使用GNU / Linux系统调用`splice`在Haskell中进行零拷贝Socket到Socket数据传输

更新：Nemo先生的回答有助于解决问题！下面的代码包含修复程序！请参阅下面的nb False和nb True调用.

还有一个名为splice的新Haskell软件包(它具有特定于操作系统的便携式实现,是最着名的套接字到套接字数据传输循环).

我有以下(Haskell)代码：

#ifdef LINUX_SPLICE
#include <fcntl.h>
{-# LANGUAGE CPP #-}
{-# LANGUAGE ForeignFunctionInterface #-}
#endif

module Network.Socket.Splice (
    Length,zeroCopy,splice
#ifdef LINUX_SPLICE,c_splice
#endif
  ) where

import Data.Word
import Foreign.Ptr

import Network.Socket
import Control.Monad
import Control.Exception
import System.Posix.Types
import System.Posix.IO

#ifdef LINUX_SPLICE
import Data.Int
import Data.Bits
import Unsafe.Coerce
import Foreign.C.Types
import Foreign.C.Error
import System.Posix.Internals
#else
import System.IO
import Foreign.Marshal.Alloc
#endif


zeroCopy :: Bool
zeroCopy =
#ifdef LINUX_SPLICE
  True
#else
  False
#endif


type Length =
#ifdef LINUX_SPLICE
  (#type size_t)
#else
  Int
#endif


-- | The 'splice' function pipes data from
--   one socket to another in a loop.
--   On Linux this happens in kernel space with
--   zero copying between kernel and user spaces.
--   On other operating systems,a portable
--   implementation utilizes a user space buffer
--   allocated with 'mallocBytes'; 'hGetBufSome'
--   and 'hPut' are then used to avoid repeated 
--   tiny allocations as would happen with 'recv'
--   'sendAll' calls from the 'bytestring' package.
splice :: Length -> Socket -> Socket -> IO ()
splice l (MkSocket x _ _ _ _) (MkSocket y _ _ _ _) = do

  let e  = error "splice ended"

#ifdef LINUX_SPLICE

  (r,w) <- createPipe
  print ('+',r,w)
  let s  = Fd x -- source
  let t  = Fd y -- target
  let c  = throwErrnoIfMinus1 "Network.Socket.Splice.splice"
  let u  = unsafeCoerce :: (#type ssize_t) -> (#type size_t)
  let fs = sPLICE_F_MOVE .|. sPLICE_F_MORE
  let nb v = do setNonBlockingFD x v
                setNonBlockingFD y v
  nb False
  finally
    (forever $do 
       b <- c $c_splice s nullPtr w nullPtr    l  fs
       if b > 0
         then   c_splice r nullPtr t nullPtr (u b) fs)
         else   e
    (do closeFd r
        closeFd w
        nb True
        print ('-',w))

#else

  -- ..    

#endif


#ifdef LINUX_SPLICE
-- SPLICE

-- fcntl.h
-- ssize_t splice(
--   int          fd_in,--   loff_t*      off_in,--   int          fd_out,--   loff_t*      off_out,--   size_t       len,--   unsigned int flags
-- );

foreign import ccall "splice"
  c_splice
  :: Fd
  -> Ptr (#type loff_t)
  -> Fd
  -> Ptr (#type loff_t)
  -> (#type size_t)
  -> Word
  -> IO (#type ssize_t)

sPLICE_F_MOVE :: Word
sPLICE_F_MOVE = (#const "SPLICE_F_MOVE")

sPLICE_F_MORE :: Word
sPLICE_F_MORE = (#const "SPLICE_F_MORE")
#endif

注意：上面的代码现在正常工作！由于Nemo,以下不再有效！

我用上面定义的splice调用了两个打开并连接的套接字(它们已经用于使用套接字API发送和接收调用传输最少量的握手数据或转换为句柄并与hGetLine和hPut一起使用)并且我一直得到：

Network.Socket.Splice.splice: resource exhausted (Resource temporarily unavailable)

在第一个c_splice调用站点：c_splice返回-1并将一些errno设置为一个读取资源耗尽的值(可能是EAGAIN)查找时资源暂时不可用.

我测试了不同长度值的调用splice：1024,8192.

解决方法

它默认看起来像Haskell sets its sockets to non-blocking mode.因此,如果您在没有数据时尝试从一个读取数据,或者在其缓冲区已满时尝试写入一个数据,那么您将失败并使用EAGAIN.

弄清楚如何将套接字更改为阻止模式,我打赌你会解决你的问题.

[更新]

或者,在尝试读取或写入套接字之前调用select或poll.但是你仍然需要处理EAGAIN,因为有一些罕见的极端情况,其中Linux选择将指示套接字已准备就绪,而实际上并非如此.

（编辑：安卓应用网）

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