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/