--- CBOR-XS/README 2013/11/22 16:18:59 1.9 +++ CBOR-XS/README 2016/04/27 09:40:18 1.17 @@ -23,38 +23,31 @@ } DESCRIPTION - WARNING! This module is very new, and not very well tested (that's up to - you to do). Furthermore, details of the implementation might change - freely before version 1.0. And lastly, most extensions depend on an IANA - assignment, and until that assignment is official, this implementation - is not interoperable with other implementations (even future versions of - this module) until the assignment is done. - - You are still invited to try out CBOR, and this module. - This module converts Perl data structures to the Concise Binary Object Representation (CBOR) and vice versa. CBOR is a fast binary - serialisation format that aims to use a superset of the JSON data model, - i.e. when you can represent something in JSON, you should be able to - represent it in CBOR. + serialisation format that aims to use an (almost) superset of the JSON + data model, i.e. when you can represent something useful in JSON, you + should be able to represent it in CBOR. - In short, CBOR is a faster and very compact binary alternative to JSON, + In short, CBOR is a faster and quite compact binary alternative to JSON, with the added ability of supporting serialisation of Perl objects. (JSON often compresses better than CBOR though, so if you plan to - compress the data later you might want to compare both formats first). + compress the data later and speed is less important you might want to + compare both formats first). To give you a general idea about speed, with texts in the megabyte range, "CBOR::XS" usually encodes roughly twice as fast as Storable or JSON::XS and decodes about 15%-30% faster than those. The shorter the data, the worse Storable performs in comparison. - As for compactness, "CBOR::XS" encoded data structures are usually about - 20% smaller than the same data encoded as (compact) JSON or Storable. + Regarding compactness, "CBOR::XS"-encoded data structures are usually + about 20% smaller than the same data encoded as (compact) JSON or + Storable. In addition to the core CBOR data format, this module implements a - number of extensions, to support cyclic and self-referencing data - structures (see "allow_sharing"), string deduplication (see - "allow_stringref") and scalar references (always enabled). + number of extensions, to support cyclic and shared data structures (see + "allow_sharing" and "allow_cycles"), string deduplication (see + "pack_strings") and scalar references (always enabled). The primary goal of this module is to be *correct* and the secondary goal is to be *fast*. To reach the latter goal it was written in C. @@ -149,48 +142,137 @@ This means that such values will only be encoded once, and will not result in a deep cloning of the value on decode, in decoders - supporting the value sharing extension. + supporting the value sharing extension. This also makes it possible + to encode cyclic data structures (which need "allow_cycles" to ne + enabled to be decoded by this module). It is recommended to leave it off unless you know your communication partner supports the value sharing extensions to CBOR - (http://cbor.schmorp.de/value-sharing). + (), as without decoder + support, the resulting data structure might be unusable. Detecting shared values incurs a runtime overhead when values are encoded that have a reference counter large than one, and might unnecessarily increase the encoded size, as potentially shared - values are encode as sharable whether or not they are actually + values are encode as shareable whether or not they are actually shared. At the moment, only targets of references can be shared (e.g. scalars, arrays or hashes pointed to by a reference). Weirder constructs, such as an array with multiple "copies" of the *same* string, which are hard but not impossible to create in Perl, are not - supported (this is the same as for Storable). + supported (this is the same as with Storable). - If $enable is false (the default), then "encode" will encode - exception when it encounters anything it cannot encode as CBOR. + If $enable is false (the default), then "encode" will encode shared + data structures repeatedly, unsharing them in the process. Cyclic + data structures cannot be encoded in this mode. This option does not affect "decode" in any way - shared values and references will always be decoded properly if present. - $cbor = $cbor->allow_stringref ([$enable]) - $enabled = $cbor->get_allow_stringref + $cbor = $cbor->allow_cycles ([$enable]) + $enabled = $cbor->get_allow_cycles + If $enable is true (or missing), then "decode" will happily decode + self-referential (cyclic) data structures. By default these will not + be decoded, as they need manual cleanup to avoid memory leaks, so + code that isn't prepared for this will not leak memory. + + If $enable is false (the default), then "decode" will throw an error + when it encounters a self-referential/cyclic data structure. + + FUTURE DIRECTION: the motivation behind this