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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 handling 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 function 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 (usually required in the child 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 occurence 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/