GHC/IO/Handle/Internals.hs

{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude
 , RecordWildCards
 , BangPatterns
 , PatternGuards
 , NondecreasingIndentation
 , Rank2Types
 #-}
{-# OPTIONS_GHC -fno-warn-unused-matches #-}
{-# OPTIONS_GHC -fno-warn-name-shadowing #-}
{-# OPTIONS_HADDOCK hide #-}

-----------------------------------------------------------------------------
-- |
-- Module : GHC.IO.Handle.Internals
-- Copyright : (c) The University of Glasgow, 1994-2001
-- License : see libraries/base/LICENSE
-- 
-- Maintainer : libraries@haskell.org
-- Stability : internal
-- Portability : non-portable
--
-- This module defines the basic operations on I\/O \"handles\". All
-- of the operations defined here are independent of the underlying
-- device.
--
-----------------------------------------------------------------------------

-- #hide
module GHC.IO.Handle.Internals (
 withHandle, withHandle', withHandle_,
 withHandle__', withHandle_', withAllHandles__,
 wantWritableHandle, wantReadableHandle, wantReadableHandle_, 
 wantSeekableHandle,

 mkHandle, mkFileHandle, mkDuplexHandle,
 openTextEncoding, closeTextCodecs, initBufferState,
 dEFAULT_CHAR_BUFFER_SIZE,

 flushBuffer, flushWriteBuffer, flushCharReadBuffer,
 flushCharBuffer, flushByteReadBuffer, flushByteWriteBuffer,

 readTextDevice, writeCharBuffer, readTextDeviceNonBlocking,
 decodeByteBuf,

 augmentIOError,
 ioe_closedHandle, ioe_EOF, ioe_notReadable, ioe_notWritable,
 ioe_finalizedHandle, ioe_bufsiz,

 hClose_help, hLookAhead_,

 HandleFinalizer, handleFinalizer,

 debugIO,
 ) where

import GHC.IO
import GHC.IO.IOMode
import GHC.IO.Encoding as Encoding
import GHC.IO.Handle.Types
import GHC.IO.Buffer
import GHC.IO.BufferedIO (BufferedIO)
import GHC.IO.Exception
import GHC.IO.Device (IODevice, SeekMode(..))
import qualified GHC.IO.Device as IODevice
import qualified GHC.IO.BufferedIO as Buffered

import GHC.Conc.Sync
import GHC.Real
import GHC.Base
import GHC.Exception
import GHC.Num ( Num(..) )
import GHC.Show
import GHC.IORef
import GHC.MVar
import Data.Typeable
import Control.Monad
import Data.Maybe
import Foreign.Safe
import System.Posix.Internals hiding (FD)

import Foreign.C

c_DEBUG_DUMP :: Bool
c_DEBUG_DUMP = False

-- ---------------------------------------------------------------------------
-- Creating a new handle

type HandleFinalizer = FilePath -> MVar Handle__ -> IO ()

newFileHandle :: FilePath -> Maybe HandleFinalizer -> Handle__ -> IO Handle
newFileHandle filepath mb_finalizer hc = do
 m <- newMVar hc
 case mb_finalizer of
 Just finalizer -> addMVarFinalizer m (finalizer filepath m)
 Nothing -> return ()
 return (FileHandle filepath m)

-- ---------------------------------------------------------------------------
-- Working with Handles

{-
In the concurrent world, handles are locked during use. This is done
by wrapping an MVar around the handle which acts as a mutex over
operations on the handle.

To avoid races, we use the following bracketing operations. The idea
is to obtain the lock, do some operation and replace the lock again,
whether the operation succeeded or failed. We also want to handle the
case where the thread receives an exception while processing the IO
operation: in these cases we also want to relinquish the lock.