option is to avoid + *real* cycles - future versions of this module might chose to decode + cyclic data structures using weak references when this option is + off, instead of throwing an error. + + This option does not affect "encode" in any way - shared values and + references will always be encoded properly if present. + + $cbor = $cbor->pack_strings ([$enable]) + $enabled = $cbor->get_pack_strings If $enable is true (or missing), then "encode" will try not to encode the same string twice, but will instead encode a reference to - the string instead. Depending on your data format. this can save a + the string instead. Depending on your data format, this can save a lot of space, but also results in a very large runtime overhead (expect encoding times to be 2-4 times as high as without). It is recommended to leave it off unless you know your communications partner supports the stringref extension to CBOR - (http://cbor.schmorp.de/stringref). + (), as without decoder support, + the resulting data structure might not be usable. - If $enable is false (the default), then "encode" will encode - exception when it encounters anything it cannot encode as CBOR. + If $enable is false (the default), then "encode" will encode strings + the standard CBOR way. This option does not affect "decode" in any way - string references will always be decoded properly if present. + $cbor = $cbor->text_keys ([$enable]) + $enabled = $cbor->get_text_keys + If $enabled is true (or missing), then "encode" will encode all perl + hash keys as CBOR text strings/UTF-8 string, upgrading them as + needed. + + If $enable is false (the default), then "encode" will encode hash + keys normally - upgraded perl strings (strings internally encoded as + UTF-8) as CBOR text strings, and downgraded perl strings as CBOR + byte strings. + + This option does not affect "decode" in any way. + + This option is useful for interoperability with CBOR decoders that + don't treat byte strings as a form of text. It is especially useful + as Perl gives very little control over hash keys. + + Enabling this option can be slow, as all downgraded hash keys that + are encoded need to be scanned and converted to UTF-8. + + $cbor = $cbor->text_strings ([$enable]) + $enabled = $cbor->get_text_strings + This option works similar to "text_keys", above, but works on all + strings (including hash keys), so "text_keys" has no further effect + after enabling "text_strings". + + If $enabled is true (or missing), then "encode" will encode all perl + strings as CBOR text strings/UTF-8 strings, upgrading them as + needed. + + If $enable is false (the default), then "encode" will encode strings + normally (but see "text_keys") - upgraded perl strings (strings + internally encoded as UTF-8) as CBOR text strings, and downgraded + perl strings as CBOR byte strings. + + This option does not affect "decode" in any way. + + This option has similar advantages and disadvantages as "text_keys". + In addition, this option effectively removes the ability to encode + byte strings, which might break some "FREEZE" and "TO_CBOR" methods + that rely on this, such as bignum encoding, so this option is mainly + useful for very simple data. + + $cbor = $cbor->validate_utf8 ([$enable]) + $enabled = $cbor->get_validate_utf8 + If $enable is true (or missing), then "decode" will validate that + elements (text strings) containing UTF-8 data in fact contain valid + UTF-8 data (instead of blindly accepting it). This validation + obviously takes extra time during decoding. + + The concept of "valid UTF-8" used is perl's concept, which is a + superset of the official UTF-8. + + If $enable is false (the default), then "decode" will blindly accept + UTF-8 data, marking them as valid UTF-8 in the resulting data + structure regardless of whether that's true or not. + + Perl isn't too happy about corrupted UTF-8 in strings, but should + generally not crash or do similarly evil things. Extensions might be + not so forgiving, so it's recommended to turn on this setting if you + receive untrusted CBOR. + + This option does not affect "encode" in any way - strings that are + supposedly valid UTF-8 will simply be dumped into the resulting CBOR + string without checking whether that is, in fact, true or not. + $cbor = $cbor->filter ([$cb->($tag, $value)]) $cb_or_undef = $cbor->get_filter Sets or replaces the tagged value decoding filter (when $cb is @@ -220,7 +302,7 @@ returns no values. Example: decode all tags not handled internally into - CBOR::XS::Tagged objects, with no other special handling (useful + "CBOR::XS::Tagged" objects, with no other special handling (useful when working with potentially "unsafe" CBOR data). CBOR::XS->new->filter (sub { })->decode ($cbor_data); @@ -255,6 +337,64 @@ CBOR::XS->new->decode_prefix ("......") => ("...", 3) + INCREMENTAL PARSING + In some cases, there is the need for incremental parsing of JSON texts. + While this module always has to keep both CBOR text and resulting Perl + data structure in memory at one time, it does allow you to