Name

File::Replace - Perl extension for replacing files by renaming a temp file over the original

Synopsis

This module provides three interfaces:

 use File::Replace 'replace2';
 
 my ($infh,$outfh) = replace2($filename);
 while (<$infh>) {
     # write whatever you like to $outfh here
     print $outfh "X: $_";
 }
 close $infh;   # closing both handles will
 close $outfh;  # trigger the replace

Or the more magical single filehandle, in which print, printf, and syswrite go to the output file; binmode to both; fileno only reports open/closed status; and the other I/O functions go to the input file:

 use File::Replace 'replace';
 
 my $fh = replace($filename);
 while (<$fh>) {
     # can read _and_ write from/to $fh
     print $fh "Y: $_";
 }
 close $fh;

Or the object oriented:

 use File::Replace;
 
 my $repl = File::Replace->new($filename);
 my $infh = $repl->in_fh;
 while (<$infh>) {
     print {$repl->out_fh} "Z: $_";
 }
 $repl->finish;

Description

This module implements and hides the following pattern for you:

  1. Open a temporary file for output

  2. While reading from the original file, write output to the temporary file

  3. rename the temporary file over the original file

In many cases, in particular on many UNIX filesystems, the rename operation is atomic*. This means that in such cases, the original filename will always exist, and will always point to either the new or the old version of the file, so a user attempting to open and read the file will always be able to do so, and never see an unfinished version of the file while it is being written.

* Unfortunately, whether or not a rename will actually be atomic in your specific circumstances is not always an easy question to answer, as it depends on exact details of the operating system and file system. Consult your system's documentation and search the Internet for "atomic rename" for more details.

Version

This documentation describes version 0.02 of this module.

This is an alpha version. While the module works and has a full test suite, I may still decide to change the API as I gain experience with it. I will try to make such changes compatible, but can't guarantee that just yet.

Constructors and Overview

The functions File::Replace->new(), replace(), and replace2() take exactly the same arguments, and differ only in their return values. Note that replace() and replace2() are normal functions and not methods, don't attempt to call them as such. If you don't want to import them you can always call them as, for example, File::Replace::replace().

 File::Replace->new( $filename );
 File::Replace->new( $filename, $layers );
 File::Replace->new( $filename, option => 'value', ... );
 File::Replace->new( $filename, $layers, option => 'value', ... );
 # replace(...) and replace2(...) take the same arguments

The options are described in "Options". The constructors will die in case of errors. It is strongly recommended that you use warnings;, as then this module will issue warnings which may be of interest to you.

File::Replace->new

 use File::Replace;
 my $replace_object = File::Replace->new($filename, ...);

Returns a new File::Replace object. The central methods provided are ->in_fh and ->out_fh, which return the input resp. output filehandle which you can read resp. write, and ->finish, which causes the files to be closed and the replace operation to be performed. There is also ->cancel, which just discards the temporary output file without touching the input file. Additional helper methods are mentioned below.

finish will die on errors, while cancel will only return a false value on errors. This module will try to clean up after itself (remove temporary files) as best it can, even when things go wrong.

Please don't re-open the in_fh and out_fh handles, as this may lead to confusion.

The method ->is_open will return a false value if the replace operation has been finished or canceled, or a true value if it is still active. The method ->filename returns the filename passed to the constructor. The method ->options in list context returns the options this object has set (including defaults) as a list of key/value pairs, in scalar context it returns a hashref of these options.

replace

 use File::Replace 'replace';
 my $magic_handle = replace($filename, ...);

Returns a single, "magical" tied filehandle. The operations print, printf, and syswrite are passed through to the output filehandle, binmode operates on both the input and output handle, and fileno only reports -1 if the File::Replace object is still active or undef if the replace operation has finished or been canceled. All other I/O functions, such as <$handle>, readline, sysread, seek, tell, eof, etc. are passed through to the input handle. You can still access these operations on the output handle via e.g. eof( tied(*$handle)->out_fh ) or tied(*$handle)->out_fh->tell(). The replace operation (finish) is performed when you close the handle.

