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Module Version: 0.35   Source   Latest Release: Sereal-Decoder-3.001_002

NAME ^

Sereal::Decoder - Fast, compact, powerful binary deserialization

SYNOPSIS ^

  use Sereal::Decoder qw(decode_sereal looks_like_sereal);
  
  my $decoder = Sereal::Decoder->new({...options...});
  
  my $structure;
  $decoder->decode($blob, $structure); # deserializes into $structure
  
  # or if you don't have references to the top level structure, this works, too:
  $structure = $decoder->decode($blob);
  
  # alternatively functional interface:
  decode_sereal($blob, {... options ...}, $structure);
  $structure = decode_sereal($blob, {... options ...});
  
  # Not a full validation, but just a quick check for a reasonable header:
  my $is_likely_sereal = looks_like_sereal($some_string);
  # or:
  $is_likely_sereal = $decoder->looks_like_sereal($some_string);

DESCRIPTION ^

This is an experimental module. The interface may change without notice. Before using it in production, please get in touch with the authors!

This library implements a deserializer for an efficient, compact-output, and feature-rich binary protocol called Sereal. Its sister module Sereal::Encoder implements an encoder for this format. The two are released separately to allow for independent and safer upgrading.

The Sereal protocol version that is compatible with this decoder implementation is currently protocol version 1. As it stands, it will refuse to attempt to decode future versions of the protocol, but there is likely going to be an option to decode the parts of the input that are compatible with version 1 of the protocol. The protocol was designed to allow for this.

The protocol specification and many other bits of documentation can be found in the github repository. Right now, the specification is at https://github.com/Sereal/Sereal/blob/master/sereal_spec.pod, there is a discussion of the design objectives in https://github.com/Sereal/Sereal/blob/master/README.pod, and the output of our benchmarks can be seen at https://github.com/Sereal/Sereal/wiki/Sereal-Comparison-Graphs.

CLASS METHODS ^

new

Constructor. Optionally takes a hash reference as first parameter. This hash reference may contain any number of options that influence the behaviour of the encoder.

Currently, the following options are recognized, none of them are on by default.

refuse_snappy

If set, the decoder will refuse Snappy-compressed input data. This can be desirable for robustness. See the section ROBUSTNESS below.

refuse_objects

If set, the decoder will refuse deserializing any objects in the input stream and instead throw and exception. Defaults to off. See the section ROBUSTNESS below.

no_bless_objects

If set, the decoder will deserialize any objects in the input stream but without blessing them. Defaults to off. See the section ROBUSTNESS below.

validate_utf8

If set, the decoder will refuse invalid UTF-8 byte sequences. This is off by default, but it's strongly encouraged to be turned on if you're dealing with any data that has been encoded by an external source (e.g. http cookies).

max_recursion_depth

Sereal::Decoder is recursive. If you pass it a Sereal document that is deeply nested, it will eventually exhaust the C stack. Therefore, there is a limit on the depth of recursion that is accepted. It defaults to 10000 nested calls. You may choose to override this value with the max_recursion_depth option. Beware that setting it too high can cause hard crashes.

Do note that the setting is somewhat approximate. Setting it to 10000 may break at somewhere between 9997 and 10003 nested structures depending on their types.

max_num_hash_entries

If set to a non-zero value (default: 0), then Sereal::Decoder will refuse to deserialize any hash/dictionary (or hash-based object) with more than that number of entries. This is to be able to respond quickly to any future hash-collision attacks on Perl's hash function. Chances are, you don't want or need this. For a gentle introduction to the topic from the cryptographic point of view, see http://en.wikipedia.org/wiki/Collision_attack.

INSTANCE METHODS ^

decode

Given a byte string of Sereal data, the decode call derializes that data structure. The result can be obtained in one of two ways: decode accepts a second parameter, which is a scalar to write the result to, AND decode will return the resulting data structure.

The two are subtly different in case of data structures that contain references to the root element. In that case, the return value will be a (non-recursive) copy of the reference. The pass-in style is more correct. In other words,

  $decoder->decode($sereal_string, my $out);
  # is almost the same but safer than:
  my $out = $decoder->decode($sereal_string);

This is an unfortunate side-effect of perls standard copy semantics of assignment. Possibly one day we will have an alternative to this.

decode_with_offset

Same as the decode method, except as second parameter, you must pass an integer offset into the input string, at which the decoding is to start. The optional "pass-in" style scalar (see decode above) is relegated to being the third parameter.

bytes_consumed

After using the decode method, bytes_consumed can return the number of bytes of the input string that were actually consumed by the decoder. That is, if you append random garbage to a valid Sereal document, decode will happily decode the data and ignore the garbage. If that is an error in your use case, you can use bytes_consumed to catch it.

  my $out = $decoder->decode($sereal_string);
  if (length($sereal_string) != $decoder->bytes_consumed) {
    die "Not all input data was consumed!";
  }

Chances are that if you do this, you're violating UNIX philosophy in "be strict in what you emit but lenient in what you accept".

