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
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