factor out Utility.FileSystemEncoding

Utility/FileSystemEncoding.hs

@@ -1,4 +1,4 @@
-{- File system encoding handling.
+{- GHC File system encoding handling.
  -
  - Copyright 2012 Joey Hess <joey@kitenet.net>
  -
@@ -7,8 +7,17 @@
 
 module Utility.FileSystemEncoding where
 
-import GHC.IO.Encoding (getFileSystemEncoding)
+import System.IO
+import Foreign.C
 import GHC.Foreign as GHC
+import GHC.IO.Encoding
+
+{- Sets a Handle to use the filesystem encoding. This causes data
+ - written or read from it to be encoded/decoded the same
+ - as ghc 7.4 does to filenames etc. This special encoding
+ - allows "arbitrary undecodable bytes to be round-tripped through it". -}
+fileEncoding :: Handle -> IO ()
+fileEncoding h = hSetEncoding h =<< getFileSystemEncoding
 
 {- Marshal a Haskell FilePath into a NUL terminated C string using temporary
  - storage. The FilePath is encoded using the filesystem encoding,
@@ -16,3 +25,13 @@ import GHC.Foreign as GHC
  - was obtained. -}
 withFilePath :: FilePath -> (CString -> IO a) -> IO a
 withFilePath fp f = getFileSystemEncoding >>= \enc -> GHC.withCString enc fp f
+
+{- Encodes a FilePath into a String of encoded bytes, applying the
+ - filesystem encoding.
+ -
+ - This does not do any IO, beyond allocating a C buffer. GHC does not
+ - seem to provide a pure way to do this conversion. -}
+encodeFilePath :: FilePath -> IO String
+encodeFilePath fp = do
+	enc <- getFileSystemEncoding
+	GHC.withCString enc fp $ GHC.peekCString enc