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    },
    features => {
        tx => {v=>2},
        idempotent => 1,
    },
};
sub prepend {
    my %args = @_;

    # TMP, schema
    my $tx_action  = $args{-tx_action} // '';
    my $taid       = $args{-tx_action_id}
        or return [400, "Please specify -tx_action_id"];
    my $dry_run    = $args{-dry_run};
    my $path       = $args{path};
    defined($path) or return [400, "Please specify path"];
    defined($args{string}) or return [400, "Please specify string"];
    my $string     = "$args{string}";

    my $is_sym  = (-l $path);
    my @st      = stat($path);
    my $exists  = $is_sym || (-e _);
    my $is_file = (-f _);
    my $size    = (-s _);

    if ($tx_action eq 'check_state') {
        return [412, "File $path does not exist"]        unless $exists;
        return [412, "File $path is not a regular file"] if $is_sym||!$is_file;
        return [500, "File $path can't be stat'd"]       unless @st;

        if ($size >= length($string)) {
            my $buf;
            open my($fh),"<",$path or return [500, "Can't open file $path: $!"];
            read $fh, $buf, length($string);
            if (defined($buf) && $buf eq $string) {
                return [304, "File $path already prepended with string, ".
                            "won't prepend twice"];
            }
        }
        log_info("(DRY) Prepending string to file $path ...") if $dry_run;
        return [200, "File $path needs to be prepended with a string", undef,
                {undo_actions=>[
                    ['File::Trash::Undoable::untrash', # restore original
                     {path=>$path, suffix=>substr($taid,0,8)}],
                    ['File::Trash::Undoable::trash', # trash new file
                     {path=>$path, suffix=>substr($taid,0,8)."n"}],
                ]}];
    } elsif ($tx_action eq 'fix_state') {
        log_info("Prepending string to file $path ...");
        my $res = File::Trash::Undoable::trash(
            -tx_action=>'fix_state', path=>$path, suffix=>substr($taid,0,8));
        return $res unless $res->[0] == 200 || $res->[0] == 304;
        open my($oh), "<", $res->[2]
            or return [500, "Can't open $res->[2] for reading: $!"];
        open my($nh), ">", $path
            or return [500, "Can't open $path for writing: $!"];
        print $nh $string;
        while (my $l = <$oh>) { print $nh $l }
        close $nh or return [500, "Can't close: $!"];
        chmod $st[2] & 07777, $path; # XXX ignore error?
        unless ($>) { chown $st[4], $st[5], $path } # XXX ignore error?
        return [200, "OK"];
    }
    [400, "Invalid -tx_action"];
}

1;
# ABSTRACT: Prepend string to a file, with undo support

__END__

=pod

=encoding UTF-8

=head1 NAME

File::Prepend::Undoable - Prepend string to a file, with undo support

=head1 VERSION

This document describes version 0.05 of File::Prepend::Undoable (from Perl distribution File-Prepend-Undoable), released on 2017-07-10.

=head1 FUNCTIONS


=head2 prepend

Usage:

 prepend(%args) -> [status, msg, result, meta]

Prepend string to a file, with undo support.

On do, will trash file, copy it to original location (with the same permission
and ownership as the original) and prepend the string to the beginning of file.
On undo will trash the new file and untrash the original.

Some notes:

=over

=item * Chown will not be done if we are not running as root.

=item * Symlink is currently not permitted.

=item * Since transaction requires the function to be idempotent, in the C<check_state>
phase the function will check if the string has been prepended. It will refuse
to prepend the same string twice.

=item * Take care not to use string that are too large, as the string, being a
function parameter, is entered into the transaction table.

=back

Fixed state: file exists and string has been prepended to beginning of file.

Fixable state: file exists and string has not been prepended to beginning of
file.

Unfixable state: file does not exist or path is not a regular file (directory