=encoding utf8

=head1 NAME

Mail::Box::Locker - manage the locking of mail folders

=head1 INHERITANCE

 Mail::Box::Locker
   is a Mail::Reporter

 Mail::Box::Locker is extended by
   Mail::Box::Locker::DotLock
   Mail::Box::Locker::FcntlLock
   Mail::Box::Locker::Flock
   Mail::Box::Locker::Multi
   Mail::Box::Locker::Mutt
   Mail::Box::Locker::NFS
   Mail::Box::Locker::POSIX

=head1 SYNOPSIS

 use Mail::Box::Locker;
 my $locker = new Mail::Box::Locker(folder => $folder);

 $locker->lock;
 $locker->isLocked;
 $locker->hasLock;
 $locker->unlock;

 use Mail::Box;
 my $folder = Mail::Box->new(lock_method => 'DOTLOCK');
 print $folder->locker->type;

=head1 DESCRIPTION

Each L<Mail::Box|Mail::Box> will create its own C<Mail::Box::Locker> object which
will handle the locking for it.  You can access of the object directly
from the folder, as shown in the examples below.

Extends L<"DESCRIPTION" in Mail::Reporter|Mail::Reporter/"DESCRIPTION">.
 
=head1 METHODS

Extends L<"METHODS" in Mail::Reporter|Mail::Reporter/"METHODS">.
 
=head2 Constructors

Extends L<"Constructors" in Mail::Reporter|Mail::Reporter/"Constructors">.
 
=over 4

=item Mail::Box::Locker-E<gt>B<new>(%options)

Create a new lock. You may do this directly. However, in most cases the
lock will not be separately instantiated but will be the second class in
a multiple inheritance construction with a L<Mail::Box|Mail::Box>.

Generally the client program specifies the locking behavior through
options given to the folder class.

 -Option --Defined in     --Default
  expires                   1 hour
  file                      undef
  folder                    <undef>
  log      Mail::Reporter   'WARNINGS'
  method                    'DOTLOCK'
  timeout                   10
  trace    Mail::Reporter   'WARNINGS'

=over 2

=item expires => SECONDS

How long can a lock exist?  If a different e-mail program leaves a stale
lock, then this lock will be removed automatically after the specified
number of seconds.

=item file => FILENAME

Name of the file to lock.  By default, the name of the folder is taken.

=item folder => FOLDER

Which FOLDER is to be locked, a L<Mail::Box|Mail::Box> object.

=item log => LEVEL

=item method => STRING|CLASS|ARRAY

Which kind of locking, specified as one of the following names as STRING.
You may also specify a CLASS name, or an ARRAY of names.  In case of an
ARRAY, a 'multi' locker is started with all thee 
full CLASS name.

Supported locking names are

=over 4

=item 'DOTLOCK' | 'dotlock'

The folder handler creates a file which signals that it is in use.  This
is a bit problematic, because not all mail-handling software agree on
the name of the file to be created.

On various folder types, the lockfile differs.  See the documentation for
each folder, which describes the locking strategy as well as special
options to change the default behavior.

=item 'FLOCK' | 'flock'

For some folder handlers, locking is based on a file locking mechanism
provided by the operating system.  However, this does not work on all
systems, such as network filesystems, and such. This also doesn't work on
folders based on directories (L<Mail::Box::Dir|Mail::Box::Dir> and derived).

=item 'FCNTLLOCK' | 'fcntllock'

POSIX locking via File::FcntlLock, which works on more platforms.
However, that module requires a C compiler to install.

=item 'POSIX' | 'posix'

Use the POSIX standard fcntl locking.

=item 'MULTI' | 'multi'

Use ALL available locking methods at the same time, to have a bigger
chance that the folder will not be modified by some other application
which uses an unspecified locking method.  When one of the locking
methods disallows access, the locking fails.

=item 'MUTT'| 'mutt'

Use the external program 'mutt_dotlock' to lock and unlock.

=item 'NFS' | 'nfs'

A kind of C<dotlock> file-locking mechanism, but adapted to work over
NFS.  Extra precaution is needed because an C<open O_EXCL> on NFS is
not an atomic action.

=item 'NONE' | 'none'

Do not use locking.

