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/