There are three versions of @withHandle@: corresponding to the three
possible combinations of:

 - the operation may side-effect the handle
 - the operation may return a result

If the operation generates an error or an exception is raised, the
original handle is always replaced.
-}

{-# INLINE withHandle #-}
withHandle :: String -> Handle -> (Handle__ -> IO (Handle__,a)) -> IO a
withHandle fun h@(FileHandle _ m) act = withHandle' fun h m act
withHandle fun h@(DuplexHandle _ m _) act = withHandle' fun h m act

withHandle' :: String -> Handle -> MVar Handle__
 -> (Handle__ -> IO (Handle__,a)) -> IO a
withHandle' fun h m act =
 mask_ $ do
 (h',v) <- do_operation fun h act m
 checkHandleInvariants h'
 putMVar m h'
 return v

{-# INLINE withHandle_ #-}
withHandle_ :: String -> Handle -> (Handle__ -> IO a) -> IO a
withHandle_ fun h@(FileHandle _ m) act = withHandle_' fun h m act
withHandle_ fun h@(DuplexHandle _ m _) act = withHandle_' fun h m act

withHandle_' :: String -> Handle -> MVar Handle__ -> (Handle__ -> IO a) -> IO a
withHandle_' fun h m act = withHandle' fun h m $ \h_ -> do
 a <- act h_
 return (h_,a)

withAllHandles__ :: String -> Handle -> (Handle__ -> IO Handle__) -> IO ()
withAllHandles__ fun h@(FileHandle _ m) act = withHandle__' fun h m act
withAllHandles__ fun h@(DuplexHandle _ r w) act = do
 withHandle__' fun h r act
 withHandle__' fun h w act

withHandle__' :: String -> Handle -> MVar Handle__ -> (Handle__ -> IO Handle__)
 -> IO ()
withHandle__' fun h m act =
 mask_ $ do
 h' <- do_operation fun h act m
 checkHandleInvariants h'
 putMVar m h'
 return ()

do_operation :: String -> Handle -> (Handle__ -> IO a) -> MVar Handle__ -> IO a
do_operation fun h act m = do
 h_ <- takeMVar m
 checkHandleInvariants h_
 act h_ `catchException` handler h_
 where
 handler h_ e = do
 putMVar m h_
 case () of
 _ | Just ioe <- fromException e ->
 ioError (augmentIOError ioe fun h)
 _ | Just async_ex <- fromException e -> do -- see Note [async]
 let _ = async_ex :: AsyncException
 t <- myThreadId
 throwTo t e
 do_operation fun h act m
 _otherwise ->
 throwIO e

-- Note [async]
--
-- If an asynchronous exception is raised during an I/O operation,
-- normally it is fine to just re-throw the exception synchronously.
-- However, if we are inside an unsafePerformIO or an
-- unsafeInterleaveIO, this would replace the enclosing thunk with the
-- exception raised, which is wrong (#3997). We have to release the
-- lock on the Handle, but what do we replace the thunk with? What
-- should happen when the thunk is subsequently demanded again?
--
-- The only sensible choice we have is to re-do the IO operation on
-- resumption, but then we have to be careful in the IO library that
-- this is always safe to do. In particular we should
--
-- never perform any side-effects before an interruptible operation
--
-- because the interruptible operation may raise an asynchronous
-- exception, which may cause the operation and its side effects to be
-- subsequently performed again.
--
-- Re-doing the IO operation is achieved by:
-- - using throwTo to re-throw the asynchronous exception asynchronously
-- in the current thread
-- - on resumption, it will be as if throwTo returns. In that case, we
-- recursively invoke the original operation (see do_operation above).
--
-- Interruptible operations in the I/O library are:
-- - threadWaitRead/threadWaitWrite
-- - fillReadBuffer/flushWriteBuffer
-- - readTextDevice/writeTextDevice

augmentIOError :: IOException -> String -> Handle -> IOException
augmentIOError ioe@IOError{ ioe_filename = fp } fun h
 = ioe { ioe_handle = Just h, ioe_location = fun, ioe_filename = filepath }
 where filepath
 | Just _ <- fp = fp
 | otherwise = case h of
 FileHandle path _ -> Just path
 DuplexHandle path _ _ -> Just path

-- ---------------------------------------------------------------------------
-- Wrapper for write operations.

wantWritableHandle :: String -> Handle -> (Handle__ -> IO a) -> IO a
wantWritableHandle fun h@(FileHandle _ m) act
 = wantWritableHandle' fun h m act
wantWritableHandle fun h@(DuplexHandle _ _ m) act
 = wantWritableHandle' fun h m act
 -- we know it's not a ReadHandle or ReadWriteHandle, but we have to
 -- check for ClosedHandle/SemiClosedHandle. (#4808)

wantWritableHandle'
 :: String -> Handle -> MVar Handle__
 -> (Handle__ -> IO a) -> IO a
wantWritableHandle' fun h m act
 = withHandle_' fun h m (checkWritableHandle act)

