would
first "$async->block" the interrupt object, then register a read watcher on the
"pipe_fileno" that calls "$async->handle".
This disables asynchronous interruptions, but ensures that interrupts are handled by
the event loop.
$async->signal_hysteresis ($enable)
Enables or disables signal hysteresis (default: disabled). If a POSIX signal is used
as a signal source for the interrupt object, then enabling signal hysteresis causes
Async::Interrupt to reset the signal action to "SIG_IGN" in the signal handler and
restore it just before handling the interruption.
When you expect a lot of signals (e.g. when using SIGIO), then enabling signal
hysteresis can reduce the number of handler invocations considerably, at the cost of
two extra syscalls.
Note that setting the signal to "SIG_IGN" can have unintended side effects when you
fork and exec other programs, as often they do not expect signals to be ignored by
default.
$async->block
$async->unblock
Sometimes you need a "critical section" of code that will not be interrupted by an
Async::Interrupt. This can be implemented by calling "$async->block" before the
critical section, and "$async->unblock" afterwards.
Note that there must be exactly one call of "unblock" for every previous call to
"block" (i.e. calls can nest).
Since ensuring this in the presence of exceptions and threads is usually more
difficult than you imagine, I recommend using "$async->scoped_block" instead.
$async->scope_block
This call "$async->block" and installs a handler that is called when the current scope
is exited (via an exception, by canceling the Coro thread, by calling last/goto etc.).
This is the recommended (and fastest) way to implement critical sections.
($block_func, $block_arg) = $async->scope_block_func
Returns the address of a function that implements the "scope_block" functionality.
It has the following prototype and needs to be passed the specified $block_arg, which
is a "void *" cast to "IV":
void (*block_func) (void *block_arg)
An example call would look like:
block_func (block_arg);
The function is safe to call only from within the toplevel of a perl XS function and
will call "LEAVE" and "ENTER" (in this order!).
$async->pipe_enable
$async->pipe_disable
Enable/disable signalling the pipe when the interrupt occurs (default is enabled).
Writing to a pipe is relatively expensive, so it can be disabled when you know you are
not waiting for it (for example, with EV you could disable the pipe in a check
watcher, and enable it in a prepare watcher).
Note that currently, while "pipe_disable" is in effect, no attempt to read from the
pipe will be done when handling events. This might change as soon as I realize why
this is a mistake.
$fileno = $async->pipe_fileno
Returns the reading side of the signalling pipe. If no signalling pipe is currently
attached to the object, it will dynamically create one.
Note that the only valid operation on this file descriptor is to wait until it is
readable. The fd might belong currently to a pipe, a tcp socket, or an eventfd,
depending on the platform, and is guaranteed to be "select"able.
$async->pipe_autodrain ($enable)
Enables (1) or disables (0) automatic draining of the pipe (default: enabled). When
automatic draining is enabled, then Async::Interrupt will automatically clear the
pipe. Otherwise the user is responsible for this draining.
This is useful when you want to share one pipe among many Async::Interrupt objects.
$async->pipe_drain
Drains the pipe manually, for example, when autodrain is disabled. Does nothing when
no pipe is enabled.
$async->post_fork
The object will not normally be usable after a fork (as the pipe fd is shared between
processes). Calling this method after a fork in the child ensures that the object will
work as expected again. It only needs to be called when the async object is used in
the child.
This only works when the pipe was created by Async::Interrupt.
Async::Interrupt ensures that the reading file descriptor does not change it's value.
$signum = Async::Interrupt::sig2num $signame_or_number
$signame = Async::Interrupt::sig2name $signame_or_number
These two convenience functions simply convert a signal name or number to the
corresponding name or number. They are not used by this module and exist just because
perl doesn't have a nice way to do this on its own.
They will return "undef" on illegal names or numbers.
THE Async::Interrupt::EventPipe CLASS
Pipes are the predominant utility to make asynchronous signals synchronous. However, pipes
are hard to come by: they don't exist on the broken windows platform, and on GNU/Linux
systems, you might want to use an "eventfd" instead.
This class creates selectable event pipes in a portable fashion: on windows, it will try
to create a tcp socket pair, on GNU/Linux, it will try to create an eventfd and everywhere
else it will try to use a normal pipe.
$epipe = new Async::Interrupt::EventPipe
This creates and returns an eventpipe object. This object is simply a blessed array
reference:
($r_fd, $w_fd) = $epipe->filenos
Returns the read-side file descriptor and the write-side file descriptor.
Example: pass an eventpipe object as pipe to the Async::Interrupt constructor, and
create an AnyEvent watcher for the read side.
my $epipe = new Async::Interrupt::EventPipe;
my $asy = new Async::Interrupt pipe => [$epipe->filenos];
my $iow = AnyEvent->io (fh => $epipe->fileno, poll => 'r', cb => sub { });
$r_fd = $epipe->fileno
Return only the reading/listening side.
$epipe->signal
Write something to the pipe, in a portable fashion.
$epipe->drain
Drain (empty) the pipe.
($c_func, $c_arg) = $epipe->signal_func
($c_func, $c_arg) = $epipe->drain_func
These two methods returns a function pointer and "void *" argument that can be called
to have the effect of "$epipe->signal" or "$epipe->drain", respectively, on the XS
level.
They both have the following prototype and need to be passed their $c_arg, which is a
"void *" cast to an "IV":
void (*c_func) (void *c_arg)
An example call would look like:
c_func (c_arg);
$epipe->renew
Recreates the pipe (useful after a fork). The reading side will not change it's file
descriptor number, but the writing side might.
$epipe->wait
This method blocks the process until there are events on the pipe. This is not a very
event-based or ncie way of usign an event pipe, but it can be occasionally useful.
IMPLEMENTATION DETAILS AND LIMITATIONS
This module works by "hijacking" SIGKILL, which is guaranteed to always exist, but also
cannot be caught, so is always available.
Basically, this module fakes the occurance of a SIGKILL signal and then intercepts the
interpreter handling it. This makes normal signal handling slower (probably unmeasurably,
though), but has the advantage of not requiring a special runops function, nor slowing
down normal perl execution a bit.
It assumes that "sig_atomic_t", "int" and "IV" are all async-safe to modify.
AUTHOR
Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de/