[Haskell-cafe] Fwd: Safe TCP accept loop

[Sumit Raja]

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 <omeragacan at gmail.com> 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 <sumitraja at gmail.com>:
>> 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/Run.hs#L211
>>>
>>
>> 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