Re: [Haskell-cafe] Happstack + network package issue on the Mac

Bryan O'Sullivan wrote:

On Fri, Oct 9, 2009 at 7:25 AM, Gregory Collins mailto:greg@gregorycollins.net> wrote:

There's been an open ticket for months; personally I think this is a job for the C preprocessor, but nobody's written a patch yet.

Is there an open ticket against the network package? Can someone write a simple standalone repro for me that doesn't require happstack (i.e. only depends on network)? I have access to a Mac and Johan and I co-maintain the network package, so in principle this shouldn't be hard to fix, given enough details and pointers.

I have the impression that the problem is at least partly with TH on the Mac, and there may not even be any relevant bug in the network package. It's possible that the network package could provide some additional support to help Happstack solve this another way, though. The main problem that Happstack faces here is to avoid referencing the SockAddrInet6 constructor if it doesn't exist, i.e. if the network package was compiled without IPv6 support. If Happstack had some reliable/portable way of accessing IPV6_SOCKET_SUPPORT, a C #define used by the network package, this could easily be done with the C preprocessor. I don't think Cabal is up to that task, though. Happstack would probably have to use its own configure script (it doesn't have one right now) to determine IPv6 status for itself, and hope that it matches what the network package found when it was compiled. The current solution in Happstack is to use TH to detect IPv6 support in the network package, by testing for the existence of the SockAddrInet6 constructor, as follows (reformatted slightly for email): -- find out at compile time if the SockAddr6 / HostAddress6 constructors -- are available supportsIPv6 :: Bool supportsIPv6 = $(let c = "Network.Socket.SockAddrInet6" d = ''SockAddr in do TyConI (DataD _ _ _ cs _) <- reify d if isJust (find (\(NormalC n _) -> show n == c) cs) then [| True |] else [| False |] ) If I had access to a Mac, the first thing I'd do is check what supportsIPv6 is getting set to, and whether it changes depending on the version of the network package being used. However, some comments many months back seemed to indicate that supportsIPv6 might be getting set correctly, and that the problem was happening later. In that case, it would be in Happstack/Server/HTTP/Socket.hs, which contains this test: let peer = $(if supportsIPv6 If supportsIPv6 is False, or somehow incorrectly treated as False, then the error message being reported could happen if the provided socket is actually IPv6. Alternatively, if the True branch is taken, then a TH-encoded case expression is used to match the SockAddr value, without having to reference the SockAddrInet6 constructor directly in code. But if this the part that executes, then for some mysterious reason, it is failing, only on the Mac, to match the appropriate constructor. That sounds like a fascinating bug! I hope someone with a Mac will track it down and let us know what was happening... Anton