[GHC] #3815: System.Win32.Types.failWith segfaults on unknown error code

[GHC]

#3815: System.Win32.Types.failWith segfaults on unknown error code
----------------------------------+-----------------------------------------
    Reporter:  dherington         |        Owner:                         
        Type:  bug                |       Status:  new                    
    Priority:  normal             |    Milestone:                         
   Component:  libraries (other)  |      Version:  6.10.1                 
    Keywords:  getErrorMessage    |   Difficulty:  Easy (less than 1 hour)
          Os:  Windows            |     Testcase:                         
Architecture:  Unknown/Multiple   |      Failure:  Runtime crash          
----------------------------------+-----------------------------------------
 If a (Windows) system call returns an error code not known to
 FormatMessage, System.Win32.Types.failWith segfaults.  failWith
 incorrectly assumes that getErrorMessage never returns a NULL pointer.

 A simple fix in failWith is to replace:

   msg <- peekTString c_msg
   -- We ignore failure of freeing c_msg, given we're already failing
   _ <- localFree c_msg

 by:

   msg <- if c_msg == nullPtr
            then return $ "Error 0x" ++ Numeric.showHex err_code ""
            else do msg <- peekTString c_msg
                    -- We ignore failure of freeing c_msg, given we're
 already failing
                    _ <- localFree c_msg
                    return msg

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3815>
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