Fix corrupted config file header for non-ASCII package names

The config-state header is a human readable line prepended to the
binary serialisation which looks like

    Saved package config for pkgname-1.2.3 written by Cabal-2.5.0.0 using ghc-8.6

However, the functions generating and parsing this header didn't take into
account that package names are not limited to the ASCII subset and blindly used
the ByteString `pack` function which truncates away the high bits of the `Char`
code point resulting in a corrupted header with a non-sensical package-name.

The fix is simply to serialise the package-name with the UTF-8 encoding which
works nicely with the rest of the UTF-8 unaware string handling functions.
Hence the fix is a lot shorter than this commit message.

Fixes haskell#2557

Cabal/ChangeLog.md

@@ -18,6 +18,8 @@
       supports fully static linking;
       [`glibc` has some issues](https://sourceware.org/glibc/wiki/FAQ#Even_statically_linked_programs_need_some_shared_libraries_which_is_not_acceptable_for_me.__What_can_I_do.3F)
       with fully static linking.
+  * Fix corrupted config file header for non-ASCII package names
+    ([2557](https://github.com/haskell/cabal/issues/2557)).
 
 ----
 

Cabal/Distribution/Simple/Configure.hs

@@ -275,7 +275,7 @@ parseHeader header = case BLC8.words header of
   ["Saved", "package", "config", "for", pkgId, "written", "by", cabalId,
    "using", compId] ->
       fromMaybe (throw ConfigStateFileBadHeader) $ do
-          _ <- simpleParsec (BLC8.unpack pkgId) :: Maybe PackageIdentifier
+          _ <- simpleParsec (fromUTF8LBS pkgId) :: Maybe PackageIdentifier
           cabalId' <- simpleParsec (BLC8.unpack cabalId)
           compId' <- simpleParsec (BLC8.unpack compId)
           return (cabalId', compId')
@@ -286,7 +286,7 @@ showHeader :: PackageIdentifier -- ^ The processed package.
             -> ByteString
 showHeader pkgId = BLC8.unwords
     [ "Saved", "package", "config", "for"
-    , BLC8.pack $ prettyShow pkgId
+    , toUTF8LBS $ prettyShow pkgId
     , "written", "by"
     , BLC8.pack $ prettyShow currentCabalId
     , "using"