Installing the Haskell Network library on Windows

Summary: This post describes how to install the Haskell network library on Windows, again.

I recently bought a new computer, and tried to install GHC 8.0.1 then upgrade the network library using Cabal. As I have come to expect, it didn't work. Using Git Bash, I got the error:

$ cabal install network-2.6.3.1
Resolving dependencies...
Configuring network-2.6.3.1...
Failed to install network-2.6.3.1
Build log ( C:\Users\Neil\AppData\Roaming\cabal\logs\network-2.6.3.1.log ):
Configuring network-2.6.3.1...
configure: WARNING: unrecognized options: --with-compiler
checking for gcc... C:\ghc\GHC-80~1.1┼║
checking whether the C compiler works... no
configure: error: in `C:/Neil':
configure: error: C compiler cannot create executables
See `config.log' for more details
cabal: Leaving directory '.'
cabal.exe: Error: some packages failed to install:
old-time-1.1.0.3 failed during the configure step. The exception was:
ExitFailure 77

Running -v3 shows the CC variable is being set to C:\ghc\GHC-80~1.1┼║, which looks like a buffer corruption or encoding issue. I tried my previous solution, but it didn't work. My new solution is:

$ cabal unpack network-2.6.3.1
$ cd network-2.6.3.1
$ cabal configure
... fails with a similar error to above ...
$ sh ./configure
$ cabal build
$ cabal copy
$ cabal register

I had to repeat the same pattern for the latest version of old-time, and the same pattern worked.

Another way that works is to use Stack.

Making withSocketsDo unnecessary

Summary: Currently you have to call withSocketsDo before using the Haskell network library. In the next version you won't have to.

The Haskell network library has always had a weird and unpleasant invariant. Under Windows, you must call withSocketsDo before calling any other functions. If you forget, the error message isn't particularly illuminating (e.g. getAddrInfo, does not exist, error 10093). Calling withSocketsDo isn't harmful under Linux, but equally isn't necessary, and thus easy to accidentally omit. The network library has recently merged some patches so that in future versions there is no requirement to call withSocketsDo, even on Windows.

Existing versions of network

The reason for requiring withSocketsDo is so that the network library can initialise the Windows Winsock library. The code for withSocketsDo was approximately:

withSocketsDo :: IO a -> IO a
#if WINDOWS
withSocketsDo act = do
    initWinsock
    act `finally` termWinsock
#else
withSocketsDo act = act
#endif

Where initWinsock and termWinsock were C functions. Both checked a mutable variable so they only initialised/terminated once. The initWinsock function immediately initialised the Winsock library. The termWinsock function did not terminate the library, but merely installed an atexit handler, providing a function that ran when the program shut down which terminated the Winsock library.

As a result, in all existing versions of the network library, it is fine to nest calls to withSocketsDo, call withSocketsDo multiple times, and to perform networking operations after withSocketsDo has returned.

Future versions of network

My approach to removing the requirement to call withSocketsDo was to make it very cheap, then sprinkle it everywhere it might be needed. Making such a function cheap on non-Windows just required an INLINE pragma (although its very likely GHC would have always inlined the function anyway).

For Windows, I changed to:

withSocketsDo act = do evaluate withSocketsInit; act 

{-# NOINLINE withSocketsInit #-}
withSocketsInit = unsafePerformIO $ do
    initWinsock
    termWinsock

Now withSocketsDo is very cheap, with subsequent calls requiring no FFI calls, and thanks to pointer tagging, just a few cheap instructions. When placing additional withSocketsDo calls my strategy was to make sure I called it before constructing a Socket (which many functions take as an argument), and when taking one of the central locks required for the network library. In addition, I identified a few places not otherwise covered.

In newer versions of the network library it is probably never necessary to call withSocketsDo - if you find a place where one is necessary, let me know. However, for compatibility with older versions on Windows, it is good practice to always call withSocketsDo. Libraries making use of the network library should probably call withSocketsDo on their users behalf.