How to call event loop from callback?

Question

That is, let the event loop process the events currently in queue. In VBA the call is named DoEvents.

I want to do this in Perl. Preferably to do this in AnyEvent, because I have a script written with it. But I couldn't find any function like that in documentation.

I naively tried to implement it with a condvar, because documentation says that recv calls event loop, but it failed, here is sample code:

use strict;
use warnings;

use AnyEvent;
use AnyEvent::Strict;

sub _long_task;

my $t1 = AE::timer 0, 3, sub{print"task 1, with interval 3 seconds\n";};
my $t2 = AE::timer 0, 7, sub{print"task 2, with interval 7 seconds\n";};
my $t3 = AE::timer 0, 11, sub{print"task 3, with interval 11 seconds\n";};
my $t_long = AE::timer 0, 0, \&_long_task;

sub DoEvents()
{
    my $cv = AE::cv;
    my $t = AE::timer 0, 0, sub { $cv->send; };
    $cv->recv;
}

sub _long_task {
    print"long task: ENTERING\n";
    for(1..5) {
        print"long task: doing some work for 2 seconds\n";
        sleep 2;
        print"long task: calling DoEvents\n";
        DoEvents();
    }
    print"long task: EXITING, resheduling after 10 seconds\n";
    $t_long = AE::timer 10, 0, \&_long_task;
}

AE::cv->recv;

The output is:

task 1, with interval 3 seconds
task 2, with interval 7 seconds
task 3, with interval 11 seconds
long task: ENTERING
long task: doing some work for 2 seconds
long task: calling DoEvents
AnyEvent::CondVar: recursive blocking wait attempted at C:\Users\andreyi\Desktop\try_event_loop.pl line 18.

UPDATE: There are lines in AnyEvent.pm:

  $WAITING
     and Carp::croak "AnyEvent::CondVar: recursive blocking wait attempted";

If you comment them, the DoEvents() works.

However, it will be better to have solution that does not involve mofidication of this CPAN module.

I think you want Coro, a co-operative mutlitasking system that works with AnyEvent. Change `my $t_long = AE::timer 0, 0, \&_long_task;` to `async { _long_task() };` and change `DoEvents()` to `cede()`. — ikegami, Aug 25 '15 at 16:12
By the way, `$cv` is equivalent to `sub { $cv->send; }` when used as a callback. — ikegami, Aug 25 '15 at 16:40
The problem is that you are blocking inside a callback. [My solution](http://pasted.co/c79c919c) ...which doesn't work either :( — ikegami, Aug 25 '15 at 19:37
Forgot to mention: Do not comment those lines in AnyEvent.pm. They are correctly detecting a problem. — ikegami, Aug 26 '15 at 03:39

my_stk_oflw_account · Answer 1 · 2015-08-26T15:22:56.013

Every problem has at least one simple solution (sometimes it is a dirty hack).

In my case this one seems to work. I added it to production code.

BEGIN { our $orig_Carp_croak = \&Carp::croak; }
sub DoEvents()
{
    my $cv = AE::cv;
    my $t = AE::timer 0, 0, $cv;
    no warnings 'redefine';
    local *Carp::croak = sub{
        (caller 1)[3] eq 'AnyEvent::CondVar::Base::recv'
            && $_[0] =~ /recursive blocking wait attempted/
            && return;
        goto \&{our $orig_Carp_croak};
    };
    $cv->recv;
}

UPDATE: For all callbacks that call DoEvents you will need to ensure that it they are not reentered. Like this:

our $entered_callbacks = {};
# ...
sub some_callback {
    $entered_callbacks->{some_callback} && return;
    local $entered_callbacks->{some_callback} = 1;
    ...;
}

The loops EV and AnyEvent::Loop have flaw that the callback is removed from queue only when it returned, not before it is called. It makes event loop not safe for reentry.

How to call event loop from callback?

1 Answers1