Provided by: libsereal-decoder-perl_2.03-1_amd64 
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 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 versions that are compatible with this decoder implementation are
currently protocol versions 1 and 2. As it stands, it will refuse to attempt to decode
future versions of the protocol, but if necessary there is likely going to be an option to
decode the parts of the input that are compatible with version 2 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>.
incremental
If set to a non-zero value (default: 0), then "Sereal::Decoder" will destructively parse
Sereal documents out of a variable. Every time a Sereal document is successfully parsed it
is removed from the front of the string it is parsed from.
This means you can do this:
while (length $buffer) {
my $data= decode_sereal($buffer,{incremental=>1});
}
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. Furthermore, if the encoder emitted any objects using
"FREEZE" callbacks, the "THAW" class method may be invoked on the respective classes. If
you can't trust the source of your Sereal documents, you may want to use the
"refuse_objects" option. For more details on the "FREEZE/THAW" mechanism, please refer to
Sereal::Encoder.
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.
BUGS, CONTACT AND SUPPORT
For reporting bugs, please use the github bug tracker at
<http://github.com/Sereal/Sereal/issues>.
For support and discussion of Sereal, there are two Google Groups:
Announcements around Sereal (extremely low volume):
<https://groups.google.com/forum/?fromgroups#!forum/sereal-announce>
Sereal development list: <https://groups.google.com/forum/?fromgroups#!forum/sereal-dev>
AUTHORS
Yves Orton <demerphq@gmail.com>
Damian Gryski
Steffen Mueller <smueller@cpan.org>
Rafaeel Garcia-Suarez
AEvar Arnfjoer` 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, 2014 by Steffen Mueller Copyright (C) 2012, 2013, 2014 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.