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

    Rob B Brown (bbb@cpan.org) - 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 (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/

      Copyright (C) 2002-2018,
      Rob B Brown
      bbb@cpan.org

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

      All rights reserved.