Re: [Haskell-cafe] Fwd: Safe TCP accept loop

My apologies I've only just managed to get back to this. I've used your method described above, thanks for the clear explanation Now am unable to terminate the async thread that is running the accept call. Seems to me that Warp relies on the termination of the main thread to terminate the accept loop - is this correct? On 25 October 2017 at 17:30, Ömer Sinan Ağacan wrote:

Your pseudo code doesn't look right although I couldn't completely understand it. You need something like this:

{-# LANGUAGE ScopedTypeVariables #-}

import Network.Socket import Control.Concurrent import Control.Exception

acceptLoop :: Socket -> IO () acceptLoop sock = mask_ loop where loop = do -- only safe point in the loop for exceptions allowInterrupt

(connected_sock, _) <- accept sock -- use forkIOWithUnmask: we want the thread to be interruptable no matter -- what the inherited masking state is _thr_id <- forkIOWithUnmask (handle_conn connected_sock)

loop

handle_conn connected_sock unmask = -- register cleanup action, run the handler in interruptable state to be -- able to kill the thread. catch (unmask (handler connected_sock)) (\(_exc :: SomeException) -> close connected_sock)

handler connected_sock = -- fill here return ()

Ömer

2017-10-25 8:52 GMT+03:00 Sumit Raja :

...

Hi Ömer

...

You need to mask async exceptions between `accept()` and cleanup action registration, because an exception in between these operations will cause the socket to leak.

You can take a look at warp's accept loop:

https://github.com/yesodweb/wai/blob/master/warp/Network/Wai/Handler/Warp/Ru...

Trying to map steps for the code you've pointed me to in bad pseudo code:

finally (mask >> acceptLoop serverSocket) (close serverSocket)

acceptLoop = unmask sock <- accept serverSock mask forkIO $ do mask finally (unmask >> process sock) (close sock) acceptLoop

Is this correct?

Thanks Sumit