Haskell concurrency and Handles

问题

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!

回答1:

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.

来源：https://stackoverflow.com/questions/5136375/haskell-concurrency-and-handles