=back

The other option is to produce your own C<Mail::Box::Locker> derived class,
which implements the desired locking method. (Please consider offering it
for inclusion in the public Mail::Box module!) Create an instance of that
class with this parameter:

 my $locker = Mail::Box::Locker::MyOwn->new;
 $folder->open(locker => $locker);

=item timeout => SECONDS|'NOTIMEOUT'

How long to wait while trying to acquire the lock. The lock request will
fail when the specified number of seconds is reached.  If C<'NOTIMEOUT'> is
specified, the module will wait until the lock can be taken.

Whether it is possible to limit the wait time is platform- and
locking-method-specific.  For instance, the `dotlock' method on Windows
will always wait until the lock has been received.

=item trace => LEVEL

=back

=back

=head2 Attributes

=over 4

=item $obj-E<gt>B<expires>( [SECONDS] )

Get/Set the expiration time.  Not available for all lockers.

=item $obj-E<gt>B<timeout>( [SECONDS] )

Get/Set the timeout.  Not available for all lockers.

=back

=head2 The Locker

=over 4

=item $obj-E<gt>B<filename>( [$filename] )

Returns the filename which is used to lock the folder, optionally after
setting it to the specified $filename.

example: 

 print $locker->filename;

=item $obj-E<gt>B<folder>( [$folder] )

Returns the folder object which is locker.

=item $obj-E<gt>B<name>()

Returns the method used to lock the folder. See the L<new(method)|Mail::Box::Locker/"METHODS"> for
details on how to specify the lock method.  The name of the method is
returned in upper-case.

example: 

 if($locker->name eq 'FLOCK') ...

=back

=head2 Locking

=over 4

=item $obj-E<gt>B<hasLock>()

Check whether the folder has the lock.

example: 

 if($locker->hasLock) {...}
 if($folder->locker->hasLock) {...}

=item $obj-E<gt>B<isLocked>()

Test if the folder is locked by this or a different application.

example: 

 if($locker->isLocked) {...}
 if($folder->locker->isLocked) {...}

=item $obj-E<gt>B<lock>($folder)

Get a lock on a folder.  This will return false if the lock fails.

example: 

 die unless $locker->lock;
 if($folder->locker->lock) {...}

=item $obj-E<gt>B<unlock>()

Undo the lock on a folder.

example: 

 $locker->unlock;
 $folder->locker->unlock;

=back

=head2 Error handling

Extends L<"Error handling" in Mail::Reporter|Mail::Reporter/"Error handling">.
 
=over 4

=item $obj-E<gt>B<AUTOLOAD>()

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<addReport>($object)

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<defaultTrace>( [$level]|[$loglevel, $tracelevel]|[$level, $callback] )

=item Mail::Box::Locker-E<gt>B<defaultTrace>( [$level]|[$loglevel, $tracelevel]|[$level, $callback] )

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<errors>()

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<log>( [$level, [$strings]] )

=item Mail::Box::Locker-E<gt>B<log>( [$level, [$strings]] )

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<logPriority>($level)

=item Mail::Box::Locker-E<gt>B<logPriority>($level)

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<logSettings>()

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<notImplemented>()

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<report>( [$level] )

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<reportAll>( [$level] )

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<trace>( [$level] )

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<warnings>()

Inherited, see L<Mail::Reporter/"Error handling">

=back

=head2 Cleanup

Extends L<"Cleanup" in Mail::Reporter|Mail::Reporter/"Cleanup">.
 
=over 4

=item $obj-E<gt>B<DESTROY>()

When the locker is destroyed, for instance when the folder is closed
or the program ends, the lock will be automatically removed.

=back

=head1 DIAGNOSTICS

=over 4

=item Error: Package $package does not implement $method.

Fatal error: the specific package (or one of its superclasses) does not
implement this method where it should. This message means that some other
related classes do implement this method however the class at hand does
not.  Probably you should investigate this and probably inform the author
of the package.

=back

=head1 SEE ALSO

This module is part of Mail-Box distribution version 3.008,
built on October 04, 2019. Website: F<http://perl.overmeer.net/CPAN/>

=head1 LICENSE

Copyrights 2001-2019 by [Mark Overmeer]. For other contributors see ChangeLog.

This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
See F<http://dev.perl.org/licenses/>