checkWritableHandle :: (Handle__ -> IO a) -> Handle__ -> IO a
checkWritableHandle act h_@Handle__{..}
 = case haType of
 ClosedHandle -> ioe_closedHandle
 SemiClosedHandle -> ioe_closedHandle
 ReadHandle -> ioe_notWritable
 ReadWriteHandle -> do
 buf <- readIORef haCharBuffer
 when (not (isWriteBuffer buf)) $ do
 flushCharReadBuffer h_
 flushByteReadBuffer h_
 buf <- readIORef haCharBuffer
 writeIORef haCharBuffer buf{ bufState = WriteBuffer }
 buf <- readIORef haByteBuffer
 buf' <- Buffered.emptyWriteBuffer haDevice buf
 writeIORef haByteBuffer buf'
 act h_
 _other -> act h_

-- ---------------------------------------------------------------------------
-- Wrapper for read operations.

wantReadableHandle :: String -> Handle -> (Handle__ -> IO (Handle__,a)) -> IO a
wantReadableHandle fun h act = withHandle fun h (checkReadableHandle act)

wantReadableHandle_ :: String -> Handle -> (Handle__ -> IO a) -> IO a
wantReadableHandle_ fun h@(FileHandle _ m) act
 = wantReadableHandle' fun h m act
wantReadableHandle_ fun h@(DuplexHandle _ m _) act
 = wantReadableHandle' fun h m act
 -- we know it's not a WriteHandle or ReadWriteHandle, but we have to
 -- check for ClosedHandle/SemiClosedHandle. (#4808)

wantReadableHandle'
 :: String -> Handle -> MVar Handle__
 -> (Handle__ -> IO a) -> IO a
wantReadableHandle' fun h m act
 = withHandle_' fun h m (checkReadableHandle act)

checkReadableHandle :: (Handle__ -> IO a) -> Handle__ -> IO a
checkReadableHandle act h_@Handle__{..} =
 case haType of
 ClosedHandle -> ioe_closedHandle
 SemiClosedHandle -> ioe_closedHandle
 AppendHandle -> ioe_notReadable
 WriteHandle -> ioe_notReadable
 ReadWriteHandle -> do
 -- a read/write handle and we want to read from it. We must
 -- flush all buffered write data first.
 bbuf <- readIORef haByteBuffer
 when (isWriteBuffer bbuf) $ do
 when (not (isEmptyBuffer bbuf)) $ flushByteWriteBuffer h_
 cbuf' <- readIORef haCharBuffer
 writeIORef haCharBuffer cbuf'{ bufState = ReadBuffer }
 bbuf <- readIORef haByteBuffer
 writeIORef haByteBuffer bbuf{ bufState = ReadBuffer }
 act h_
 _other -> act h_

-- ---------------------------------------------------------------------------
-- Wrapper for seek operations.

wantSeekableHandle :: String -> Handle -> (Handle__ -> IO a) -> IO a
wantSeekableHandle fun h@(DuplexHandle _ _ _) _act =
 ioException (IOError (Just h) IllegalOperation fun
 "handle is not seekable" Nothing Nothing)
wantSeekableHandle fun h@(FileHandle _ m) act =
 withHandle_' fun h m (checkSeekableHandle act)

checkSeekableHandle :: (Handle__ -> IO a) -> Handle__ -> IO a
checkSeekableHandle act handle_@Handle__{haDevice=dev} =
 case haType handle_ of
 ClosedHandle -> ioe_closedHandle
 SemiClosedHandle -> ioe_closedHandle
 AppendHandle -> ioe_notSeekable
 _ -> do b <- IODevice.isSeekable dev
 if b then act handle_
 else ioe_notSeekable

-- -----------------------------------------------------------------------------
-- Handy IOErrors

ioe_closedHandle, ioe_EOF,
 ioe_notReadable, ioe_notWritable, ioe_cannotFlushNotSeekable,
 ioe_notSeekable :: IO a