parse a CBOR + stream incrementally, using a similar to using "decode_prefix" to see if + a full CBOR object is available, but is much more efficient. + + It basically works by parsing as much of a CBOR string as possible - if + the CBOR data is not complete yet, the pasrer will remember where it + was, to be able to restart when more data has been accumulated. Once + enough data is available to either decode a complete CBOR value or raise + an error, a real decode will be attempted. + + A typical use case would be a network protocol that consists of sending + and receiving CBOR-encoded messages. The solution that works with CBOR + and about anything else is by prepending a length to every CBOR value, + so the receiver knows how many octets to read. More compact (and + slightly slower) would be to just send CBOR values back-to-back, as + "CBOR::XS" knows where a CBOR value ends, and doesn't need an explicit + length. + + The following methods help with this: + + @decoded = $cbor->incr_parse ($buffer) + This method attempts to decode exactly one CBOR value from the + beginning of the given $buffer. The value is removed from the + $buffer on success. When $buffer doesn't contain a complete value + yet, it returns nothing. Finally, when the $buffer doesn't start + with something that could ever be a valid CBOR value, it raises an + exception, just as "decode" would. In the latter case the decoder + state is undefined and must be reset before being able to parse + further. + + This method modifies the $buffer in place. When no CBOR value can be + decoded, the decoder stores the current string offset. On the next + call, continues decoding at the place where it stopped before. For + this to make sense, the $buffer must begin with the same octets as + on previous unsuccessful calls. + + You can call this method in scalar context, in which case it either + returns a decoded value or "undef". This makes it impossible to + distinguish between CBOR null values (which decode to "undef") and + an unsuccessful decode, which is often acceptable. + + @decoded = $cbor->incr_parse_multiple ($buffer) + Same as "incr_parse", but attempts to decode as many CBOR values as + possible in one go, instead of at most one. Calls to "incr_parse" + and "incr_parse_multiple" can be interleaved. + + $cbor->incr_reset + Resets the incremental decoder. This throws away any saved state, so + that subsequent calls to "incr_parse" or "incr_parse_multiple" start + to parse a new CBOR value from the beginning of the $buffer again. + + This method can be caled at any time, but it *must* be called if you + want to change your $buffer or there was a decoding error and you + want to reuse the $cbor object for future incremental parsings. + MAPPING This section describes how CBOR::XS maps Perl values to CBOR values and vice versa. These mappings are designed to "do the right thing" in most @@ -271,7 +411,7 @@ support, 64 bit integers will be truncated or otherwise corrupted. byte strings - Byte strings will become octet strings in Perl (the byte values + Byte strings will become octet strings in Perl (the Byte values 0..255 will simply become characters of the same value in Perl). UTF-8 strings @@ -299,7 +439,7 @@ Tagged items consists of a numeric tag and another CBOR value. See "TAG HANDLING AND EXTENSIONS" and the description of "->filter" - for details. + for details on which tags are handled how. anything else Anything else (e.g. unsupported simple values) will raise a decoding @@ -307,13 +447,14 @@ PERL -> CBOR The mapping from Perl to CBOR is slightly more difficult, as Perl is a - truly typeless language, so we can only guess which CBOR type is meant - by a Perl value. + typeless language. That means this module can only guess which CBOR type + is meant by a perl value. hash references Perl hash references become CBOR maps. As there is no inherent ordering in hash keys (or CBOR maps), they will usually be encoded - in a pseudo-random order. + in a pseudo-random order. This order can be different each time a + hash is encoded. Currently, tied hashes will use the indefinite-length format, while normal hashes will use the fixed-length format. @@ -322,14 +463,17 @@ Perl array references become fixed-length CBOR arrays. other references - Other unblessed references are generally not allowed and will cause - an exception to be thrown, except for references to the integers 0 - and 1, which get turned into false and true in CBOR. + Other unblessed references will be represented using the indirection + tag extension (tag value 22098, + ). CBOR decoders are guaranteed + to be able to decode these values somehow, by either "doing the + right thing", decoding into a generic tagged object, simply ignoring + the tag, or something else. CBOR::XS::Tagged objects Objects of this type must be arrays consisting of a single "[tag, value]" pair. The (numerical) tag will be encoded as a CBOR tag, the - value will be encoded as appropriate for the value. You cna use + value will be encoded as appropriate for the value. You must use "CBOR::XS::tag" to create such objects. Types::Serialiser::true, Types::Serialiser::false, @@ -355,7 +499,7 @@ encode_cbor [-3.0e17] # yields [-3e+17] my $value = 5; encode_cbor [$value] # yields [5] - # used as string, so dump as string + # used as string, so dump as string (either byte or text) print $value; encode_cbor [$value] # yields ["5"] @@ -369,6 +513,19 @@ $x .