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

SYNOPSIS
     use File::NFSLock qw(uncache);
     use Fcntl qw(LOCK_EX LOCK_NB);

     my $file = "somefile";

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

       ### OR
       ### my $lock = File::NFSLock->new($file,LOCK_EX|LOCK_NB,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

       Or else one or more of the following joined with '|':

         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 as long as there
       is not already an exclusive lock obtained.

   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).

METHODS
   After the $lock object is instantiated with new, as outlined above, some
   methods may be used for additional functionality.

 unlock
     $lock->unlock;

   This method may be used to explicitly release a lock that is acquired.
   In most cases, it is not necessary to call unlock directly since it will
   implicitly be called when the object leaves whatever scope it is in.

 uncache
     $lock->uncache;
     $lock->uncache("otherfile1");
     uncache("otherfile2");

   This method is used to freshen up the contents of a file across NFS,
   ignoring what is contained in the NFS client cache. It is always called
   from within the new constructor on the file that the lock is being
   attempted. uncache may be used as either an object method or as a stand
   alone subroutine.

 fork
     my $pid = $lock->fork;
     if (!defined $pid) {
       # Fork Failed
     } elsif ($pid) {
       # Parent ...
     } else {
       # Child ...
     }

   fork() is a convenience method that acts just like the normal
   CORE::fork() except it safely ensures the lock is retained within both
   parent and child processes. WITHOUT this, then when either the parent or
   child process releases the lock, then the entire lock will be lost,
   allowing external processes to re-acquire a lock on the same file, even
   if the other process still has the lock object in scope. This can cause
   corruption since both processes might think they have exclusive access
   to the file.

 newpid
     my $pid = fork;
     if (!defined $pid) {
       # Fork Failed
     } elsif ($pid) {
       $lock->newpid;
       # Parent ...
     } else {
       $lock->newpid;
       # Child ...
     }

   The newpid() synopsis shown above is equivalent to the one used for the
   fork() method, but it's not intended to be called directly. It is called
   internally by the fork() method. To be safe, it is recommended to use
   $lock->fork() from now on.

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 extension of ".NFSLock".
   This is in a global variable $File::NFSLock::LOCK_EXTENSION that may be
   changed to suit other purposes (such as compatibility in mail systems).

REPO
   The source is now on github:

   git clone https://github.com/hookbot/File-NFSLock

BUGS
   If you spot anything, please submit a pull request on github and/or
   submit a ticket with RT:
   https://rt.cpan.org/Dist/Display.html?Queue=File-NFSLock

 FIFO
   Locks are not necessarily obtained on a first come first serve basis.
   Not only does this not seem fair to new processes trying to obtain a
   lock, but it may cause a process starvation condition on heavily locked
   files.

 DIRECTORIES
   Locks cannot be obtained on directory nodes, nor can a directory node be
   uncached with the uncache routine because hard links do not work with
   directory nodes. Some other algorithm might be used to uncache a
   directory, but I am unaware of the best way to do it. The biggest use I
   can see would be to avoid NFS cache of directory modified and last
   accessed timestamps.

INSTALL
   Download and extract tarball before running these commands in its base
   directory:

     perl Makefile.PL
     make
     make test
     make install

   For RPM installation, download tarball before running these commands in
   your _topdir:

     rpm -ta SOURCES/File-NFSLock-*.tar.gz
     rpm -ih RPMS/noarch/perl-File-NFSLock-*.rpm

AUTHORS
   Paul T Seamons ([email protected]) - Performed majority of the
   programming with copious amounts of input from Rob Brown.

   Rob B Brown ([email protected]) - In addition to helping in the programming,
   Rob Brown provided most of the core testing to make sure implementation
   worked properly. He is now the current maintainer.

   Also Mark Overmeer ([email protected]) - Author of Mail::Box::Locker,
   from which some key concepts for File::NFSLock were taken.

   Also Kevin Johnson ([email protected]) - Author of Mail::Folder::Maildir,
   from which Mark Overmeer based Mail::Box::Locker.

COPYRIGHT
     Copyright (C) 2001
     Paul T Seamons
     [email protected]
     http://seamons.com/

     Copyright (C) 2002-2018,
     Rob B Brown
     [email protected]

     This package may be distributed under the terms of either the
     GNU General Public License
       or the
     Perl Artistic License

     All rights reserved.

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