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

[Henning Thielemann]

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'.