[Haskell-cafe] warp and http-conduit on concurrent threads on windows

28 Mar 2013

      Hi,

I'd like to know what is wrong with the following program on windows8 
(GHC 7.4.2, 32bit):

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}

module Main where

import Control.Concurrent.Async
import qualified Control.Exception as E
import Network.HTTP.Conduit
import Network.HTTP.Types
import Network.Wai
import Network.Wai.Handler.Warp

query port = E.catch
     (simpleHttp ("http://haskell.org:" ++ show port) >>= print . take 
10 . show)
     (\(e :: HttpException) -> print $ "caught: " ++ show e)

listen = run 8080 $ \_ ->
     return $ responseLBS ok200 [] "abc"

main = do
     withAsync (query 12345) $ \a -> do
     withAsync listen $ \b -> do
     wait a
     wait b

I compile the program with "ghc --make -threaded Main.hs" and run it as 
"./Main +RTS -N".

On POSIX systems this works as expected. Even if the failing "query" 
runs in a forever loop the "listen" thread responds promptly to 
requests. On windows the "listen" thread seems blocked by the failing 
"query" thread. Sometimes the query returns (relatively) prompt. But 
sometimes (about a third of all runs) it takes very long (about 20 sec). 
Also, sometimes it returns with "Connection timed out (WSAETIMEDOUT)", 
sometimes with "getAddrInfo: does not exist (error 11003)", and 
sometimes just with "FailedConnectionException".

The fact that the "listen" thread is blocked seems to contradict the 
following quote form the documentation of Control.Concurrent:

-- Quote from Control.Concurrent --
Using forkOS instead of forkIO makes no difference at all to the 
scheduling behaviour of the Haskell runtime system. It is a common 
misconception that you need to use forkOS instead of forkIO to avoid 
blocking all the Haskell threads when making a foreign call; this isn't 
the case. To allow foreign calls to be made without blocking all the 
Haskell threads (with GHC), it is only necessary to use the -threaded 
option when linking your program, and to make sure the foreign import is 
not marked unsafe.
-- End Quote --

By the way: using withAsyncBound instead of withAsync seems to improve 
(but not completely solve) the issue.

Thanks,
Lars

[Haskell-cafe] warp and http-conduit on concurrent threads on windows

Lars Kuhtz