Fix self-contained handling of ASCII encoding

D898 was primarily intended to fix hangs in the event that iconv was
unavailable (namely #10298 and #7695). In addition to this fix, it also
introduced self-contained handling of ANSI terminals to allow compiled
executables to run in minimal environments lacking iconv.

However, the behavior that the patch introduced is highly suspicious.
Specifically, it gives the user a UTF-8 encoding even if they requested
ASCII.

This has the potential to break quite a lot of code. At very least it
breaks GHC's Unicode terminal detection logic, which attempts to catch
an invalid character when encoding a pair of smart-quotes. Of course,
this exception will never be thrown if a UTF-8 encoder is used.

Here we use the `char8` encoding to handle requests for ASCII encodings
in the event that we find iconv to be non-functional.

Fixes #10623.

Test Plan: Validate with T8959a

Reviewers: rwbarton, hvr, austin, hsyl20

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1059

GHC Trac Issues: #10623

3 files changed, 49 insertions, 18 deletions

diff --git a/libraries/base/GHC/IO/Encoding.hs b/libraries/base/GHC/IO/Encoding.hs
index 014b61b8b0..76c7f55e02 100644
--- a/libraries/base/GHC/IO/Encoding.hs
+++ b/libraries/base/GHC/IO/Encoding.hs
@@ -235,14 +235,9 @@ mkTextEncoding e = case mb_coding_failure_mode of
         _             -> Nothing
 
 mkTextEncoding' :: CodingFailureMode -> String -> IO TextEncoding
-mkTextEncoding' cfm enc
-  -- First, specifically match on ASCII encodings directly using
-  -- several possible aliases (specified by RFC 1345 & co), which
-  -- allows us to handle ASCII conversions without iconv at all (see
-  -- trac #10298).
-  | any (== enc) ansiEncNames = return (UTF8.mkUTF8 cfm)
-  -- Otherwise, handle other encoding needs via iconv.
-  | otherwise = case [toUpper c | c <- enc, c /= '-'] of
+mkTextEncoding' cfm enc =
+  case [toUpper c | c <- enc, c /= '-'] of
+  -- UTF-8 and friends we can handle ourselves
     "UTF8"    -> return $ UTF8.mkUTF8 cfm
     "UTF16"   -> return $ UTF16.mkUTF16 cfm
     "UTF16LE" -> return $ UTF16.mkUTF16le cfm
@@ -254,13 +249,31 @@ mkTextEncoding' cfm enc
     'C':'P':n | [(cp,"")] <- reads n -> return $ CodePage.mkCodePageEncoding cfm cp
     _ -> unknownEncodingErr (enc ++ codingFailureModeSuffix cfm)
 #else
-    _ -> Iconv.mkIconvEncoding cfm enc
-#endif
+    -- Otherwise, handle other encoding needs via iconv.
+
+    -- Unfortunately there is no good way to determine whether iconv is actually
+    -- functional without telling it to do something.
+    _ -> do res <- Iconv.mkIconvEncoding cfm enc
+            let isAscii = any (== enc) ansiEncNames
+            case res of
+              Just e -> return e
+              -- At this point we know that we can't count on iconv to work
+              -- (see, for instance, Trac #10298). However, we still want to do
+              --  what we can to work with what we have. For instance, ASCII is
+              -- easy. We match on ASCII encodings directly using several
+              -- possible aliases (specified by RFC 1345 & Co) and for this use
+              -- the 'char8' encoding
+              Nothing
+                | isAscii   -> return char8
+                | otherwise ->
+                    unknownEncodingErr (enc ++ codingFailureModeSuffix cfm)
   where
     ansiEncNames = -- ASCII aliases
       [ "ANSI_X3.4-1968", "iso-ir-6", "ANSI_X3.4-1986", "ISO_646.irv:1991"
       , "US-ASCII", "us", "IBM367", "cp367", "csASCII", "ASCII", "ISO646-US"
       ]
+#endif
+
 
 latin1_encode :: CharBuffer -> Buffer Word8 -> IO (CharBuffer, Buffer Word8)
 latin1_encode input output = fmap (\(_why,input',output') -> (input',output')) $ Latin1.latin1_encode input output -- unchecked, used for char8
diff --git a/libraries/base/GHC/IO/Encoding/Iconv.hs b/libraries/base/GHC/IO/Encoding/Iconv.hs
index 89ca71e084..061bd60c20 100644
--- a/libraries/base/GHC/IO/Encoding/Iconv.hs
+++ b/libraries/base/GHC/IO/Encoding/Iconv.hs
@@ -34,9 +34,10 @@ import GHC.Base () -- For build ordering
 #else
 
 import Foreign
-import Foreign.C
+import Foreign.C hiding (charIsRepresentable)
 import Data.Maybe
 import GHC.Base
+import GHC.Foreign (charIsRepresentable)
 import GHC.IO.Buffer
 import GHC.IO.Encoding.Failure
 import GHC.IO.Encoding.Types
@@ -96,15 +97,27 @@ char_shift :: Int
 char_shift | charSize == 2 = 1
            | otherwise     = 2
 
-iconvEncoding :: String -> IO TextEncoding
+iconvEncoding :: String -> IO (Maybe TextEncoding)
 iconvEncoding = mkIconvEncoding ErrorOnCodingFailure
 
-mkIconvEncoding :: CodingFailureMode -> String -> IO TextEncoding
+-- | Construct an iconv-based 'TextEncoding' for the given character set and
+-- 'CodingFailureMode'.
+--
+-- As iconv is missing in some minimal environments (e.g. #10298), this
+-- checks to ensure that iconv is working properly before returning the
+-- encoding, returning 'Nothing' if not.
+mkIconvEncoding :: CodingFailureMode -> String -> IO (Maybe TextEncoding)
 mkIconvEncoding cfm charset = do
-  return (TextEncoding {
-                textEncodingName = charset,
-                mkTextDecoder = newIConv raw_charset (haskellChar ++ suffix) (recoverDecode cfm) iconvDecode,
-                mkTextEncoder = newIConv haskellChar charset                 (recoverEncode cfm) iconvEncode})
+    let enc = TextEncoding {
+                  textEncodingName = charset,
+                  mkTextDecoder = newIConv raw_charset (haskellChar ++ suffix)
+                                           (recoverDecode cfm) iconvDecode,
+                  mkTextEncoder = newIConv haskellChar charset
+                                           (recoverEncode cfm) iconvEncode}
+    good <- charIsRepresentable enc 'a'
+    return $ if good
+               then Just enc
+               else Nothing
   where
     -- An annoying feature of GNU iconv is that the //PREFIXES only take
     -- effect when they appear on the tocode parameter to iconv_open:
diff --git a/libraries/base/GHC/TopHandler.hs b/libraries/base/GHC/TopHandler.hs
index e725196cdf..05c905f7c1 100644
--- a/libraries/base/GHC/TopHandler.hs
+++ b/libraries/base/GHC/TopHandler.hs
@@ -176,7 +176,12 @@ disasterHandler exit _ =
   withCAString "%s" $ \fmt ->
     withCAString msgStr $ \msg ->
       errorBelch fmt msg >> exit 1
-  where msgStr = "encountered an exception while trying to report an exception"
+  where
+    msgStr =
+        "encountered an exception while trying to report an exception." ++
+        "One possible reason for this is that we failed while trying to " ++
+        "encode an error message. Check that your locale is configured " ++
+        "properly."
 
 {- Note [Disaster with iconv]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~