Haskell并发性和Handles [英] Haskell concurrency and Handles

问题描述

我正在写一个通知服务器将数据推送到客户端。基本体系结构看起来像这样（缩减伪代码）：

  acceptConnections sock =永远$ do 
连接<  - 接受袜子
 forkIO（handleConnection连接）
 
 handleConnection连接= do 
 connectionHandle<  -  socketToHandle连接ReadWriteMode 
 handleMessage connectionHandle 
 hClose connectionHandle 
 
 handleMessage connectionHandle = forever $ do 
消息<  -  hGetLine connectionHandle 
 if shouldPushMessage消息
然后hPutStrLn targetConnection消息
 else return（）
  

其中targetConnection（在handleMessage中）来自单独的连接，并挂起handleMessage在不同的线程中等待其缓冲区变得充实。我认为这会导致一个问题，因为我有2个线程访问相同的句柄。所以，我的问题是，为什么这不是问题？或者是，我只是没有看到它变成了一个问题呢？在我的实际应用程序中，当我抓取targetConnection时，我通过一个通过MVar访问的地图，但在hGetLine调用中没有安全地访问它。

免责声明：我是一个完整的Haskell和多线程的newb

感谢您的任何解释/见解！

解决作为

Handle /html/src/GHC-IO-Handle-Types.html#Handlerel =nofollow>在GHC中实现，已经是一个MVar包装在底层的 IODevice 。我没有完全明白你在做什么（不是说它不清楚，我有点不舒服，所以我可能会很慢），但我猜测GHC是建立在线程安全处理 Handle 正在节省你。

I'm writing a little notification server to push data to a client. The basic architecture looks something like this (pared down pseudo-code):

acceptConnections sock = forever $ do
    connection <- accept sock
    forkIO (handleConnection connection)

handleConnection connection = do
    connectionHandle <- socketToHandle connection ReadWriteMode
    handleMessage connectionHandle
    hClose connectionHandle

handleMessage connectionHandle = forever $ do
    message <- hGetLine connectionHandle 
    if shouldPushMessage message
        then hPutStrLn targetConnection message
        else return () 

Where targetConnection (in handleMessage) is from a separate connection and is hanging up handleMessage in a different thread waiting for its buffer to be filled. I would think this would cause a problem as I have 2 threads accessing the same Handle. So, my question is, why isn't this a problem? Or is it, and I just haven't seen it turn into an issue yet? In my actual application, when I grab targetConnection, I do so through a map I access via an MVar, but it's not being safely accessed at the hGetLine call.

Disclaimer: I'm a complete Haskell and multi-threaded newb

Thanks for any explanations/insight!

解决方案

Handle, as implemented in GHC, is already an MVar wrapping over the underlying IODevice. I didn't quite get what you're doing (not saying it was unclear, I'm a little ill so perhaps I'm slow) but am guessing GHCs built in thread-safe handling of Handle is saving you.

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