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=head1 NAME

Lucy::Store::Lock - Abstract class representing an interprocess mutex lock.

=head1 SYNOPSIS

    my $lock = $lock_factory->make_lock(
        name    => 'write',
        timeout => 5000,
    );
    $lock->obtain or die "can't get lock for " . $lock->get_name;
    do_stuff();
    $lock->release;



=head1 DESCRIPTION

The Lock class produces an interprocess mutex lock.  The default subclass
uses dot-lock files, but alternative implementations are possible.

Each lock must have a name which is unique per resource to be locked.  Each
lock also has a "host" id which should be unique per machine; it is used to
help clear away stale locks.

=head1 CONSTRUCTORS

=head2 new( I<[labeled params]> )

    my $lock = Lucy::Store::Lock->new(
        name     => 'commit',     # required
        folder   => $folder,      # required
        host     => $hostname,    # required
        timeout  => 5000,         # default: 0
        interval => 1000,         # default: 100
    );

Abstract constructor.

=over

=item *

B<folder> - A Folder.

=item *

B<name> - String identifying the resource to be locked, which must
consist solely of characters matching [-_.A-Za-z0-9].

=item *

B<host> - A unique per-machine identifier.

=item *

B<timeout> - Time in milliseconds to keep retrying before abandoning
the attempt to obtain() a lock.

=item *

B<interval> - Time in milliseconds between retries.

=back



=head1 ABSTRACT METHODS

=head2 request()

Make one attempt to acquire the lock.

The semantics of request() differ depending on whether shared() returns
true.  If the Lock is shared(), then request() should not fail if
another lock is held against the resource identified by
C<< name >> (though it might fail for other reasons).  If it is
not shared() -- i.e. it's an exclusive (write) lock -- then other locks
should cause request() to fail.

Returns: true on success, false on failure (sets Lucy->error).

=head2 release()

Release the lock.

=head2 is_locked()

Indicate whether the resource identified by this lock's name is
currently locked.

Returns: true if the resource is locked, false otherwise.

=head2 clear_stale()

Release all locks that meet the following three conditions: the lock
name matches, the host id matches, and the process id that the lock
was created under no longer identifies an active process.



=head1 METHODS

=head2 obtain()

Call request() once per C<< interval >> until request() returns
success or the C<< timeout >> has been reached.

Returns: true on success, false on failure (sets Lucy->error).



=head1 INHERITANCE

Lucy::Store::Lock isa L<Lucy::Object::Obj>.


=cut