Question

Perl script needs to receive ajax request, send a “success” message back to browser, and then “spawn” a process to run in the background. (this spawned process could take 10 minutes to run, so I don’t want the browser to wait for that – or for an apache timeout to occur)

I was under the impression that the Perl exec() function would do this. Better than using fork() or system() since those are supposed to wait for a response.

But, the script seems to be waiting for the exec() program to finish before sending the success message to the browser. Here’s the very end of my Perl script:

print "Content-type: text/html\n\n";
print "success"; # this gets returned via ajax

exec $script_filename, $var1, $var2, $var3; 

Everything works, but the browser isn’t receiving the “success” message until $script_filename finishes running.

What I want to do seems similar to this previous post, but there aren’t enough specifics there.

Any ideas? Thanks.

Answer 1

I found an answer to another StackOverflow question which seems to solve my problem – see Paul Tomblin’s response here.

It seems that if the perl script does exec(), then apache waits. But if the perl script first does a fork, and then the fork does the exec(), then everything works as expected.

And here’s what it looks like in my code, replacing what was in my original post:

use POSIX 'setsid';

$SIG{CHLD} = 'IGNORE';

# This should flush stdout.
my $ofh = select(STDOUT);$| = 1;select $ofh;

my $kpid = fork;
if ($kpid)
{
    # Parent process
    print "Content-type: text/html\n\n";
    print "success"; # this gets returned via ajax
}
else
{
    close STDIN;
    close STDOUT;
    close STDERR;
    setsid();
    my $gpid = fork;
    if (!$gpid)
    {
        open(STDIN, "</dev/null") ;
        open(STDOUT, ">/dev/null") ;
        open(STDERR, ">/dev/null") ;
        # Child process
        exec($script_filename, $var1, $var2, $var3) ;
    }
    exit 0;
}

Thanks to Adam for confirming right away that what I was originally trying to do was flawed.

Thanks to Jonathan for pointing me in the right direction.