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File::NFSLock(3pm) User Contributed Perl Documentation File::NFSLock(3pm)
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 AT seamons.com) - Performed majority of the programming with copious
amounts of input from Rob Brown.
Rob B Brown (bbb AT 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 AT overmeer.net) - Author of Mail::Box::Locker, from which some key
concepts for File::NFSLock were taken.
Also Kevin Johnson (kjj AT pobox.com) - Author of Mail::Folder::Maildir, from which Mark
Overmeer based Mail::Box::Locker.
COPYRIGHT
Copyright (C) 2001
Paul T Seamons
paul AT seamons.com
http://seamons.com/
Copyright (C) 2002-2018,
Rob B Brown
bbb AT 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.
perl v5.32.0 2021-01-01 File::NFSLock(3pm)
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On Apache
Under GNU General Public License
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