[commit: ghc] ghc-7.10: Fix self-contained handling of ASCII encoding (677552f)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : ghc-7.10
Link       : http://ghc.haskell.org/trac/ghc/changeset/677552f21690761b89255d05e42976679be4d374/ghc

>---------------------------------------------------------------

commit 677552f21690761b89255d05e42976679be4d374
Author: Ben Gamari <bgamari.foss at gmail.com>
Date:   Thu Jul 9 08:07:54 2015 -0400

    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.
    
    Fixes #10623.


>---------------------------------------------------------------

677552f21690761b89255d05e42976679be4d374
 libraries/base/GHC/IO/Encoding.hs       | 35 +++++++++++++++++++++++----------
 libraries/base/GHC/IO/Encoding/Iconv.hs |  2 ++
 libraries/base/GHC/TopHandler.hs        |  6 +++++-
 testsuite/tests/driver/Makefile         |  2 +-
 4 files changed, 33 insertions(+), 12 deletions(-)

diff --git a/libraries/base/GHC/IO/Encoding.hs b/libraries/base/GHC/IO/Encoding.hs
index 014b61b..108b0fc 100644
--- a/libraries/base/GHC/IO/Encoding.hs
+++ b/libraries/base/GHC/IO/Encoding.hs
@@ -30,6 +30,7 @@ module GHC.IO.Encoding (
     ) where
 
 import GHC.Base
+import GHC.Foreign (charIsRepresentable)
 import GHC.IO.Exception
 import GHC.IO.Buffer
 import GHC.IO.Encoding.Failure
@@ -235,14 +236,9 @@ 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 +250,32 @@ 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
+            good <- charIsRepresentable res 'a'
+            let isAscii = any (== enc) ansiEncNames
+            case good of
+              True -> return res
+              -- 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 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' encodeing
+              False
+                | 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 89ca71e..f64d245 100644
--- a/libraries/base/GHC/IO/Encoding/Iconv.hs
+++ b/libraries/base/GHC/IO/Encoding/Iconv.hs
@@ -99,6 +99,8 @@ char_shift | charSize == 2 = 1
 iconvEncoding :: String -> IO TextEncoding
 iconvEncoding = mkIconvEncoding ErrorOnCodingFailure
 
+-- | Construct an iconv-based 'TextEncoding' for the given character set and
+-- 'CodingFailureMode'.
 mkIconvEncoding :: CodingFailureMode -> String -> IO TextEncoding
 mkIconvEncoding cfm charset = do
   return (TextEncoding {
diff --git a/libraries/base/GHC/TopHandler.hs b/libraries/base/GHC/TopHandler.hs
index e725196..d901069 100644
--- a/libraries/base/GHC/TopHandler.hs
+++ b/libraries/base/GHC/TopHandler.hs
@@ -176,7 +176,11 @@ 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 configured properly."
 
 {- Note [Disaster with iconv]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/testsuite/tests/driver/Makefile b/testsuite/tests/driver/Makefile
index 4418ba4..e12f3a5 100644
--- a/testsuite/tests/driver/Makefile
+++ b/testsuite/tests/driver/Makefile
@@ -551,7 +551,7 @@ T7563:
 	-"$(TEST_HC)" $(TEST_HC_OPTS_NO_RECOMP) -C T7563.hs
 
 # Below we set LC_ALL=C to request standard ASCII output in the resulting error
-# messagse. Unfortunately, Mac OS X still uses a Unicode encoding even with
+# messages. Unfortunately, Mac OS X still uses a Unicode encoding even with
 # LC_ALL=C, so we expect these tests to fail there.
 
 .PHONY: T6037