Author image thrig
and 1 contributors

NAME

File::AtomicWrite - writes files atomically via rename()

SYNOPSIS

  use File::AtomicWrite ();

  # Standalone method: requires filename and
  # input data (filehandle or scalar ref)
  File::AtomicWrite->write_file(
    { file  => 'data.dat',
      input => $filehandle
    }
  );

  # how paranoid are you?
  File::AtomicWrite->write_file(
    { file     => '/etc/passwd',
      input    => \$scalarref,
      CHECKSUM => 1,
      min_size => 100
    }
  );

  # OO interface
  my $aw = File::AtomicWrite->new({ file => 'name' });

  my $tmp_fh   = $aw->fh;
  my $tmp_file = $aw->filename;

  print $tmp_fh ...

  $aw->checksum($sha1_hexdigest);
  $aw->commit;

DESCRIPTION

This module offers atomic file writes via a temporary file created in the same directory (and therefore, probably the same partition) as the specified file. After data has been written to the temporary file, the rename call is used to replace the target file. The module optionally supports various sanity checks (min_size, CHECKSUM) that help ensure the data is written without errors.

Should anything go awry, the module will die or croak. All error messages created by the module will end with a newline, though those from submodules (File::Temp, File::Path) may not. All calls should be wrapped in eval blocks:

  eval {
    File::AtomicWrite->write_file(...);
  };
  if ($@) {
    die "uh oh: $@";
  }

The module attempts to flush and sync the temporary filehandle prior to the rename call. This may cause portability problems. If so, please let the author know. Also notify the author if false positives from the close call are observed.

METHODS

write_file

Class method. Requires a hash reference that contains at minimum both the input and file options. Performs the various required steps in a single method call. Only if all checks pass will the input data be moved to the file file via rename. If not, the module will throw an error, and attempt to cleanup any temporary files created.

See "OPTIONS" for details on the various required and optional values that can be passed to write_file in a hash reference.

new

Takes the same options as write_file (excepting the input option), returns an object.

In the event a rollback is required, undef the File::AtomicWrite object. The object destructor should then unlink the temporary file. However, should the process receive a TERM, INT, or some other catchable signal that causes it to exit, the cleanup will not be run. This edge case will need to be handled by the caller. Consult perlipc(1) for more information on signal handling.

  my $aw     = File::AtomicWrite->new({file => 'somefile'});
  $SIG{TERM} = sub { undef $aw };
  $SIG{INT}  = sub { undef $aw };
  ...
fh

Instance method, returns a filehandle for the temporary file.

filename

Instance method, returns the file name of the temporary file.

checksum

Instance method. Takes a single argument that should contain the Digest::SHA1 hexdigest of the data written to the temporary file. Enables the CHECKSUM option.

commit

Instance method. Call once finished with the temporary file. A number of sanity checks (if enabled via the appropriate option) will be performed. If these pass, the temporary file will be renamed to the real filename.

OPTIONS

The write_file and new methods accept a number of options, supplied via a hash reference:

file

Mandatory. A filename in the current working directory, or a path to the file that will be eventually created. By default, the temporary file will be written into the parent directory of the file path. This default can be changed by using the tmpdir option.

If the MKPATH option is true, the module will attempt to create any missing directories. If the MKPATH option is false or not set, the module will throw an error should any parent directories of the file not exist.

input

Mandatory for the write_file method, illegal for the new method. Scalar reference, or otherwise some filehandle reference that can be looped over via <>. Supplies the data to be written to file.

template

Template to supply to File::Temp. Defaults to a hopefully reasonable value if unset. NOTE: if customized, the template must contain a sufficient number of X that terminate the template string, as otherwise File::Temp will throw an error.

min_size

Specify a minimum size (in bytes) that the data written must exceed. If not, the module throws an error.

mode

Accepts a Unix mode for chmod to be applied to the file. Usual throwing of error. NOTE: depending on the source of the mode, oct may be first required to convert it into an octal number:

  my $orig_mode = (stat $source_file)[2] & 07777;
  ...->write_file({ ..., mode => $orig_mode });

  my $mode = '0644';
  ...->write_file({ ..., mode => oct($mode) });

The module does not change umask, nor is there a means to specify the permissions on directories created if MKPATH is set.

owner

Accepts similar arguments to chown(1) to be applied via chown to the file. Usual throwing of error.

  ...->write_file({ ..., owner => '0'   });
  ...->write_file({ ..., owner => '0:0' });
  ...->write_file({ ..., owner => 'user:somegroup' });
tmpdir

If set to a directory, the temporary file will be written to this directory instead of by default to the parent directory of the target file. If the tmpdir is on a different partition than the parent directory for file, or if anything else goes awry, the module will throw an error, as rename(2) cannot operate across partition boundaries.

This option is advisable when writing files to include directories such as /etc/logrotate.d, as the programs that read include files from these directories may read even a temporary dot file while it is being written. To avoid this (slight but non-zero) risk, use the tmpdir option to write the configuration out in full under a different directory on the same partition.

checksum

If this option exists, and CHECKSUM is true, the module will not create a Digest::SHA1 hexdigest of the data being written out to disk, but instead will rely on the value passed by the caller.

CHECKSUM

If true, Digest::SHA1 will be used to checksum the data read back from the disk against the checksum derived from the data written out to the temporary file.

Use the checksum option (or checksum method) to supply a Digest::SHA1 hexdigest checksum. This will spare the module the task of computing the checksum on the data being written.

BINMODE

If true, binmode is set on the temporary filehandle prior to writing the input data to it. Default is note to set binmode.

MKPATH

If true (default is false), attempt to create the parent directories of file should that directory not exist. If false, and the parent directory does not exist, the module throws an error. If the directory cannot be created, the module throws an error.

If true, this option will also attempt to create the tmpdir directory, if that option is set.

BUGS

No known bugs.

Reporting Bugs

Newer versions of this module may be available from CPAN.

If the bug is in the latest version, send a report to the author. Patches that fix problems or add new features are welcome.

http://github.com/thrig/File-AtomicWrite/tree/master

Known Issues

See perlport(1) for various portability problems possible with the rename call. Consult rename(2) or equivalent for the system for any caveats about this system call.

SEE ALSO

Supporting modules:

File::Temp, File::Path, File::Basename, Digest::SHA1

Alternatives, depending on the need, include:

IO::Atomic, File::Transaction, File::Transaction::Atomic, Directory::Transactional

AUTHOR

Jeremy Mates, <jmates@sial.org>

COPYRIGHT

Copyright 2009 by Jeremy Mates.

This program is free software; you can redistribute it and/or modify it under the Artistic license.