Properly detect if GHCi is running in MinTTY

Since the user experience of GHCi is known to be rather poor when run in MinTTY (e.g., see #12720), GHCi emits a warning when it thinks it's being run in a terminal that uses MinTTY (such as Cygwin or MSYS2). Here is the code that checks for this in ghci.c:

if (getenv("_")) {
    printf("WARNING: GHCi invoked via 'ghci.exe' in *nix-like shells (cygwin-bash, in particular)\n");
    printf("         doesn't handle Ctrl-C well; use the 'ghcii.sh' shell wrapper instead\n");
    fflush(stdout);
}

This is a rather shoddy way to detect MinTTY's presence, however. It's quite easy to define an environmental variable named _ in terminals that don't use MinTTY (e.g., ConEmu).

After seeing D2809, I learned of a much more reliable way to detect for MinTTY's presence:

queryCygwinTerminal :: Win32.HANDLE -> IO Bool
queryCygwinTerminal h = do
    fileType <- Win32.getFileType h
    if fileType /= Win32.fILE_TYPE_PIPE
      then pure False
      else do
        fn <- getFileNameByHandle h
        pure (("\\cygwin-" `isPrefixOf` fn || "\\msys-" `isPrefixOf` fn) &&
              "-pty" `isInfixOf` fn &&
              "-master" `isSuffixOf` fn)
  `catch` \ (_ :: IOError) ->
    pure False

getFileNameByHandle :: Win32.HANDLE -> IO String
getFileNameByHandle h = do
  let sizeOfDWORD = sizeOf (undefined :: Win32.DWORD)
  let sizeOfWchar = sizeOf (undefined :: CWchar)
  -- note: implicitly assuming that DWORD has stronger alignment than wchar_t
  let bufSize = sizeOfDWORD + mAX_PATH * sizeOfWchar
  allocaBytes bufSize $ \ buf -> do
    getFileInformationByHandleEx h fileNameInfo buf (fromIntegral bufSize)
    len :: Win32.DWORD <- peek buf
    let len' = fromIntegral len `div` sizeOfWchar
    peekCWStringLen (buf `plusPtr` sizeOfDWORD, min len' mAX_PATH)

getFileInformationByHandleEx
  :: Win32.HANDLE -> CInt -> Ptr a -> Win32.DWORD -> IO ()
getFileInformationByHandleEx h cls buf bufSize = do
  lib <- Win32.getModuleHandle (Just "kernel32.dll")
  ptr <- Win32.getProcAddress lib "GetFileInformationByHandleEx"
  let c_GetFileInformationByHandleEx =
        mk_GetFileInformationByHandleEx (castPtrToFunPtr ptr)
  Win32.failIfFalse_ "getFileInformationByHandleEx"
    (c_GetFileInformationByHandleEx h cls buf bufSize)

type F_GetFileInformationByHandleEx a =
  Win32.HANDLE -> CInt -> Ptr a -> Win32.DWORD -> IO Win32.BOOL

foreign import WINAPI "dynamic"
  mk_GetFileInformationByHandleEx
  :: FunPtr (F_GetFileInformationByHandleEx a)
  -> F_GetFileInformationByHandleEx a

We should port this to C and use this in ghci.c instead of what we have now.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	Phyx-
Operating system
Architecture