Re-opening the handle causes a new underlying File::Replace object to be created. You should explicitly close the filehandle first so that the previous replace operation is performed (or cancel that operation). The "mode" argument (or filename in the case of a two-argument open) may not contain a read/write indicator (<, >, etc.), only PerlIO layers.

You can access the underlying File::Replace object via tied(*$handle)->replace. You can also access the original, untied filehandles via tied(*$handle)->in_fh and tied(*$handle)->out_fh, but please don't close or re-open these handles as this may lead to confusion.

replace2

 use File::Replace 'replace2';
 my ($input_handle, $output_handle) = replace2($filename, ...);
 my $output_handle = replace2($filename, ...);

In list context, returns a two-element list of two tied filehandles, the first being the input filehandle, and the second the output filehandle, and the replace operation (finish) is performed when both handles are closed. In scalar context, it returns only the output filehandle, and the replace operation is performed when this handle is closed.

You cannot re-open these tied filehandles.

You can access the underlying File::Replace object via tied(*$handle)->replace on both the input and output handle. You can also access the original, untied filehandles via tied(*$handle)->in_fh and tied(*$handle)->out_fh, but please don't close or re-open these handles as this may lead to confusion.

Options

Filename

A filename. The temporary output file will be created in the same directory as this file, its name will be based on the original filename, but prefixed with a dot (.) and suffixed with a random string and an extension of .tmp.

If the input file does not exist (ENOENT), then the behavior will depend on the options "devnull" (enabled by default) and "create". If either of these options are set, the input file will be created (just at different times), if neither are enabled, then attempting to open a nonexistent file will fail.

layers

This option can either be specified as the second argument to the constructors, or as the layers => '...' option in the options hash, but not both. It is a list of PerlIO layers such as ":utf8", ":raw:crlf", or ":encoding(UTF-16)". Note that the default layers differ based on operating system, see "open" in perlfunc.

devnull

This option, which is enabled by default, causes the case of nonexistent input files to be handled by opening /dev/null or its equivalent instead of the input file. This means that while the output file is being written, the input file name will not exist, and only come into existence when the rename operation is performed. If you disable this option, and attempt to open a nonexistent file, then the constructor will die.

The option "create" being set overrides this option.

create

Enabling this option causes the case of nonexistent input files to be handled by opening the input file name with a mode of +>, meaning that it is created and opened in read-write mode. However, it is strongly recommended that you don't take advantage of the read-write mode by writing to the input file, as that contradicts the purpose of this module - instead, the input file will exist and remain empty until the replace operation.

Setting this option overrides "devnull". If this option is disabled (the default), "devnull" takes precedence.

perms

 perms => 0640       # ok
 perms => oct("640") # ok
 perms => "0640"     # WRONG!

Normally, just before the rename is performed, File::Replace will chmod the temporary file to those permissions that the original file had when it was opened, or, if the original file did not yet exist, default permissions based on the current umask. Setting this option to an octal value (a number, not a string!) will override those permissions. See also "chmod", which can be used to disable the chmod operation.

chmod

This option is enabled by default, unless you set $File::Replace::DISABLE_CHMOD to a true value. When you disable this option, the chmod operation that is normally performed just before the rename will not be attempted. This is mostly intended for systems where you know the chmod will fail. See also "perms", which allows you to define what permissions will be used.

autocancel

If the File::Replace object is destroyed (e.g. when it goes out of scope), and the replace operation has not been performed yet, normally it will cancel the replace operation and issue a warning. Enabling this option makes that implicit canceling explicit, silencing the warning.

This option cannot be used together with autofinish.

autofinish

When set, causes the finish operation to be attempted when the object is destroyed (e.g. when it goes out of scope).

However, using this option is actually not recommended unless you know what you are doing. This is because the replace operation will also be attempted when your script is dieing, in which case the output file may be incomplete, and you may not want the original file to be replaced. A second reason is that the replace operation may be attempted during global destruction, and it is not a good idea to rely on this always going well. In general it is better to finish the replace operation explicitly.

This option cannot be used together with autocancel.

debug

Enables some debug output for new, finish, and cancel.

Author, Copyright, and License

Copyright (c) 2017 Hauke Daempfling (haukex@zero-g.net) at the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany, http://www.igb-berlin.de/

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.