How do I fork properly with mod_perl2?

Question

Hi there. I'm having trouble forking a long-running process from some code running under mod_perl2.

Everything works for the most part, but it seems that the forked process is holding open handles to Apache's logfiles - this means Apache won't restart while the process is running (I get a 'failed to open logfiles' message).

Here's the code I'm using:

use POSIX; # required for setsid

# Do not wait for child processes to complete
$SIG{CHLD} = 'IGNORE';

# fork (and make sure we did!)
defined (my $kid = fork) or die "Cannot fork: $!\n";

if ($kid) {
 return (1, $kid);
}else {
 # chdir to /, stops the process from preventing an unmount
 chdir '/' or die "Can't chdir to /: $!";

 # dump our STDIN and STDOUT handles
 open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
 open STDOUT, '>/dev/null' or die "Can't write to /dev/null: $!";

 # redirect for logging
 open STDERR, '>', $log_filename or die "Can't write to log: $!";

 # Prevent locking to apache process
 setsid or die "Can't start a new session: $!";

 # execute the command
 exec( $cmd, @args );

 die "Failed to exec";
}

Back in the mod_perl1 days, I recall using $r->cleanup_for_exec to solve this problem, but it doesn't seem to be supported under mod_perl2. (Edit: Apparently it's not required any more..)

Any advice on how to correctly start a long-running process from mod_perl2 without these problems would be greatly appreciated!

Answer 1

Try closing your STDIN/STDOUT handles before the fork.

Answer 2

In my (formerly mod_perl, now FCGI) code, I have in the "else" clause of the "if ($kpid)",

    close STDIN;
    close STDOUT;
    close STDERR;
    setsid();

Also, for reasons that I forgot, I immediately fork again, and then in that child re-open STDIN, STDOUT, and STDERR.

So it looks like:

$SIG{CHLD} = 'IGNORE';

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

my $kpid = fork;
if ($kpid)
{
    # Parent process
    waitpid($kpid, 0);
}
else
{
    close STDIN;
    close STDOUT;
    close STDERR;
    setsid();
    my $gpid = fork;
    if (!$gpid)
    {
        open(STDIN, "</dev/null") ;#or print DEBUG2 "can't redirect stdin\n";
        open(STDOUT, ">/dev/null") ;#or print DEBUG2 "can't redirect stdout\n";
        open(STDERR, ">/dev/null") ;#or print DEBUG2 "can't redirect stderr\n";
        # Child process
        exec($pgm, @execargs) ;# or print DEBUG2 "exec failed\n";
    }
    exit 0;
}

Answer 3

You probably want to read this discussion. It seems you shouldn't fork on mod_perl unless you know how to prepare things. You have to use a module such as Apache2::SubProcess