NAME
    File::NFSLock - perl module to do NFS (or not) locking

SYNOPSIS
      use File::NFSLock (uncache);

      my $file = "somefile";

      ### set up a lock - lasts until object looses scope
      if( defined(my $lock = File::NFSLock->new({
        file      => $file,
        lock_type => "NONBLOCKING"
        blocking_timeout   => 10,      # 10 sec
        stale_lock_timeout => 30 * 60, # 60 min
        })) ){
        
        ### OR
        ### my $lock = File::NFSLock->new($file,"NONBLOCKING",10,30*60)

        
        ### do write protected stuff on $file
        ### at this point $file is uncached from NFS (most recent)
        open(FILE, "+<$file") || die $!;

        ### or open it any way you like
        ### my $fh = IO::File->open( $file, 'w' ) || die $!

        ### update (uncache across NFS) other files
        uncache("someotherfile1");
        uncache("someotherfile2");
        # open(FILE2,"someotherfile1");

        ### unlock it
        $lock->unlock();
        ### OR
        ### undef $lock;
        ### OR let $lock go out of scope
      }else{
        die "I couldn't lock the file [$File::NFSLock::errstr]";
      }

DESCRIPTION
    Program based of concept of hard linking of files being atomic
    across NFS. This concept was mentioned in Mail::Box::Locker
    (which was originally presented in Mail::Folder::Maildir). Some
    routine flow is taken from there -- particularly the idea of
    creating a random local file, hard linking a common file to the
    local file, and then checking the nlink status. Some ideologies
    were not complete (uncache mechanism, shared locking) and some
    coding was even incorrect (wrong stat index). File::NFSLock was
    written to be light, generic, and fast.

USAGE
    Locking occurs by creating a File::NFSLock object. If the object
    is created successfully, a lock is currently in place and
    remains in place until the lock object goes out of scope (or
    calls the unlock method).

    A lock object is created by calling the new method and passing
    two to four parameters in the following manner:

      my $lock = File::NFSLock->new($file,
                                    $lock_type,
                                    $blocking_timeout,
                                    $stale_lock_timeout,
                                    );

    Additionally, parameters may be passed as a hashref: my $lock =
    File::NFSLock->new({ file => $file, lock_type => $lock_type,
    blocking_timeout => $blocking_timeout, stale_lock_timeout =>
    $stale_lock_timeout, });

PARAMETERS
    Parameter 1: file
        Filename of the file upon which it is anticipated that a
        write will happen to. Locking will provide the most recent
        version (uncached) of this file upon a successful file lock.
        It is not necessary for this file to exist.

    Parameter 2: lock_type
        Lock type must be one of the following: BLOCKING BL
        EXCLUSIVE (BLOCKING) EX NONBLOCKING NB SHARED SH
        Fcntl::LOCK_EX() (BLOCKING) Fcntl::LOCK_NB() (NONBLOCKING)
        Fcntl::LOCK_SH() (SHARED)

        Lock type determines whether the lock will be blocking, non
        blocking, or shared. Blocking locks will wait until other
        locks are removed before the process continues. Non blocking
        locks will return undef if another process currently has the
        lock. Shared will allow other process to do a shared lock at
        the same time (shared is not yet implemented).

    Parameter 3: blocking_timeout (optional)
        Timeout is used in conjunction with a blocking timeout. If
        specified, File::NFSLock will block up to the number of
        seconds specified in timeout before returning undef (could
        not get a lock).

    Parameter 4: stale_lock_timeout (optional)
        Timeout is used to see if an existing lock file is older
        than the stale lock timeout. If do_lock fails to get a lock,
        the modified time is checked and do_lock is attempted again.
        If the stale_lock_timeout is set to low, a recursion load
        could exist so do_lock will only recurse 10 times (this is
        only a problem if the stale_lock_timeout is set too low --
        on the order of one or two seconds).

FAILURE
        On failure, a global variable, $File::NFSLock::errstr,
        should be set and should contain the cause for the failure
        to get a lock. Useful primarily for debugging.

LOCK_EXTENSION
        By default File::NFSLock will use a lock file extenstion of
        ".NFSLock". This is now in a global variable
        $File::NFSLock::LOCK_EXTENSION that may be changed to suit
        other purposes (such as compatibility in mail systems).

TODO
        Features yet to be implemented...

    SHARED locks
            Need to allow for shared locking. This will allow for
            safe reading on files. Underway.

    Tests   Improve the test suite.

AUTHORS
        Paul T Seamons (paul@seamons.com) - Performed majority of
        the programming with copious amounts of input from Rob
        Brown.

        Rob B Brown (rob@roobik.com) - In addition to helping in the
        programming, Rob Brown provided most of the core testing to
        make sure implementation worked properly.

        Also Mark Overmeer (mark@overmeer.net) - Author of
        Mail::Box::Locker, from which some key concepts for
        File::NFSLock were taken.

        Also Kevin Johnson (kjj@pobox.com) - Author of
        Mail::Folder::Maildir, from which Mark Overmeer based
        Mail::Box::Locker.

COPYRIGHT
          Copyright (C) 2001, Paul T Seamons
                              paul@seamons.com
                              http://seamons.com/

                              Rob B Brown
                              rob@roobik.com
          
          This package may be distributed under the terms of either the
          GNU General Public License 
            or the
          Perl Artistic License

          All rights reserved.

