Temporarily Redirect STDOUT to /dev/null - Not Working All The Time

Question

I am using

Server version: Apache/1.3.34 (Debian)
mod_perl - 1.29

By refering to STDIN, STDOUT, and STDERR Streams

#!/usr/bin/perl5
package main;

use strict 'vars';

{
    # Our mighty holy legacy code love to print out message in the middle of operation. Shihh....
    # Let's quietly redirect those message to /dev/null.
    my $nullfh = Apache::gensym( );
    open $nullfh, '>/dev/null' or warn "Can't open /dev/null: $!";
    local *STDOUT = $nullfh;
    print "BYE BYE WORLD";    # Shouldn't show in webpage.
    close $nullfh;
}

print "X BEGIN HELLO WORLD";  # Should show in webpage.

I realize that it is not working all the time. For example, I refresh the page for 10 times. x times it will print out "X BEGIN HELLO WORLD". (10-x) time it just print out nothing.

I cannot find any reason why it behave this way. May I know anyone of you encounter similar problem as me?

Answer 1

I bet it is an interaction of mod_perl, and the reassignment of the STDOUT glob - you are effectively running one instance of perl in the webserver, then this will lead to a race condition on when the local goes out of scope, and when the various print's and close's happen.

This is basically a guess of mine, I don't know for sure that it is happening so keep that in mind. Roughly speaking the race condition would be between when you do this:

local *STDOUT = $nullfh;

and when this local goes out of scope. I think that requests to the web server are handled as different threads (since we are using mod_perl), and each thread may be able to see the new value for the glob.

Answer 2

Try:

local $|=1;

before print. This bypass the buffering.

See http://perldoc.perl.org/perlvar.html#HANDLE-%3Eautoflush%28EXPR%29

Answer 3

Buffering is the most likely problem, as noted already, but there's another approach if that doesn't work for you for whatever reason. You can use the oft-ignored one-arg select() built-in to change the default output filehandle for print output.

Answer 4

I need to explicitly store and restore. It works for my case. But I am not sure why.

# Take copies of the file descriptors
open OLDOUT, '>&STDOUT';
my $returned_values = 0;
{
    # Our mighty holy legacy code love to print out message in the middle of operation. Shihh....
    # Let's quietly redirect those message to /dev/null.
    local *STDOUT;
    open STDOUT, '>/dev/null' or warn "Can't open /dev/null: $!";
    print "BYE BYE WORLD";    # Shouldn't show in webpage.
    close STDOUT;
}
# Restore stdout.
open STDOUT, '>&OLDOUT' or die "Can't restore stdout: $!";
# Avoid leaks by closing the independent copies.
close OLDOUT or die "Can't close OLDOUT: $!";