#13903: KQueue evtmgr backend fails to register for write events -------------------------------------+------------------------------------- Reporter: waldheinz | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Runtime | Version: 8.0.2 System | Keywords: | Operating System: FreeBSD Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The root of the problem is that the `GHC.Event.KQueue.toFilter` function has type `GHC.Event.Internal.Event -> Filter` with GHC's `Event` being a bitmask which can represent read events, write events or a combination of those. It happens that the event manager requests it's backend to be notified about read ''and'' write events on some fd, and because the kqueue `EVFILT_*`s are ''not'' bitmasks, the above function cannot capture this, silently dropping the desire to be notified about write events. The following program triggers the problematic behaviour: {{{ import Control.Concurrent ( forkIO, killThread ) import Control.Exception ( finally ) import Control.Monad.Trans.Resource ( runResourceT ) import Control.Monad.IO.Class ( liftIO ) import qualified Data.ByteString as BS import Data.Conduit ( ($$), ($=) ) import qualified Data.Conduit.Binary as CB import qualified Data.Conduit.List as CL import qualified Data.Conduit.Network as CN import Data.IORef ( newIORef, modifyIORef', readIORef) main :: IO () main = CN.runTCPClient (CN.clientSettings 5555 "192.168.2.11") client where logMsg cnt = CL.mapM $ \bs -> liftIO $ do modifyIORef' cnt (+ 1) readIORef cnt >>= \x -> putStrLn $ "msg #" ++ show x ++ " of size: " ++ show (BS.length bs) return bs client ad = do reader <- forkIO (runResourceT $ CN.appSource ad $$ CL.mapM_ ( \bs -> (liftIO . putStrLn) $ "read " ++ show (BS.length bs) ++ " bytes")) cnt <- newIORef ( 0 :: Int ) let runPipe = runResourceT $ CB.sourceFile "cool-linux-distro.iso" $$ logMsg cnt $= CN.appSink ad runPipe `finally` (killThread reader) }}} Having a `nc -l -p 5555 > /dev/null` running on another machine is sufficient to sink the data. Assuming that we can read `bigfile.iso` faster than we can send out over the socket, the `send` syscall will at some point give an `EAGAIN` as can be seen in the `truss` output: {{{ write(1,"msg #20 of size: 32752\n",23) = 23 (0x17) sendto(12,"\f\2409\0\M^RA\^T\M-&A\M-'\M-d8"...,32752,0x0,NULL,0x0) = 32752 (0x7ff0) read(13,"\M^?\0'\\\M-B\M-:+\^]D\M-0\M-="...,32752) = 32752 (0x7ff0) poll({ 1/POLLOUT },1,0) = 1 (0x1) msg #21 of size: 32752 write(1,"msg #21 of size: 32752\n",23) = 23 (0x17) sendto(12,"\M^?\0'\\\M-B\M-:+\^]D\M-0\M-="...,32752,0x0,NULL,0x0) = 19204 (0x4b04) sendto(12,"\M-j$2\M^BH\M-#-\^A\M-E\^O\M^Y\a"...,13548,0x0,NULL,0x0) ERR#35 'Resource temporarily unavailable' SIGNAL 26 (SIGVTALRM) sigprocmask(SIG_SETMASK,{ SIGVTALRM },0x0) = 0 (0x0) sigreturn(0x7fffffff9c60) ERR#35 'Resource temporarily unavailable' kevent(3,{ 12,EVFILT_READ,EV_ADD|EV_ONESHOT,0x0,0x0,0x0 },1,0x0,0,{ 0.000000000 }) = 0 (0x0) _umtx_op(0x800c71ed0,UMTX_OP_WAIT_UINT_PRIVATE,0x0,0x0,0x0) ERR#4 'Interrupted system call' SIGNAL 26 (SIGVTALRM) sigprocmask(SIG_SETMASK,{ SIGVTALRM },0x0) = 0 (0x0) sigreturn(0x7fffffffdc00) ERR#4 'Interrupted system call' _umtx_op(0x800c71ed0,UMTX_OP_WAIT_UINT_PRIVATE,0x0,0x0,0x0) ERR#4 'Interrupted system call' SIGNAL 26 (SIGVTALRM) }}} Not the `sendto` call on fd 12 resulting in `ERR#35`, soon followed by an `kevent` call for that same fd and only `EVFILT_READ` set. This makes little sense as it was an attempt to ''write'' that just failed. This is caused by `toFilter` giving precedence to `read` events, dropping the write event. Not starting the `reader` thread prevents bad things from happening as then the `write` events are properly passed thru to kqueue. I have an initial version of a patch fixing this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13903 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler