[commit: ghc] master: base: Make it less likely for fdReady() to fail on Windows sockets. (ba4dcc7)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/ba4dcc7cb77a37486368911fec854d112a1db93c/ghc

>---------------------------------------------------------------

commit ba4dcc7cb77a37486368911fec854d112a1db93c
Author: Niklas Hambüchen <mail at nh2.me>
Date:   Tue Sep 19 15:12:56 2017 -0400

    base: Make it less likely for fdReady() to fail on Windows sockets.
    
    See the added comment for details.
    
    It's "less likely" because it can still fail if the socket happens to
    have an FD larger than 1023, which can happen if many files are opened.
    
    Until now, basic socket programs that use `hWaitForInput` were 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 the following (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?
    
    The workaround for now is to increase `FD_SETSIZE` on Windows;
    increasing it is possible on Windows and BSD, see
    
    https://stackoverflow.com/questions/7976388/increasing-limit-of-fd-setsi
    ze-and-select
    
    A real fix would be to move to IO Completion Ports on Windows, and thus
    get rid of the last uses of `select()` (the other platforms already use
    `poll()` but Windows doesn't have that).
    
    Reviewers: bgamari, austin, hvr, erikd, simonmar
    
    Reviewed By: bgamari
    
    Subscribers: rwbarton, thomie
    
    Differential Revision: https://phabricator.haskell.org/D3959


>---------------------------------------------------------------

ba4dcc7cb77a37486368911fec854d112a1db93c
 libraries/base/cbits/inputReady.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/libraries/base/cbits/inputReady.c b/libraries/base/cbits/inputReady.c
index ddeee66..916633c 100644
--- a/libraries/base/cbits/inputReady.c
+++ b/libraries/base/cbits/inputReady.c
@@ -4,6 +4,22 @@
  * hWaitForInput Runtime Support
  */
 
+/* FD_SETSIZE defaults to 64 on Windows, which makes even the most basic
+ * programs break that use select() on a socket FD.
+ * Thus we raise it here (before any #include of network-related headers)
+ * to 1024 so that at least those programs would work that would work on
+ * Linux if that used select() (luckily it uses poll() by now).
+ * See https://ghc.haskell.org/trac/ghc/ticket/13497#comment:23
+ * The real solution would be to remove all uses of select()
+ * on Windows, too, and use IO Completion Ports instead.
+ * Note that on Windows, one can simply define FD_SETSIZE to the desired
+ * size before including Winsock2.h, as described here:
+ *   https://msdn.microsoft.com/en-us/library/windows/desktop/ms740141(v=vs.85).aspx
+ */
+#if defined(_WIN32)
+#define FD_SETSIZE 1024
+#endif
+
 /* select and supporting types is not Posix */
 /* #include "PosixSource.h" */
 #include "HsBase.h"