brian d foy brian d foy - 6 months ago 4x
Perl Question

Shutting down a Mojo::IOLoop recurring event connected to a Mojo websocket

I'm playing around with Mojolicious and websockets. I want to send the output of multiple external commands on the server to the webpage. I have no problems with connecting and receiving messages, but I also want to send a message back to the server to stop an external command while letting the others keep sending messages back to the client. I also want to stop checking the external command once it exits.

The external command is simply a one-liner that spits out an integer every few seconds. I have two websockets that display the numbers in separate

s. Clicking either of the stop buttons sends the message, but that's where I need to figure out how to shut down that websocket (and only that websocket) and shut down the external command.

enter image description here

When I connect the websocket, I run the external command and set up a
to check if there's output.

When I want to stop, I figure that I should call
, but that doesn't seem to completely remove it and I get error messages like
Mojo::Reactor::Poll: Timer failed: Can't call method "is_websocket" on an undefined value

If I call
on the controller object to shut down the websocket, it seems to stop everything.

I have the entire Mojolicious::Lite app as a gist, but here's the parts where I

use feature qw(signatures);
no warnings qw(experimental::signatures);
## other boilerplate redacted

websocket '/find' => sub ( $c ) {
state $loop = Mojo::IOLoop->singleton;

app->log->debug( "websocket for find" );
$c->inactivity_timeout( 50 );

my $id;
$c->on( message => sub ( $ws, $message ) {
my $json = decode_json( $message );
my $command = $json->{c};
my $name = $json->{n};

app->log->debug( "Got $command command for $name" );
if( $command eq "start" ) {
$id = run_command( $ws );
app->log->debug( "run_command for $name returned [$id]" );
elsif( $command eq "stop" ) {
app->log->debug( "stopping loop for $name [$id]" );
# XXX What should I do here?
# $ws->finish;
# $loop->remove( $id );
elsif( $command eq "open" ) {
app->log->debug( "opening websocket for $name" );
finish => sub ( $c, $code ) {
app->log->debug("WebSocket closed with status $code");


sub run_command ( $ws ) {
app->log->debug( "In run_command: $ws" );
open my $fh, "$^X -le '\$|++; while(1) { print int rand(100); sleep 3 }' |";

my $id;
$id = Mojo::IOLoop->recurring( 1 => sub ($loop) {
my $m = <$fh>;
unless( defined $m ) {
app->log->debug( "Closing down recurring loop from the inside [$id]" );
# XXX: what should I do here?
close $fh;
chomp $m;
app->log->debug( "Input [$m] for [$id] from $fh" );
$ws->send( encode_json( { 'm' => $m } ) );

return $id;

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I played around with this a bit. Logioniz's answer made me think that I shouldn't be polling or handling the filehandle details myself. I still don't know where it was hanging.

Instead, I used Mojo::Reactor's io to set a filehandle to monitor:

sub run_command ( $ws ) {
    my $pid = open my $fh, "$^X -le '\$|++; print \$\$; while(1) { print int rand(100); sleep 3 }' |";

    my $reactor = Mojo::IOLoop->singleton->reactor->io(
        $fh => sub ($reactor, $writeable) {
            my $m = <$fh>;
            chomp $m;
            $ws->send( encode_json( { 'm' => $m } ) );

    return ( $fh, $pid );

When I'm done with that command, I can unwatch that filehandle and kill the process. I finish the websocket:

    elsif( $command eq "stop" ) {
        $loop->reactor->watch( $fh, 0, 0 );
        kill 'KILL', $pid or app->log->debug( "Could not kill $pid: $!" );

I still don't know why remove($fh) doesn't work. I figure I'm leaking some IOLoop things doing it this way.