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
        aquired. 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.

  newpid

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

        If fork() is called after a lock has been aquired, then when the
        lock object leaves scope in either the parent or child, it will be
        released. This behavior may be inappropriate for your application.
        To delegate ownership of the lock from the parent to the child, both
        the parent and child process must call the newpid() method after a
        successful fork() call. This will prevent the parent from releasing
        the lock when unlock is called or when the lock object leaves scope.
        This is also useful to allow the parent to fail on subsequent lock
        attempts if the child lock is still aquired.

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

BUGS
        Notify paul@seamons.com or bbb@cpan.org if you spot anything.

  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.

        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
                              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.

