haskell – 使用管道的顺序二进制数据解码

目标是使用具有以下类型签名的管道

protobufConduit :: MonadResource m => (ByteString -> a) -> Conduit ByteString m a

管道应该重复解析通过TCP / IP(使用network-conduit包)接收的协议缓冲区(使用ByteString – >一个函数).

有线消息格式是

{length (32 bits big endian)}{protobuf 1}{length}{protobuf 2}...

(花括号不是协议的一方,仅用于分隔实体).

第一个想法是使用sequenceSink重复应用一个能够解析一个ProtoBuf的Sink：

[...]
import qualified Data.Binary         as B
import qualified Data.Conduit.Binary as CB
import qualified Data.Conduit.Util   as CU

protobufConduit :: MonadResource m => (ByteString -> a) -> Conduit ByteString m a
protobufConduit protobufDecode =
    CU.sequenceSink () $\() ->
        do lenBytes <- CB.take 4                                -- read protobuf length
           let len :: Word32
               len = B.decode lengthBytes                       -- decode ProtoBuf length
               intLen = fromIntegral len
           protobufBytes <- CB.take intLen                      -- read the ProtoBuf bytes
           return $CU.Emit () [ protobufDecode protobufBytes ] -- emit decoded ProtoBuf

它不起作用(仅适用于第一个协议缓冲区),因为似乎已经从源读取了许多“剩余”字节,但没有通过CB.take消耗掉这些字节.

我发现无法将“其余部分推回源头”.

我的概念完全错了吗？

PS：即使我在这里使用协议缓冲区,问题也与协议缓冲区无关.为了调试这个问题,我总是使用{length} {UTF8编码的字符串} {length} {UTF8编码的字符串} …和类似于上面的管道(utf8StringConduit :: MonadResource m => Conduit ByteString m Text).

更新：

我只是尝试用剩余的字节替换状态(上面的示例中没有state())并通过调用一个首先消耗已经读取的字节(来自状态)的函数来替换CB.take调用,并且仅调用await作为需要(当状态不够大时).不幸的是,这不起作用,因为只要Source没有剩余字节,sequenceSink就不会执行代码,但状态仍然包含剩余的字节:-(.

如果你应该对代码感兴趣(没有优化或非常好,但应该足以测试)：

utf8StringConduit :: forall m. MonadResource m => Conduit ByteString m Text
utf8StringConduit =
    CU.sequenceSink [] $\st ->
        do (lengthBytes, st') <- takeWithState BS.empty st 4
           let len :: Word32
               len = B.decode $BSL.fromChunks [lengthBytes]
               intLength = fromIntegral len
           (textBytes, st'') <- takeWithState BS.empty st' intLength
           return $CU.Emit st'' [ TE.decodeUtf8 $textBytes ]

takeWithState :: Monad m
              => ByteString
              -> [ByteString]
              -> Int
              -> Pipe l ByteString o u m (ByteString, [ByteString])
takeWithState acc state 0 = return (acc, state)
takeWithState acc state neededLen =
    let stateLenSum = foldl' (+) 0 $map BS.length state
     in if stateLenSum >= neededLen
           then do let (firstChunk:state') = state
                       (neededChunk, pushBack) = BS.splitAt neededLen firstChunk
                       acc' = acc `BS.append` neededChunk
                       neededLen' = neededLen - BS.length neededChunk
                       state'' = if BS.null pushBack
                                    then state'
                                    else pushBack:state'
                   takeWithState acc' state'' neededLen'
           else do aM <- await
                   case aM of
                     Just a -> takeWithState acc (state ++ [a]) neededLen
                     Nothing -> error "to be fixed later"

对于协议缓冲区解析和序列化,我们使用messageWithLengthPutM和messageWithLengthGetM(见下文),但我认为它使用了长度的varint编码,这不是你需要的.我可能会尝试通过用类似的东西替换messageWithLength Get / Put来调整我们的实现

myMessageWithLengthGetM = 
   do size <- getWord32be 
      getMessageWithSize size

但是我不知道如何使用协议缓冲包中的可用函数来实现getMessageWithSize.另一方面,你可以getByteString,然后“重新分析”bytestring.

关于管道：您是否尝试过在没有Data.Conduit.Util的情况下实施管道？就像是

protobufConduit protobufDecode = loop
   where
      loop = 
         do len <- liftM convertLen (CB.take 4)
            bs <- CB.take len
            yield (protobufDecode bs)
            loop

这是我们使用的代码：

pbufSerialize :: (ReflectDescriptor w, Wire w) => Conduit w IO ByteString
pbufSerialize = awaitForever f
    where f pb = M.mapM_ yield $BSL.toChunks $runPut (messageWithLengthPutM pb)

pbufParse :: (ReflectDescriptor w, Wire w, Show w) => Conduit ByteString IO w
pbufParse = new
    where
      new = read (runGet messageWithLengthGetM . BSL.fromChunks . (:[]))
      read parse =
          do mbs <- await
             case mbs of
               Just bs -> checkResult (parse bs)
               Nothing -> return ()
      checkResult result =
          case result of
            Failed _ errmsg -> fail errmsg
            Partial cont -> read (cont . Just . BSL.fromChunks . (:[]))
            Finished rest _ msg ->
                do yield msg
                   checkResult (runGet messageWithLengthGetM rest)