= ""; # another, more awkward way to stringify print $x; # perl does it for you, too, quite often + You can force whether a string is encoded as byte or text string by + using "utf8::upgrade" and "utf8::downgrade" (if "text_strings" is + disabled): + + utf8::upgrade $x; # encode $x as text string + utf8::downgrade $x; # encode $x as byte string + + Perl doesn't define what operations up- and downgrade strings, so if + the difference between byte and text is important, you should up- or + downgrade your string as late as possible before encoding. You can + also force the use of CBOR text strings by using "text_keys" or + "text_strings". + You can force the type to be a CBOR number by numifying it: my $x = "3"; # some variable containing a string @@ -387,10 +544,15 @@ might suffer loss of precision. OBJECT SERIALISATION + This module implements both a CBOR-specific and the generic + Types::Serialier object serialisation protocol. The following + subsections explain both methods. + + ENCODING This module knows two way to serialise a Perl object: The CBOR-specific way, and the generic way. - Whenever the encoder encounters a Perl object that it cnanot serialise + Whenever the encoder encounters a Perl object that it cannot serialise directly (most of them), it will first look up the "TO_CBOR" method on it. @@ -405,12 +567,17 @@ The "FREEZE" method can return any number of values (i.e. zero or more). These will be encoded as CBOR perl object, together with the classname. + These methods *MUST NOT* change the data structure that is being + serialised. Failure to comply to this can result in memory corruption - + and worse. + If an object supports neither "TO_CBOR" nor "FREEZE", encoding will fail with an error. - Objects encoded via "TO_CBOR" cannot be automatically decoded, but - objects encoded via "FREEZE" can be decoded using the following - protocol: + DECODING + Objects encoded via "TO_CBOR" cannot (normally) be automatically + decoded, but objects encoded via "FREEZE" can be decoded using the + following protocol: When an encoded CBOR perl object is encountered by the decoder, it will look up the "THAW" method, by using the stored classname, and will fail @@ -441,7 +608,7 @@ my ($self) = @_; my $uri = "$self"; # stringify uri utf8::upgrade $uri; # make sure it will be encoded as UTF-8 string - CBOR::XS::tagged 32, "$_[0]" + CBOR::XS::tag 32, "$_[0]" } This will encode URIs as a UTF-8 string with tag 32, which indicates an @@ -462,7 +629,6 @@ sub URI::THAW { my ($class, $serialiser, $uri) = @_; - $class->new ($uri) } @@ -568,28 +734,41 @@ ENFORCED TAGS These tags are always handled when decoding, and their handling cannot - be overriden by the user. + be overridden by the user. - (perl-object, ) + 26 (perl-object, ) These tags are automatically created (and decoded) for serialisable objects using the "FREEZE/THAW" methods (the Types::Serialier object serialisation protocol). See "OBJECT SERIALISATION" for details. - , (sharable, sharedref, L - ) - These tags are automatically decoded when encountered, resulting in - shared values in the decoded object. They are only encoded, however, - when "allow_sharable" is enabled. + 28, 29 (shareable, sharedref, ) + These tags are automatically decoded when encountered (and they do + not result in a cyclic data structure, see "allow_cycles"), + resulting in shared values in the decoded object. They are only + encoded, however, when "allow_sharing" is enabled. + + Not all shared values can be successfully decoded: values that + reference themselves will *currently* decode as "undef" (this is not + the same as a reference pointing to itself, which will be + represented as a value that contains an indirect reference to itself + - these will be decoded properly). + + Note that considerably more shared value data structures can be + decoded than will be encoded - currently, only values pointed to by + references will be shared, others will not. While non-reference + shared values can be generated in Perl with some effort, they were + considered too unimportant to be supported in the encoder. The + decoder, however, will decode these values as shared values. - , (stringref-namespace, stringref, L + 256, 25 (stringref-namespace, stringref, ) These tags are automatically decoded when encountered. They are only - encoded, however, when "allow_stringref" is enabled. + encoded, however, when "pack_strings" is enabled. 22098 (indirection, ) This tag is automatically generated when a reference are encountered - (with the exception of hash and array refernces). It is converted to - a reference when decoding. + (with the exception of hash and array references). It is converted + to a reference when decoding. 