package Sys::RunAlone;

# version info
$VERSION= '0.13';

# make sure we're strict and verbose as possible
use strict;
use warnings;

# make sure we know how to lock
use Fcntl ':flock';

# process local storage
my $silent;
my $retry;

# satisfy -require-
1;

#-------------------------------------------------------------------------------
#
# Standard Perl functionality
#
#-------------------------------------------------------------------------------
# import
#
#  IN: 1 class (not used)
#      2 .. N options (default: none)

sub import {
    shift;

    # support obsolete form of silencing
    $silent= 1, return if @_ == 1 and $_[0] and $_[0] eq 'silent';

    # huh?
    die "Must specify even number of parameters" if ( @_ & 1 ) == 1;

    # obtain parameters
    my %args= @_;
    $silent= delete $args{silent};
    $silent=  $ENV{SILENT_SYS_RUNALONE} if exists $ENV{SILENT_SYS_RUNALONE};
    $retry=  delete $args{retry};
    $retry=  $ENV{RETRY_SYS_RUNALONE} if exists $ENV{RETRY_SYS_RUNALONE};

    # sanity check
    if ( my @huh= sort keys %args ) {
        die "Don't know what to do with: @huh";
    }

    return;
} #import

#-------------------------------------------------------------------------------

INIT {
    no warnings;

    # skipping
    if ( my $skip= $ENV{SKIP_SYS_RUNALONE} ) {
        print STDERR "Skipping " . __PACKAGE__ . " check for '$0'\n"
          if !$silent and $skip > 1;
    }

    # no data handle, we're screwed
    elsif ( tell( *main::DATA ) == -1 ) {
        print STDERR "Add __END__ to end of script '$0'"
          . " to be able use the features of Sys::RunALone\n";
        exit 2;
    }

    # we're not alone!
    elsif ( !flock main::DATA, LOCK_EX | LOCK_NB ) {

        # need to retry
        if ($retry) {
            my ( $times, $sleep )= split ',', $retry;
            $sleep ||= 1;
            while ( $times-- ) {
                sleep $sleep;

                # we're alone!
                goto ALLOK if flock main::DATA, LOCK_EX | LOCK_NB;
            }
        }

        # we're done
        print STDERR "A copy of '$0' is already running\n" if !$silent;
        exit 1;
    }

  ALLOK:
} #INIT

#-------------------------------------------------------------------------------

__END__

=head1 NAME

Sys::RunAlone - make sure only one invocation of a script is active at a time

=head1 SYNOPSIS

 use Sys::RunAlone;
 # code of which there may only be on instance running on system

 use Sys::RunAlone silent => 1;
 # be silent if other running instance detected

 use Sys::RunAlone retry => 50;
 # retry execution 50 times with wait time of 1 second in between

 use Sys::RunAlone retry => '55,60';
 # retry execution 55 times with wait time of 60 seconds in between

 use Sys::RunAlone 'silent';
 # obsolete form of silent => 1

=head1 DESCRIPTION

Provide a simple way to make sure the script from which this module is
loaded, is only running once on the server.  Optionally allow for retrying
execution until the other instance of the script has finished.

=head1 VERSION

This documentation describes version 0.12.

=head1 METHODS

There are no methods.

=head1 THEORY OF OPERATION

The functionality of this module depends on the availability of the DATA
handle in the script from which this module is called (more specifically:
in the "main" namespace).

At INIT time, it is checked whethere there is a DATA handle: if not, it
exits with an error message on STDERR and an exit value of 2.

If the DATA handle is available, and it cannot be C<flock>ed, it exits
with an error message on STDERR and an exit value of 1.  The error message
will be surpressed when C<silent => 1> was specified in the C<use> statement.
This can be overridden with the environment variable C<SILENT_SYS_RUNALONE>.

If there is a DATA handle, and it could be C<flock>ed, execution continues
without any further interference.

=head1 TRYING MORE THAN ONCE

Optionally, it is possibly to specify a number of retries to be done if the
first C<flock> fails.  This can be done by either specifying the retry value
in the C<use> statement as e.g. C<retry => 55>, or with the environment
variable C<RETRY_SYS_RUNALONE>.  There are two forms of the retry value:

=over 4

=item times

 use Sys::RunAlone retry => 55;  # retry 55 times, with 1 second intervals

Specify the number of times to retry, with 1 second intervals.

=item times,seconds

 use Sys::RunAlone retry => '55,60'; # retry 55 times, with 60 second intervals

Specify both the number of retries as well as the number of seconds interval
between tries.

=back

This is particularly useful for minutely and hourly scripts that run a long
and sometimes run into the next period.  Instead of then not doing anything
for the next period, it will start processing again as soon as it is possible.
This makes the chance of catching up so that the period after the next period
everything is in sync again.

=head1 OVERRIDING CHECK

In some cases, the same script may need to be run simultaneously with another
incarnation (but possibly with different parameters).  In order to simplify
this type of usage, it is possible to specify the environment variable
C<SKIP_SYS_RUNALONE> with a true value.

 SKIP_SYS_RUNALONE=1 yourscript.pl

will run the script always.

 SKIP_SYS_RUNALONE=2 yourscript.pl

will actually be verbose about this and say:

 Skipping Sys::RunAlone check for 'yourscript.pl'

=head1 REQUIRED MODULES

 Fcntl (any)

=head1 CAVEATS

=head2 symlinks

Execution of scripts that are (sym)linked to another script, will all be seen
as execution of the same script, even though the error message will only show
the specified script name.  This could be considered a bug or a feature.

=head2 changing a running script

If you change the script while it is running, the script will effectively
lose its lock on the file.  Causing any subsequent run of the same script
to be successful, causing two instances of the same script to run at the
same time (which is what you wanted to prevent by using Sys::RunAlone in
the first place).  Therefore, make sure that no instances of the script are
running (and won't be started by cronjobs while making changes) if you really
want to be 100% sure that only one instance of the script is running at the
same time.

=head1 ACKNOWLEDGEMENTS

Inspired by Randal Schwartz's mention of using the DATA handle as a semaphore
on the London PM mailing list.

Booking.com for using this heavily in production and allowing me to improve
this module.

=head1 SEE ALSO

L<Sys::RunAlways>.

=head1 AUTHOR

 Elizabeth Mattijsen

=head1 COPYRIGHT

Copyright (c) 2005, 2006, 2008, 2009, 2011, 2012 Elizabeth Mattijsen
<liz@dijkmat.nl>.  Copyright (c) 2017 Ben Tilly <btilly@gmail.com>.  All
rights reserved.  This program is free software; you can redistribute it
and/or modify it under the same terms as Perl itself.

=cut