danstuken has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks,

I fear this may well be a FAQ or the combination of a couple of them. Forgive me if this is the case.

All I want is to lock a file. Truthfully. Its got so bad that this is the limit of my ambition. In the main I'm not a perl numpty, but this is driving my looney.

Running on an FC4 box I have the following code to reset (i.e. remove) an entry from the current user's ssh known_hosts file. I cannot get the lock to work. I have commented some of the artifacts of what I've already tried. Please guide me to enlightenment. 

#!/usr/bin/perl

my $host = $ARGV[0];
my $known_hosts = './kh.tt';

##USE '>>' OTHERWISE WE WILL TRUNCATE AT open() - BEFORE WE HAVE THE L
+OCK!!
#do{
#   print 'Error opening ssh known hosts file : ', $!, "\n";
#   exit;
#}unless( open KNOWN_HOSTS, ">> $known_hosts" );

open LOCK, "> $known_hosts.sem" or die "Can't open semaphore: $!\n";

##lock the file
my $lock_tries = 3;
my $got_lock = 0;
GETLOCK: while( $lock_tries )
{
   print "TRYING FOR LOCK ($lock_tries)\n";
   #if( flock(KNOWN_HOSTS, LOCK_EX|LOCK_NB) )
   my $ret = flock(LOCK, LOCK_EX|LOCK_NB);
   print "Flock ret : $ret\n";
   if( $ret )
   {
      print "GOT LOCK\n";
      $got_lock = 1;
      last GETLOCK;
   }
   sleep(1);
   $lock_tries --;
}

do{
   print 'Unable to lock known hosts file for updates', "\n";
   close LOCK;
   exit;
} unless($got_lock == 1);


#open KNOWN_HOSTS_RD, "< $known_hosts";
open KNOWN_HOSTS, "+< $known_hosts" or die "Can't open data file: $!\n
+";
#my @host_data = <KNOWN_HOSTS_RD>;
my @host_data = <KNOWN_HOSTS>;
#close KNOWN_HOSTS_RD;
my $host_pattern = $host;
$host_pattern =~ s/\./\\\./;
my @new_data = grep(!/^$host_pattern/,@host_data);
truncate KNOWN_HOSTS, 0;
print KNOWN_HOSTS join("\n", @new_data);

##This releases the lock.
close(KNOWN_HOSTS);

close LOCK;

[download]
Many thanks for any help.

Replies are listed 'Best First'.
Re: flock(), dodgy return?
by radiantmatrix (Parson) on Aug 01, 2007 at 13:46 UTC

    Well, the first thing I note is that the constants LOCK_EX and LOCK_NB are never defined or imported. Try including use Fcntl ':flock'; to get them (see the Fcntl module docs).

    It's worth mentioning that perl would have pointed this out to you had you used strict and warnings. I know it seems like we harp on it, but use strict; use warnings; should really be part of anything you write.

    <–radiant.matrix–>
    Ramblings and references
    The Code that can be seen is not the true Code
    I haven't found a problem yet that can't be solved by a well-placed trebuchet
[reply]
[d/l]
[select]
      Thank you radiantmatrix

      I'm appalled at myself. Of all the.... The code was taken from a 'test' script I wrote in parallel with the existing code to make sure everything I wanted to do worked. The 'parent' script does use strict and Fcntl ':flock'.

      Thank you again.

      Out of interest, you started with "...first thing I note..." do I have any other lurkers?

[reply]
        I suspect he means he didn't look any further, since it's a big chunk of code and nothing obvious stood out.

        Update: This is not a complaint. We'd be happy to look further into it if you are still having problems.

[reply]
Re: flock(), dodgy return?
by oxone (Friar) on Aug 01, 2007 at 14:21 UTC
    Hi. Wow what a mess! Try this to open a file with an exclusive lock. 
    use strict;
use warnings;

use Fcntl qw(:DEFAULT :flock);

sysopen(my $fh, 'filename', O_RDWR | O_CREAT)
  or die "Can't sysopen";    
    
flock($fh, LOCK_EX)
  or die "Can't get lock";

print "OK - opened file with lock in place.\n";

    [download]
    You don't need to use a 'semaphore' file to lock a single file. Also, it's probably best to let 'flock' do it's job, rather than putting it in a loop and trying over and over. For lots more info see the 'File Locking' section of The Camel book!

    oxone | www.datasalon.com

[reply]
[d/l]
      I suspect alarm will do the trick if a timeout is desired, at least on a unix host.
[reply]
[d/l]

        Yeah but that brings in more complexity than is really warrented in this situation, no?

[reply]

      Thanks oxone,

      I agree with you, its a bit of a state - rest assured it wont end that way. The semaphore was a product of desperation. Its been the sort of week where everything has gone wrong (and its only wednesday!?) and so I was kinda scrabbling at the problem rather than calmly dealing with it.

      With regards to not using LOCK_NB, I don't think I can go that way. This script is called, eventually, from a GUI app and I'd rather not leave the user hanging on indefinitely, even if in 99.99% of cases the lock will only exist for fractions of a second. I keep coming over sysopen but have never used it - tbh open() works fine and is what all the scripts here use and I'd rather not move away from that.

[reply]
Re: flock(), dodgy return?
by Corion (Patriarch) on Aug 01, 2007 at 14:28 UTC
[reply]
Re: flock(), dodgy return?
by moritz (Cardinal) on Aug 01, 2007 at 13:59 UTC
    I don't know much about locks, but for testing purposes please ensure that you are on a local partition, not one mounted via nfs or other network file systems.

    NFS has notorious problems with locks, perhaps other network fs' have the same caveats.

    Perl 6 in German

[reply]

      Definitely, played that game in a different life. *shudders*

[reply]

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