Re: [GHC] #13497: GHC does not use select()/poll() correctly on non-Linux platforms

30 Jul 2017

      #13497: GHC does not use select()/poll() correctly on non-Linux platforms
-------------------------------------+-------------------------------------
        Reporter:  nh2               |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Runtime System    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  8684
 Related Tickets:  #8684, #12912     |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by nh2):

 When testing my change on Windows, I found that the `if (isSock){
 select(...) }` part seems completely broken on Windows.

 That is because on Windows `FD_SETSIZE` defaults to 64, but pretty much
 all GHC programs seem to have > 64 FDs open, so you can't actually create
 a socket on which you can `select()`.

 It errors with `fdReady: fd is too big` even with an example as simple as
 [https://github.com/nh2/ghc-socket-
 test/blob/af74bb348e88f3f7ca717c70540936fea1293257/ghc-socket-test.hs
 this] (in this case, on my machine the `fd` is `284`):

 {{{
 {-# LANGUAGE OverloadedStrings #-}

 import Control.Monad (forever)
 import Network.Socket
 import System.IO

 -- Simple echo server: Reads up to 10 chars from network, echoes them
 back.
 -- Uses the Handle API so that `hWaitForInput` can be used.
 main :: IO ()
 main = do
   sock <- socket AF_INET Stream 0
   setSocketOption sock ReuseAddr 1
   bind sock (SockAddrInet 1234 0x0100007f) -- 0x0100007f == 127.0.0.1
 localhost
   listen sock 2
   forever $ do
     (connSock, _connAddr) <- accept sock
     putStrLn "Got connection"

     h <- socketToHandle connSock ReadWriteMode
     hSetBuffering h NoBuffering

     ready <- hWaitForInput h (5 * 1000) -- 5 seconds
     putStrLn $ "Ready: " ++ show ready

     line <- hGetLine h
     putStrLn "Got line"
     hPutStrLn h ("Got: " ++ line)
     hClose h
 }}}

 I'm not sure how this was not discovered earlier; for #13525 (where
 `fdReady()` breaking completely was also discovered late) at least it
 failed only when the timeout was non-zero, which is not used in ghc beyond
 in `hWaitForInput`, but in this Windows socket case it breaks even on the
 0-timeout.

 Maybe there is not actually anybody who uses sockets as handles on
 Windows?

 It seems an approriate workaround for now is to increase `FD_SETSIZE`
 (which is possible on Windows and BSD, see
 [https://stackoverflow.com/questions/7976388/increasing-limit-of-fd-
 setsize-and-select here]) on Windows.

 A real fix would be to move to IO Completion Ports, and thus get rid of
 the last use of `select()` (the other platforms already use `poll()` but
 Windows doesn't have that).

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13497#comment:23
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