port transferkeys to windows; make stopping in progress transfers work too (probably)

transferkeys had used special FDs for communication, but that would be
quite annoying to do in Windows.

Instead, use stdin and stdout. But, to avoid commands like rsync stomping
on them and messing up the communications channel, they're duplicated to a
different handle; stdin is replaced with a null handle, and stdout is
replaced with a copy of stderr. This should all work in windows too.

Stopping in progress transfers may work on windows.. if the types unify
anyway. ;) May need some more porting.

Assistant/TransferrerPool.hs

@@ -5,8 +5,6 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-{-# LANGUAGE CPP #-}
-
 module Assistant.TransferrerPool where
 
 import Assistant.Common
@@ -14,12 +12,10 @@ import Assistant.Types.TransferrerPool
 import Logs.Transfer
 import Utility.Batch
 
-#ifndef mingw32_HOST_OS
 import qualified Command.TransferKeys as T
-#endif
 
 import Control.Concurrent.STM
-import System.Process (create_group)
+import System.Process (create_group, std_in, std_out)
 import Control.Exception (throw)
 import Control.Concurrent
 
@@ -44,46 +40,30 @@ withTransferrer program batchmaker pool a = do
  - finish. -}
 performTransfer :: Transferrer -> Transfer -> AssociatedFile -> IO Bool
 performTransfer transferrer t f = catchBoolIO $ do
-#ifndef mingw32_HOST_OS
 	T.sendRequest t f (transferrerWrite transferrer)
 	T.readResponse (transferrerRead transferrer)
-#else
-	error "TODO performTransfer not implemented on Windows"
-#endif
 
-{- Starts a new git-annex transferkeys process, setting up a pipe
+{- Starts a new git-annex transferkeys process, setting up handles
  - that will be used to communicate with it. -}
 mkTransferrer :: FilePath -> BatchCommandMaker -> IO Transferrer
 mkTransferrer program batchmaker = do
-#ifndef mingw32_HOST_OS
-	(myread, twrite) <- createPipe
-	(tread, mywrite) <- createPipe
-	mapM_ (\fd -> setFdOption fd CloseOnExec True) [myread, mywrite]
-	let params =
-		[ Param "transferkeys"
-		, Param "--readfd", Param $ show tread
-		, Param "--writefd", Param $ show twrite
-		]
 	{- It runs as a batch job. -}
-	let (program', params') = batchmaker (program, params)
+	let (program', params') = batchmaker (program, [Param "transferkeys"])
 	{- It's put into its own group so that the whole group can be
 	 - killed to stop a transfer. -}
-	(_, _, _, pid) <- createProcess (proc program' $ toCommand params')
-		{ create_group = True }
-	closeFd twrite
-	closeFd tread
-	myreadh <- fdToHandle myread
-	mywriteh <- fdToHandle mywrite
-	fileEncoding myreadh
-	fileEncoding mywriteh
+	(Just writeh, Just readh, _, pid) <- createProcess
+		(proc program' $ toCommand params')
+		{ create_group = True
+		, std_in = CreatePipe
+		, std_out = CreatePipe
+		}
+	fileEncoding readh
+	fileEncoding writeh
 	return $ Transferrer
-		{ transferrerRead = myreadh
-		, transferrerWrite = mywriteh
+		{ transferrerRead = readh
+		, transferrerWrite = writeh
 		, transferrerHandle = pid
 		}
-#else
-	error "TODO mkTransferrer not implemented on Windows"
-#endif
 
 {- Checks if a Transferrer is still running. If not, makes a new one. -}
 checkTransferrer :: FilePath -> BatchCommandMaker -> Transferrer -> IO Transferrer