Re: [Haskell-cafe] network's example echo server seems leaking socket on error binding

9 May 2020


      On Sat, 9 May 2020, Compl Yue wrote:
...
In top-level doc, the minimal example echo server uses Control.Exception.bracket like this:
E.bracket (open addr) close loop
  where
    resolve = do
        let hints = defaultHints {
                addrFlags = [AI_PASSIVE]
              , addrSocketType = Stream
              }
        head <$> getAddrInfo (Just hints) mhost (Just port)
    open addr = do
        sock <- socket (addrFamily addr) (addrSocketType addr) (addrProtocol addr)
        setSocketOption sock ReuseAddr 1
        withFdSocket sock setCloseOnExecIfNeeded
        bind sock $ addrAddress addr
        listen sock 1024
        return sock
    loop sock = forever $ do
        (conn, _peer) <- accept sock
        void $ forkFinally (server conn) (const $ gracefulClose conn 5000)
I happened to copy the configuration with another machine's IP address to run on my dev machine, then of course
it failed binding to the IP, but in this case I suspect sock above is leaked without close, as open failed at
all, so sock is not given to bracket for it to do the cleanup.
I think you are right. If 'bind' throws an exception then 'close' will not 
be called. You might move the block from 'setSocketOption' to 'listen' 
into 'loop' before 'forever'.

Re: [Haskell-cafe] network's example echo server seems leaking socket on error binding

Henning Thielemann