Add a test for rawSystemStdInOut exception handling

Not all the exception handling, just for the case fixed by the previous
patch.

Turns out of course that this is sensitive to the locale encoding, so we
just skip the test when the localeEncoding is not UTF-8.

Cabal/tests/UnitTests/Distribution/Simple/Utils.hs

@@ -9,7 +9,9 @@ import Data.IORef
 import System.Directory ( doesDirectoryExist, doesFileExist
                         , getTemporaryDirectory
                         , removeDirectoryRecursive, removeFile )
-import System.IO (hClose)
+import System.IO (hClose, localeEncoding)
+import System.IO.Error
+import qualified Control.Exception as Exception
 
 import Test.Tasty
 import Test.Tasty.HUnit
@@ -46,6 +48,24 @@ withTempDirRemovedTest = do
   withTempDirectory normal tempDir "foo" $ \dirPath -> do
     removeDirectoryRecursive dirPath
 
+rawSystemStdInOutTextDecodingTest :: Assertion
+rawSystemStdInOutTextDecodingTest
+    -- We can only get this exception when the locale encoding is UTF-8
+    -- so skip the test if it's not.
+  | show localeEncoding /= "UTF-8" = return ()
+  | otherwise = do
+  res <- Exception.try $
+    rawSystemStdInOut normal
+      -- hopefully this is sufficiently portable, we just need to execute a
+      -- program that will produce non-unicode output:
+      "ghc" ["-e", "Data.ByteString.putStr (Data.ByteString.pack [255])"]
+      Nothing Nothing Nothing
+      False -- not binary mode output, ie utf8 text mode so try to decode
+  case res of
+    Right _ -> assertFailure "expected IO decoding exception"
+    Left err | isDoesNotExistError err -> Exception.throwIO err -- no ghc!
+             | otherwise               -> return ()
+
 tests :: [TestTree]
 tests =
     [ testCase "withTempFile works as expected" $
@@ -56,4 +76,6 @@ tests =
       withTempDirTest
     , testCase "withTempDirectory can handle removed directories" $
       withTempDirRemovedTest
+    , testCase "rawSystemStdInOut reports text decoding errors" $
+      rawSystemStdInOutTextDecodingTest
     ]