You can also use this to deserialize a list of Sereal documents that is concatenated into the same string (code not very robust...):

  my @out;
  my $pos = 0;
  eval {
    while (1) {
      push @out, $decoder->decode_with_offset($sereal_string, $pos);
      $pos += $decoder->bytes_consumed;
      last if $pos >= length($sereal_string)
           or not $decoder->bytes_consumed;
    }
  };

looks_like_sereal

Given a string (or undef), checks whether it looks like it starts with a valid Sereal packet. This is not a full-blown validation. Instead, this just checks the magic string and some header properties to provide a quick and efficient way to distinguish multiple well-formed serialization methods instead of really making sure it's valid Sereal. For reference, sereal's magic string is a four byte string =srl.

EXPORTABLE FUNCTIONS ^

decode_sereal

The functional interface that is equivalent to using new and decode. Expects a byte string to deserialize as first argument, optionally followed by a hash reference of options (see documentation for new()). Finally, decode_sereal supports a third parameter, which is the output scalar to write to. See the documentation for decode above for details.

The functional interface is marginally slower than the OO interface since it cannot reuse the decoder object.

looks_like_sereal

Same as the object method of the same name.

ROBUSTNESS ^

This implementation of a Sereal decoder tries to be as robust to invalid input data as reasonably possible. This means that it should never (though read on) segfault. It may, however, cause a large malloc to fail. Generally speaking, invalid data should cause a Perl-trappable exception. The one exception is that for Snappy-compressed Sereal documents, the Snappy library may cause segmentation faults (invalid reads orwrites). This should only be a problem if you do not checksum your data (internal checksum support is a To-Do) or if you accept data from potentially malicious sources.

It requires a lot of run-time boundary checks to prevent decoder segmentation faults on invalid data. We implemented them in the lightest way possible. Adding robustness against running out of memory would cause an very significant run-time overhead. In most cases of random garbage (with valid header no less) when a malloc() fails due to invalid data, the problem was caused by a very large array or string length. This kind of very large malloc can then fail, being trappable from Perl. Only when packet causes many repeated allocations do you risk causing a hard OOM error from the kernel that cannot be trapped because Perl may require some small allocations to succeed before the now-invalid memory is released. It is at least not entirely trivial to craft a Sereal document that causes this behaviour.

Finally, deserializing proper objects is potentially a problem because classes can define a destructor. Thus, the data fed to the decoder can cause the (deferred) execution of any destructor in your application. That's why the refuse_objects option exists and what the no_bless_objects can be used for as well. Later on, we may or may not provide a facility to whitelist classes.

PERFORMANCE ^

The exact performance in time and space depends heavily on the data structure to be serialized. For ready-made comparison scripts, see the author_tools/bench.pl and author_tools/dbench.pl programs that are part of this distribution. Suffice to say that this library is easily competitive in both time and space efficiency with the best alternatives.

THREAD-SAFETY ^

Sereal::Decoder is thread-safe on Perl's 5.8.7 and higher. This means "thread-safe" in the sense that if you create a new thread, all Sereal::Decoder objects will become a reference to undef in the new thread. This might change in a future release to become a full clone of the decoder object.

AUTHOR ^

Yves Orton <demerphq@gmail.com>

Damian Gryski

Steffen Mueller <smueller@cpan.org>

Rafaël Garcia-Suarez

Ævar Arnfjörð Bjarmason <avar@cpan.org>

Daniel Dragan <bulkdd@cpan.org> (Windows support and bugfixes)

Some inspiration and code was taken from Marc Lehmann's excellent JSON::XS module due to obvious overlap in problem domain.

ACKNOWLEDGMENT ^

This module was originally developed for Booking.com. With approval from Booking.com, this module was generalized and published on CPAN, for which the authors would like to express their gratitude.

COPYRIGHT AND LICENSE ^

Copyright (C) 2012, 2013 by Steffen Mueller Copyright (C) 2012, 2013 by Yves Orton

The license for the code in this distribution is the following, with the exceptions listed below:

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

Except portions taken from Marc Lehmann's code for the JSON::XS module, which is licensed under the same terms as this module. (Many thanks to Marc for inspiration, and code.)

Also except the code for Snappy compression library, whose license is reproduced below and which, to the best of our knowledge, is compatible with this module's license. The license for the enclosed Snappy code is:

  Copyright 2011, Google Inc.
  All rights reserved.

  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are
  met:

    * Redistributions of source code must retain the above copyright
  notice, this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above
  copyright notice, this list of conditions and the following disclaimer
  in the documentation and/or other materials provided with the
  distribution.
    * Neither the name of Google Inc. nor the names of its
  contributors may be used to endorse or promote products derived from
  this software without specific prior written permission.

  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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