network-3.2.8 is broken on Windows: fails with WSAENOPROTOOPT due to DontFragment socket option

[harendra-kumar]

We are seeing this error in the CI:

In the test streamly/test/Streamly/Test/Network/Socket.hs (https://ci.appveyor.com/project/harendra-kumar/streamly/builds/53663341):

Network.Socket
    Read/Write
  Network.Socket.exe: Network.Socket.setSockOpt: failed (Bad protocol option (WSAENOPROTOOPT))
  HasCallStack backtrace:
    collectBacktraces, called at libraries\ghc-internal\src\GHC\Internal\Exception.hs:169:13 in ghc-internal:GHC.Internal.Exception
    toExceptionWithBacktrace, called at libraries\ghc-internal\src\GHC\Internal\IO.hs:260:11 in ghc-internal:GHC.Internal.IO
    throwIO, called at libraries\ghc-internal\src\GHC\Internal\Control\Exception\Base.hs:195:43 in ghc-internal:GHC.Internal.Control.Exception.Base . The relevant

We are not setting any socket option in the test, so it has to be some code inside network package.

I think the culprit is this change #603 :

Version 3.2.8.0

    sockopt: add IP_DONTFRAG/IP_MTU_DISCOVER option. https://github.com/haskell/network/pull/603

The following code is unguarded for windows:

    setDontFragment s = when (family == AF_INET) $
#if HAVE_DECL_IP_DONTFRAG || HAVE_DECL_IP_MTU_DISCOVER
      setSocketOption s DontFragment 1
#else
      -- do nothing
      return ()
#endif

What I see in the configure step in the failing build:

configure output in failing build:

[00:06:49] checking whether IP_DONTFRAG is declared... no
[00:06:50] checking whether IP_MTU_DISCOVER is declared... yes

IP_MTU_DISCOVER is causing the DontFragment option to be set which at runtime the OS is not allowing. A potential fix could be:

#if HAVE_DECL_IP_DONTFRAG
    setSocketOption s DontFragment 1
#elif HAVE_DECL_IP_MTU_DISCOVER && !defined(mingw32_HOST_OS)
    setSocketOption s DontFragment 1
#endif

[kazu-yamamoto]

@harendra-kumar Thank you for this report. Could you send a PR by yourself?
Cc: @thehajime

[harendra-kumar]

If anyone else can't take it up I will try in some time.