NAME
    File::Lockfile::Emacs - Create/check/delete Emacs-style lockfiles

VERSION
    This document describes version 0.002 of File::Lockfile::Emacs (from
    Perl distribution File-Lockfile-Emacs), released on 2025-03-13.

SYNOPSIS
     use File::Lockfile::Emacs qw(
         emacs_lockfile_lock
         emacs_lockfile_get
         emacs_lockfile_locked
         emacs_lockfile_unlock
     );

     # create an Emacs-style lockfile; return 304 status if the same process already
     # acquires the lock, returns 409 if other process is holding the lock.
     my $res = emacs_lockfile_lock(target_file => "target.txt");
     if    ($res->[0] == 200) { say "Locked" }
     elsif ($res->[0] == 304) { say "Already locked" }
     else                     { die "Can't lock target.txt: $res->[0] - $res->[1]" }

     # get information on the emacs lockfile of a file
     my $res = emacs_lockfile_get(target_file => "../target.txt");
     if    ($res->[0] != 200) { die "Can't check lockfile: $res->[0] - $res->[1]" }
     elsif ($res->[0] != 404) { die "Lockfile does not exist" }
     elsif ($res->[0] != 200) { die "Can't get: $res->[0] - $res->[1]" }
     say "user = $res->[2]{user}\nhost = $res->[2]{host}\npid = $res->[2]{pid}";
     say "boot = $res->[2]{boot}" if $res->[2]{boot};

     # check if there is an emacs lockfile to a file. return 200 status and boolean
     # value.
     my $res = check_emacs_locked(target_file => "../target.txt");
     if ($res->[0] != 200) { die "Can't check lockfile: $res->[0] - $res->[1]" }
     say "File is ".($res->[2] ? "locked" : "NOT locked").
         " by ".($res->[2]{"func.pid"} == $$ ? "us" : "pid $res->[3]{'func.pid'}");

     # unlock a file. normally will only unlock if we (the same process) holds the
     # lock.
     my $res = emacs_lockfile_unlock(target_file => "../target.txt");
     if    ($res->[0] == 304) { say "File was not locked" }
     elsif ($res->[0] == 409) { say "Won't lock, file is not locked by us" }
     elsif ($res->[0] != 200) { die "Can't unlock: $res->[0] - $res->[1]" }
     else { say "Unlocked" }

DESCRIPTION
    From Emacs documentation:

      When two users edit the same file at the same time, they are likely to
      interfere with each other. Emacs tries to prevent this situation from arising
      by recording a file lock when a file is being modified. Emacs can then detect
      the first attempt to modify a buffer visiting a file that is locked by another
      Emacs job, and ask the user what to do. The file lock is really a file, a
      symbolic link with a special name, stored in the same directory as the file
      you are editing. The name is constructed by prepending .# to the file name of
      the buffer. The target of the symbolic link will be of the form
      `user@host.pid:boot`, where `user` is replaced with the current username (from
      `user-login-name`), `host` with the name of the host where Emacs is running
      (from `system-name`), `pid` with Emacsâs process id, and `boot` with the time
      since the last reboot. `:boot` is omitted if the boot time is unavailable. (On
      file systems that do not support symbolic links, a regular file is used
      instead, with contents of the form `user@host.pid:boot`.)

FUNCTIONS
  emacs_lockfile_get
    Usage:

     emacs_lockfile_get(%args) -> [$status_code, $reason, $payload, \%result_meta]

    Get information on an Emacs lockfile of a target file.

    This function is not exported by default, but exportable.

    Arguments ('*' denotes required arguments):

    *   target_file* => *filename*

        Target file.

    Returns an enveloped result (an array).

    First element ($status_code) is an integer containing HTTP-like status
    code (200 means OK, 4xx caller error, 5xx function error). Second
    element ($reason) is a string containing error message, or something
    like "OK" if status is 200. Third element ($payload) is the actual
    result, but usually not present when enveloped result is an error
    response ($status_code is not 2xx). Fourth element (%result_meta) is
    called result metadata and is optional, a hash that contains extra
    information, much like how HTTP response headers provide additional
    metadata.

    Return value: (any)

  emacs_lockfile_lock
    Usage:

     emacs_lockfile_lock(%args) -> [$status_code, $reason, $payload, \%result_meta]

    Lock a file using Emacs-style lockfile.

    Will return 412 if target file does not exist (unless "force" option is
    set to true, in which case we proceed to locking anyway).

    Will return 304 if target file is already locked using Emacs-style
    lockfile by the same process as us.

    Will return 409 if target file is already locked using Emacs-style
    lockfile by another process (unless when "force" option is set to true,
    in which case will take over the lock). Note that there are race
    conditions when using the "force" option (between checking that the
    lockfile, unlinking it, and creating our own). It is not recommended to
    use the "force" option.

    Will return 500 if there's an error in reading the lockfile.

