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96fb839133
* rlp: improve nil pointer handling
In both encoder and decoder, the rules for encoding nil pointers were a
bit hard to understand, and didn't leave much choice. Since RLP allows
two empty values (empty list, empty string), any protocol built on RLP
must choose either of these values to represent the null value in a
certain context.
This change adds choice in the form of two new struct tags, "nilString"
and "nilList". These can be used to specify how a nil pointer value is
encoded. The "nil" tag still exists, but its implementation is now
explicit and defines exactly how nil pointers are handled in a single
place.
Another important change in this commit is how nil pointers and the
Encoder interface interact. The EncodeRLP method was previously called
even on nil values, which was supposed to give users a choice of how
their value would be handled when nil. It turns out this is a stupid
idea. If you create a network protocol containing an object defined in
another package, it's better to be able to say that the object should be
a list or string when nil in the definition of the protocol message
rather than defining the encoding of nil on the object itself.
As of this commit, the encoding rules for pointers now take precedence
over the Encoder interface rule. I think the "nil" tag will work fine
for most cases. For special kinds of objects which are a struct in Go
but strings in RLP, code using the object can specify the desired
encoding of nil using the "nilString" and "nilList" tags.
* rlp: propagate struct field type errors
If a struct contained fields of undecodable type, the encoder and
decoder would panic instead of returning an error. Fix this by
propagating type errors in makeStruct{Writer,Decoder} and add a test.
131 lines
5.8 KiB
Go
131 lines
5.8 KiB
Go
// Copyright 2014 The go-ethereum Authors
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// This file is part of the go-ethereum library.
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//
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// The go-ethereum library is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Lesser General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// The go-ethereum library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public License
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// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
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/*
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Package rlp implements the RLP serialization format.
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The purpose of RLP (Recursive Linear Prefix) is to encode arbitrarily nested arrays of
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binary data, and RLP is the main encoding method used to serialize objects in Ethereum.
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The only purpose of RLP is to encode structure; encoding specific atomic data types (eg.
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strings, ints, floats) is left up to higher-order protocols. In Ethereum integers must be
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represented in big endian binary form with no leading zeroes (thus making the integer
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value zero equivalent to the empty string).
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RLP values are distinguished by a type tag. The type tag precedes the value in the input
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stream and defines the size and kind of the bytes that follow.
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Encoding Rules
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Package rlp uses reflection and encodes RLP based on the Go type of the value.
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If the type implements the Encoder interface, Encode calls EncodeRLP. It does not
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call EncodeRLP on nil pointer values.
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To encode a pointer, the value being pointed to is encoded. A nil pointer to a struct
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type, slice or array always encodes as an empty RLP list unless the slice or array has
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elememt type byte. A nil pointer to any other value encodes as the empty string.
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Struct values are encoded as an RLP list of all their encoded public fields. Recursive
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struct types are supported.
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To encode slices and arrays, the elements are encoded as an RLP list of the value's
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elements. Note that arrays and slices with element type uint8 or byte are always encoded
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as an RLP string.
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A Go string is encoded as an RLP string.
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An unsigned integer value is encoded as an RLP string. Zero always encodes as an empty RLP
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string. big.Int values are treated as integers. Signed integers (int, int8, int16, ...)
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are not supported and will return an error when encoding.
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Boolean values are encoded as the unsigned integers zero (false) and one (true).
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An interface value encodes as the value contained in the interface.
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Floating point numbers, maps, channels and functions are not supported.
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Decoding Rules
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Decoding uses the following type-dependent rules:
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If the type implements the Decoder interface, DecodeRLP is called.
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To decode into a pointer, the value will be decoded as the element type of the pointer. If
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the pointer is nil, a new value of the pointer's element type is allocated. If the pointer
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is non-nil, the existing value will be reused. Note that package rlp never leaves a
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pointer-type struct field as nil unless one of the "nil" struct tags is present.
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To decode into a struct, decoding expects the input to be an RLP list. The decoded
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elements of the list are assigned to each public field in the order given by the struct's
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definition. The input list must contain an element for each decoded field. Decoding
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returns an error if there are too few or too many elements for the struct.
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To decode into a slice, the input must be a list and the resulting slice will contain the
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input elements in order. For byte slices, the input must be an RLP string. Array types
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decode similarly, with the additional restriction that the number of input elements (or
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bytes) must match the array's defined length.
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To decode into a Go string, the input must be an RLP string. The input bytes are taken
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as-is and will not necessarily be valid UTF-8.
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To decode into an unsigned integer type, the input must also be an RLP string. The bytes
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are interpreted as a big endian representation of the integer. If the RLP string is larger
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than the bit size of the type, decoding will return an error. Decode also supports
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*big.Int. There is no size limit for big integers.
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To decode into a boolean, the input must contain an unsigned integer of value zero (false)
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or one (true).
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To decode into an interface value, one of these types is stored in the value:
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[]interface{}, for RLP lists
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[]byte, for RLP strings
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Non-empty interface types are not supported when decoding.
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Signed integers, floating point numbers, maps, channels and functions cannot be decoded into.
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Struct Tags
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Package rlp honours certain struct tags: "-", "tail", "nil", "nilList" and "nilString".
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The "-" tag ignores fields.
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The "tail" tag, which may only be used on the last exported struct field, allows slurping
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up any excess list elements into a slice. See examples for more details.
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The "nil" tag applies to pointer-typed fields and changes the decoding rules for the field
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such that input values of size zero decode as a nil pointer. This tag can be useful when
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decoding recursive types.
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type StructWithOptionalFoo struct {
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Foo *[20]byte `rlp:"nil"`
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}
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RLP supports two kinds of empty values: empty lists and empty strings. When using the
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"nil" tag, the kind of empty value allowed for a type is chosen automatically. A struct
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field whose Go type is a pointer to an unsigned integer, string, boolean or byte
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array/slice expects an empty RLP string. Any other pointer field type encodes/decodes as
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an empty RLP list.
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The choice of null value can be made explicit with the "nilList" and "nilString" struct
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tags. Using these tags encodes/decodes a Go nil pointer value as the kind of empty
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RLP value defined by the tag.
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*/
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package rlp
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