Google Protocol Buffers (protobuf): Lua parsing library

Google Protocol Buffers (AKA protobuf) is a popular data serialization scheme used for communication protocols, data storage, etc. There are implementations are available for almost every popular language. The focus points of this scheme are brevity (data is encoded in a very size-efficient manner) and extensibility (one can add keys to the structure, while keeping it readable in previous version of software).

Protobuf uses semi-self-describing encoding scheme for its messages. It means that it is possible to parse overall structure of the message (skipping over fields one can't understand), but to fully understand the message, one needs a protocol definition file (.proto). To be specific:

  • "Keys" in key-value pairs provided in the message are identified only with an integer "field tag". .proto file provides info on which symbolic field names these field tags map to.
  • "Keys" also provide something called "wire type". It's not a data type in its common sense (i.e. you can't, for example, distinguish sint32 vs uint32 vs some enum, or string from bytes), but it's enough information to determine how many bytes to parse. Interpretation of the value should be done according to the type specified in .proto file.
  • There's no direct information on which fields are optional / required, which fields may be repeated or constitute a map, what restrictions are placed on fields usage in a single message, what are the fields' default values, etc, etc.

KS implementation details

License: MIT
Minimal Kaitai Struct required: 0.7

References

This page hosts a formal specification of Google Protocol Buffers (protobuf) using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Lua source code to parse Google Protocol Buffers (protobuf)

google_protobuf.lua

-- This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild
--
-- This file is compatible with Lua 5.3

local class = require("class")
require("kaitaistruct")
local enum = require("enum")

require("vlq_base128_le")
-- 
-- Google Protocol Buffers (AKA protobuf) is a popular data
-- serialization scheme used for communication protocols, data storage,
-- etc. There are implementations are available for almost every
-- popular language. The focus points of this scheme are brevity (data
-- is encoded in a very size-efficient manner) and extensibility (one
-- can add keys to the structure, while keeping it readable in previous
-- version of software).
-- 
-- Protobuf uses semi-self-describing encoding scheme for its
-- messages. It means that it is possible to parse overall structure of
-- the message (skipping over fields one can't understand), but to
-- fully understand the message, one needs a protocol definition file
-- (`.proto`). To be specific:
-- 
-- * "Keys" in key-value pairs provided in the message are identified
--   only with an integer "field tag". `.proto` file provides info on
--   which symbolic field names these field tags map to.
-- * "Keys" also provide something called "wire type". It's not a data
--   type in its common sense (i.e. you can't, for example, distinguish
--   `sint32` vs `uint32` vs some enum, or `string` from `bytes`), but
--   it's enough information to determine how many bytes to
--   parse. Interpretation of the value should be done according to the
--   type specified in `.proto` file.
-- * There's no direct information on which fields are optional /
--   required, which fields may be repeated or constitute a map, what
--   restrictions are placed on fields usage in a single message, what
--   are the fields' default values, etc, etc.
-- See also: Source (https://developers.google.com/protocol-buffers/docs/encoding)
GoogleProtobuf = class.class(KaitaiStruct)

function GoogleProtobuf:_init(io, parent, root)
  KaitaiStruct._init(self, io)
  self._parent = parent
  self._root = root or self
  self:_read()
end

function GoogleProtobuf:_read()
  self.pairs = {}
  local i = 0
  while not self._io:is_eof() do
    self.pairs[i + 1] = GoogleProtobuf.Pair(self._io, self, self._root)
    i = i + 1
  end
end

-- 
-- Key-value pairs which constitute a message.

-- 
-- Key-value pair.
GoogleProtobuf.Pair = class.class(KaitaiStruct)

GoogleProtobuf.Pair.WireTypes = enum.Enum {
  varint = 0,
  bit_64 = 1,
  len_delimited = 2,
  group_start = 3,
  group_end = 4,
  bit_32 = 5,
}

function GoogleProtobuf.Pair:_init(io, parent, root)
  KaitaiStruct._init(self, io)
  self._parent = parent
  self._root = root or self
  self:_read()
end

function GoogleProtobuf.Pair:_read()
  self.key = VlqBase128Le(self._io)
  local _on = self.wire_type
  if _on == GoogleProtobuf.Pair.WireTypes.varint then
    self.value = VlqBase128Le(self._io)
  elseif _on == GoogleProtobuf.Pair.WireTypes.len_delimited then
    self.value = GoogleProtobuf.DelimitedBytes(self._io, self, self._root)
  elseif _on == GoogleProtobuf.Pair.WireTypes.bit_64 then
    self.value = self._io:read_u8le()
  elseif _on == GoogleProtobuf.Pair.WireTypes.bit_32 then
    self.value = self._io:read_u4le()
  end
end

-- 
-- "Wire type" is a part of the "key" that carries enough
-- information to parse value from the wire, i.e. read correct
-- amount of bytes, but there's not enough informaton to
-- interprete in unambiguously. For example, one can't clearly
-- distinguish 64-bit fixed-sized integers from 64-bit floats,
-- signed zigzag-encoded varints from regular unsigned varints,
-- arbitrary bytes from UTF-8 encoded strings, etc.
GoogleProtobuf.Pair.property.wire_type = {}
function GoogleProtobuf.Pair.property.wire_type:get()
  if self._m_wire_type ~= nil then
    return self._m_wire_type
  end

  self._m_wire_type = GoogleProtobuf.Pair.WireTypes((self.key.value & 7))
  return self._m_wire_type
end

-- 
-- Identifies a field of protocol. One can look up symbolic
-- field name in a `.proto` file by this field tag.
GoogleProtobuf.Pair.property.field_tag = {}
function GoogleProtobuf.Pair.property.field_tag:get()
  if self._m_field_tag ~= nil then
    return self._m_field_tag
  end

  self._m_field_tag = (self.key.value >> 3)
  return self._m_field_tag
end

-- 
-- Key is a bit-mapped variable-length integer: lower 3 bits
-- are used for "wire type", and everything higher designates
-- an integer "field tag".
-- 
-- Value that corresponds to field identified by
-- `field_tag`. Type is determined approximately: there is
-- enough information to parse it unambiguously from a stream,
-- but further infromation from `.proto` file is required to
-- interprete it properly.

GoogleProtobuf.DelimitedBytes = class.class(KaitaiStruct)

function GoogleProtobuf.DelimitedBytes:_init(io, parent, root)
  KaitaiStruct._init(self, io)
  self._parent = parent
  self._root = root or self
  self:_read()
end

function GoogleProtobuf.DelimitedBytes:_read()
  self.len = VlqBase128Le(self._io)
  self.body = self._io:read_bytes(self.len.value)
end