55799 (self-describe CBOR, RFC 7049) This value is not generated on encoding (unless explicitly requested @@ -597,8 +776,8 @@ NON-ENFORCED TAGS These tags have default filters provided when decoding. Their handling - can be overriden by changing the %CBOR::XS::FILTER entry for the tag, or - by providing a custom "filter" callback when decoding. + can be overridden by changing the %CBOR::XS::FILTER entry for the tag, + or by providing a custom "filter" callback when decoding. When they result in decoding into a specific Perl class, the module usually provides a corresponding "TO_CBOR" method as well. @@ -608,20 +787,38 @@ to provide these modules. The decoding usually fails with an exception if the required module cannot be loaded. + 0, 1 (date/time string, seconds since the epoch) + These tags are decoded into Time::Piece objects. The corresponding + "Time::Piece::TO_CBOR" method always encodes into tag 1 values + currently. + + The Time::Piece API is generally surprisingly bad, and fractional + seconds are only accidentally kept intact, so watch out. On the plus + side, the module comes with perl since 5.10, which has to count for + something. + 2, 3 (positive/negative bignum) These tags are decoded into Math::BigInt objects. The corresponding "Math::BigInt::TO_CBOR" method encodes "small" bigints into normal CBOR integers, and others into positive/negative CBOR bignums. - 4, 5 (decimal fraction/bigfloat) + 4, 5, 264, 265 (decimal fraction/bigfloat) Both decimal fractions and bigfloats are decoded into Math::BigFloat objects. The corresponding "Math::BigFloat::TO_CBOR" method *always* - encodes into a decimal fraction. + encodes into a decimal fraction (either tag 4 or 264). + + NaN and infinities are not encoded properly, as they cannot be + represented in CBOR. - CBOR cannot represent bigfloats with *very* large exponents - - conversion of such big float objects is undefined. + See "BIGNUM SECURITY CONSIDERATIONS" for more info. - Also, NaN and infinities are not encoded properly. + 30 (rational numbers) + These tags are decoded into Math::BigRat objects. The corresponding + "Math::BigRat::TO_CBOR" method encodes rational numbers with + denominator 1 via their numerator only, i.e., they become normal + integers or "bignums". + + See "BIGNUM SECURITY CONSIDERATIONS" for more info. 21, 22, 23 (expected later JSON conversion) CBOR::XS is not a CBOR-to-JSON converter, and will simply ignore @@ -678,6 +875,35 @@ information you might want to make sure that exceptions thrown by CBOR::XS will not end up in front of untrusted eyes. +BIGNUM SECURITY CONSIDERATIONS + CBOR::XS provides a "TO_CBOR" method for both Math::BigInt and + Math::BigFloat that tries to encode the number in the simplest possible + way, that is, either a CBOR integer, a CBOR bigint/decimal fraction (tag + 4) or an arbitrary-exponent decimal fraction (tag 264). Rational numbers + (Math::BigRat, tag 30) can also contain bignums as members. + + CBOR::XS will also understand base-2 bigfloat or arbitrary-exponent + bigfloats (tags 5 and 265), but it will never generate these on its own. + + Using the built-in Math::BigInt::Calc support, encoding and decoding + decimal fractions is generally fast. Decoding bigints can be slow for + very big numbers (tens of thousands of digits, something that could + potentially be caught by limiting the size of CBOR texts), and decoding + bigfloats or arbitrary-exponent bigfloats can be *extremely* slow + (minutes, decades) for large exponents (roughly 40 bit and longer). + + Additionally, Math::BigInt can take advantage of other bignum libraries, + such as Math::GMP, which cannot handle big floats with large exponents, + and might simply abort or crash your program, due to their code quality. + + This can be a concern if you want to parse untrusted CBOR. If it is, you + might want to disable decoding of tag 2 (bigint) and 3 (negative bigint) + types. You should also disable types 5 and 265, as these can be slow + even without bigints. + + Disabling bigints will also partially or fully disable types that rely + on them, e.g. rational numbers that use bignums. + CBOR IMPLEMENTATION NOTES This section contains some random implementation notes. They do not describe guaranteed behaviour, but merely behaviour as-is implemented @@ -695,6 +921,14 @@ Strict mode and canonical mode are not implemented. +LIMITATIONS ON PERLS WITHOUT 64-BIT INTEGER SUPPORT + On perls that were built without 64 bit integer support (these are rare + nowadays, even on 32 bit architectures, as all major Perl distributions + are built with 64 bit integer support), support for any kind of 64 bit + integer in CBOR is very limited - most likely, these 64 bit values will + be truncated, corrupted, or otherwise not decoded correctly. This also + includes string, array and map sizes that are stored as 64 bit integers. + THREADS This module is *not* guaranteed to be thread safe and there are no plans to change this until Perl gets thread support (as opposed to the