    Will return 412 if we are not the same process that locks the file
    (unless "force" option is set to true, in which case we proceed to
    unlocking anyway).

    Will return 500 if there's an error in removing the lockfile.

    Will return 200 if everything goes ok.

    This function is not exported by default, but exportable.

    Arguments ('*' denotes required arguments):

    *   force => *bool*

        (No description)

    *   target_file* => *filename*

        Target file.

    Returns an enveloped result (an array).

    First element ($status_code) is an integer containing HTTP-like status
    code (200 means OK, 4xx caller error, 5xx function error). Second
    element ($reason) is a string containing error message, or something
    like "OK" if status is 200. Third element ($payload) is the actual
    result, but usually not present when enveloped result is an error
    response ($status_code is not 2xx). Fourth element (%result_meta) is
    called result metadata and is optional, a hash that contains extra
    information, much like how HTTP response headers provide additional
    metadata.

    Return value: (any)

  emacs_lockfile_locked
    Usage:

     emacs_lockfile_locked(%args) -> [$status_code, $reason, $payload, \%result_meta]

    Check whether a target file is locked using Emacs-style lockfile.

    This function is not exported by default, but exportable.

    Arguments ('*' denotes required arguments):

    *   by_us => *bool*

        If set to true, only return true when lockfile is created by us; if
        false, then will only return true when lockfile is created by
        others.

    *   target_file* => *filename*

        Target file.

    Returns an enveloped result (an array).

    First element ($status_code) is an integer containing HTTP-like status
    code (200 means OK, 4xx caller error, 5xx function error). Second
    element ($reason) is a string containing error message, or something
    like "OK" if status is 200. Third element ($payload) is the actual
    result, but usually not present when enveloped result is an error
    response ($status_code is not 2xx). Fourth element (%result_meta) is
    called result metadata and is optional, a hash that contains extra
    information, much like how HTTP response headers provide additional
    metadata.

    Return value: (any)

  emacs_lockfile_unlock
    Usage:

     emacs_lockfile_unlock(%args) -> [$status_code, $reason, $payload, \%result_meta]

    Unlock a file locked with Emacs-style lockfile.

    Note that there is a race condition between reading the lockfile and
    unlinking it.

    Will return 412 if target file does not exist (unless "force" option is
    set to true, in which case we proceed to unlocking anyway).

    Will return 304 if target file is not currently locked using Emacs-style
    lockfile.

    Will return 500 if there's an error in reading the lockfile.

    Will return 412 if we are not the same process that locks the file
    (unless "force" option is set to true, in which case we proceed to
    unlocking anyway).

    Will return 500 if there's an error in removing the lockfile.

    Will return 200 if everything goes ok.

    This function is not exported by default, but exportable.

    Arguments ('*' denotes required arguments):

    *   force => *bool*

        (No description)

    *   target_file* => *filename*

        Target file.

    Returns an enveloped result (an array).

    First element ($status_code) is an integer containing HTTP-like status
    code (200 means OK, 4xx caller error, 5xx function error). Second
    element ($reason) is a string containing error message, or something
    like "OK" if status is 200. Third element ($payload) is the actual
    result, but usually not present when enveloped result is an error
    response ($status_code is not 2xx). Fourth element (%result_meta) is
    called result metadata and is optional, a hash that contains extra
    information, much like how HTTP response headers provide additional
    metadata.

    Return value: (any)

HOMEPAGE
    Please visit the project's homepage at
    <https://metacpan.org/release/File-Lockfile-Emacs>.

SOURCE
    Source repository is at
    <https://github.com/perlancar/perl-File-Lockfile-Emacs>.

SEE ALSO
AUTHOR
    perlancar <perlancar@cpan.org>

CONTRIBUTING
    To contribute, you can send patches by email/via RT, or send pull
    requests on GitHub.

    Most of the time, you don't need to build the distribution yourself. You
    can simply modify the code, then test via:

     % prove -l

    If you want to build the distribution (e.g. to try to install it locally
    on your system), you can install Dist::Zilla,
    Dist::Zilla::PluginBundle::Author::PERLANCAR,
    Pod::Weaver::PluginBundle::Author::PERLANCAR, and sometimes one or two
    other Dist::Zilla- and/or Pod::Weaver plugins. Any additional steps
    required beyond that are considered a bug and can be reported to me.

COPYRIGHT AND LICENSE
    This software is copyright (c) 2025 by perlancar <perlancar@cpan.org>.

    This is free software; you can redistribute it and/or modify it under
    the same terms as the Perl 5 programming language system itself.

BUGS
    Please report any bugs or feature requests on the bugtracker website
    <https://rt.cpan.org/Public/Dist/Display.html?Name=File-Lockfile-Emacs>

    When submitting a bug or request, please include a test-file or a patch
    to an existing test-file that illustrates the bug or desired feature.

