avoid throwing exception when String is not encoded using the filesystem encoding

Since _encodeFilePath generates a String that doesn't use the filesystem
encoding, when this exception is caught, we know we already have such a
String, and can just return it as-is.

Utility/Base64.hs

@@ -31,7 +31,8 @@ fromB64 = fromMaybe bad . fromB64Maybe
 	bad = error "bad base64 encoded data"
 
 -- Only ascii strings are tested, because an arbitrary string may contain
--- characters not encoded using the FileSystemEncoding.
+-- characters not encoded using the FileSystemEncoding, which would thus
+-- not roundtrip, as fromB64 always generates an output encoded that way.
 prop_b64_roundtrips :: String -> Bool
 prop_b64_roundtrips s
 	| all (isAscii) s = s == fromB64 (toB64 s)

Utility/FileSystemEncoding.hs

@@ -35,6 +35,8 @@ import qualified Data.ByteString.Lazy as L
 import qualified Data.ByteString.Lazy.UTF8 as L8
 #endif
 
+import Utility.Exception
+
 {- 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
@@ -70,14 +72,14 @@ withFilePath fp f = Encoding.getFileSystemEncoding
  - effects.
  -
  - If the FilePath contains a value that is not legal in the filesystem
- - encoding, this may throw an exception. For example, "\226" is not valid
+ - encoding, rather than thowing an exception, it will be returned as-is.
- - in the C locale, but is in utf locales.
  -}
 {-# NOINLINE _encodeFilePath #-}
 _encodeFilePath :: FilePath -> String
 _encodeFilePath fp = unsafePerformIO $ do
 	enc <- Encoding.getFileSystemEncoding
-	GHC.withCString enc fp $ GHC.peekCString Encoding.char8
+	GHC.withCString enc fp (GHC.peekCString Encoding.char8)
+		`catchNonAsync` (\_ -> return fp)
 
 {- Encodes a FilePath into a Md5.Str, applying the filesystem encoding. -}
 md5FilePath :: FilePath -> MD5.Str