[Haskell-cafe] network package and SIGVTALRM

[Ruben Astudillo]

Hi all

I am doing a DCC subsystem on a irc client. After all the handshakes are
done I just connect to the server and start `recv`. The code I use for
this is:

     getPackets :: MVar Int
                -> FilePath -- ^ Name media
                -> Int      -- ^ File size
                -> AddrInfo -> ExceptT DCCError IO ()
     getPackets mvar name totalSize addr =
       do receivedSize <- lift $ bracket acquire release receive
          let delta = (totalSize - receivedSize)
          if delta > 0 then throwE (NotFullRecv delta) else return ()
       where
         bufferSize = 16384

         acquire :: IO (IO.Handle,Socket)
         acquire = (,) <$> (IO.openFile name IO.WriteMode)
                       <*> newSocket addr

         release :: (IO.Handle,Socket) -> IO ()
         release (hdl, sock) = IO.hClose hdl >> close sock

         receive :: (IO.Handle,Socket) -> IO Int
         receive (hdl, sock) =
             flip execStateT 0 . fix $ \loop -> do
                 mediaData <- lift (B.recv sock bufferSize)
                 unless (B.null mediaData) $ do
                     S.modify' (+ (B.length mediaData))
                     currentSize <- S.get
                     lift $ B.hPut hdl mediaData
                            >> B.send sock (int2BS currentSize)
                            >> swapMVar mvar currentSize
                     loop

Part of the protocol is that on each `recv` I send the current received
size on network byte order. Hence the B.send line on receive I use this
function:

     -- | given a number forms a bytestring with each digit on a separated
     -- Word8 in network byte-order
     int2BS :: Int -> B.ByteString
     int2BS i | w <- (fromIntegral i :: Word32) =
         B.pack [ (fromIntegral (shiftR w 24) :: Word8)
               , (fromIntegral (shiftR w 16) :: Word8)
               , (fromIntegral (shiftR w  8) :: Word8)
               , (fromIntegral w             :: Word8)]

Everything works correctly until around 1/2 of a test transfer (ie in a
file of 340M it gets 170). That first half is gotten in the right order
(I tested with a video and it was playable until the middle). On tinier
files the bug doesn't happen, the file is received completly. I did a
little bit of `strace` and `tcpdump` and I got this

-- strace -e trace=network -p $client
     (..)
     30439 recvfrom(13, 
"\312\255\201\337\376\355\253\r\177\276\204X]8\6\221\301#\361<>\273+\355\5\343B 
\333\366\351W"..., 16384, 0, NULL, NULL) = 1380
     30439 sendto(13, "\n\273\31l", 4, 0, NULL, 0) = 4
     30439 recvfrom(13, 0x20023f010, 16384, 0, NULL, NULL) = -1 EAGAIN 
(Resource temporarily unavailable)
     30439 recvfrom(13, 
"\222llq_H\23\17\275\f}\367\"P4\23\207\312$w\371J\354aW2\243R\32\v\n\251"..., 
16384, 0, NULL, NULL) = 1380
     30439 sendto(13, "\n\273\36\320", 4, 0, NULL, 0) = -1 EAGAIN (Resource 
temporarily unavailable)
     30438 --- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, 
si_timerid=0, si_overrun=0, si_value={int=0, ptr=0}} ---
     30438 --- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, 
si_timerid=0, si_overrun=0, si_value={int=0, ptr=0}} ---
     (..)

-- tcpdump
     05:20:22.788273 IP tapioca.36346 > 198.255.92.74.36103: Flags [.], ack 
48004680, win 489, options [nop,nop,TS val 627805332 ecr 
675358947,nop,nop,sack 1 {48006060:48066780}], length 0
     05:20:22.975627 IP 198.255.92.74.36103 > tapioca.36346: Flags [.], seq 
48004680:48006060, ack 82629, win 0, options [nop,nop,TS val 675359033 ecr 
627805248], length 1380
     05:20:23.014991 IP tapioca.36346 > 198.255.92.74.36103: Flags [.], ack 
48066780, win 4, options [nop,nop,TS val 627805559 ecr 675359033], length 0
     05:20:23.768012 IP 198.255.92.74.36103 > tapioca.36346: Flags [P.], 
seq 48066780:48067292, ack 82629, win 0, options [nop,nop,TS val 675359232 
ecr 627805559], length 512
     05:20:23.768143 IP tapioca.36346 > 198.255.92.74.36103: Flags [.], ack 
48067292, win 0, options [nop,nop,TS val 627806312 ecr 675359232], length 0
     05:20:24.523397 IP 198.255.92.74.36103 > tapioca.36346: Flags [.], ack 
82629, win 0, options [nop,nop,TS val 675359421 ecr 627806312], length 0

What bothers me is that SIGVTALRM on the strace output. I am not the
greatest unix hacker but that signal is related to settimer and I haven't
explicitly set that up. So I am scratching me head a little. Maybe
somebody has experienced something related with the network package? Do
you notice something on the logs? thanks in advance.

--
-- Ruben Astudillo