ioe_closedHandle = ioException
 (IOError Nothing IllegalOperation ""
 "handle is closed" Nothing Nothing)
ioe_EOF = ioException
 (IOError Nothing EOF "" "" Nothing Nothing)
ioe_notReadable = ioException
 (IOError Nothing IllegalOperation ""
 "handle is not open for reading" Nothing Nothing)
ioe_notWritable = ioException
 (IOError Nothing IllegalOperation ""
 "handle is not open for writing" Nothing Nothing)
ioe_notSeekable = ioException
 (IOError Nothing IllegalOperation ""
 "handle is not seekable" Nothing Nothing)
ioe_cannotFlushNotSeekable = ioException
 (IOError Nothing IllegalOperation ""
 "cannot flush the read buffer: underlying device is not seekable"
 Nothing Nothing)

ioe_finalizedHandle :: FilePath -> Handle__
ioe_finalizedHandle fp = throw
 (IOError Nothing IllegalOperation ""
 "handle is finalized" Nothing (Just fp))

ioe_bufsiz :: Int -> IO a
ioe_bufsiz n = ioException
 (IOError Nothing InvalidArgument "hSetBuffering"
 ("illegal buffer size " ++ showsPrec 9 n []) Nothing Nothing)
 -- 9 => should be parens'ified.

-- ---------------------------------------------------------------------------
-- Wrapper for Handle encoding/decoding.

-- The interface for TextEncoding changed so that a TextEncoding doesn't raise
-- an exception if it encounters an invalid sequnce. Furthermore, encoding
-- returns a reason as to why encoding stopped, letting us know if it was due
-- to input/output underflow or an invalid sequence.
--
-- This code adapts this elaborated interface back to the original TextEncoding
-- interface.
--
-- FIXME: it is possible that Handle code using the haDecoder/haEncoder fields
-- could be made clearer by using the 'encode' interface directly. I have not
-- looked into this.

streamEncode :: BufferCodec from to state
 -> Buffer from -> Buffer to
 -> IO (Buffer from, Buffer to)
streamEncode codec from to = go (from, to)
 where 
 go (from, to) = do
 (why, from', to') <- encode codec from to
 -- When we are dealing with Handles, we don't care about input/output
 -- underflow particularly, and we want to delay errors about invalid
 -- sequences as far as possible.
 case why of
 Encoding.InvalidSequence | bufL from == bufL from' -> recover codec from' to' >>= go
 _ -> return (from', to')

-- -----------------------------------------------------------------------------
-- Handle Finalizers

-- For a duplex handle, we arrange that the read side points to the write side
-- (and hence keeps it alive if the read side is alive). This is done by
-- having the haOtherSide field of the read side point to the read side.
-- The finalizer is then placed on the write side, and the handle only gets
-- finalized once, when both sides are no longer required.

-- NOTE about finalized handles: It's possible that a handle can be
-- finalized and then we try to use it later, for example if the
-- handle is referenced from another finalizer, or from a thread that
-- has become unreferenced and then resurrected (arguably in the
-- latter case we shouldn't finalize the Handle...). Anyway,
-- we try to emit a helpful message which is better than nothing.
--
-- [later; 8/2010] However, a program like this can yield a strange
-- error message:
--
-- main = writeFile "out" loop
-- loop = let x = x in x
--
-- because the main thread and the Handle are both unreachable at the
-- same time, the Handle may get finalized before the main thread
-- receives the NonTermination exception, and the exception handler
-- will then report an error. We'd rather this was not an error and
-- the program just prints "<<loop>>".

handleFinalizer :: FilePath -> MVar Handle__ -> IO ()
handleFinalizer fp m = do
 handle_ <- takeMVar m
 (handle_', _) <- hClose_help handle_
 putMVar m handle_'
 return ()

-- ---------------------------------------------------------------------------
-- Allocating buffers

-- using an 8k char buffer instead of 32k improved performance for a
-- basic "cat" program by ~30% for me. --SDM
dEFAULT_CHAR_BUFFER_SIZE :: Int
dEFAULT_CHAR_BUFFER_SIZE = 2048 -- 8k/sizeof(HsChar)

getCharBuffer :: IODevice dev => dev -> BufferState
 -> IO (IORef CharBuffer, BufferMode)
getCharBuffer dev state = do
 buffer <- newCharBuffer dEFAULT_CHAR_BUFFER_SIZE state
 ioref <- newIORef buffer
 is_tty <- IODevice.isTerminal dev

 let buffer_mode 
 | is_tty = LineBuffering 
 | otherwise = BlockBuffering Nothing

 return (ioref, buffer_mode)

mkUnBuffer :: BufferState -> IO (IORef CharBuffer, BufferMode)
mkUnBuffer state = do
 buffer <- newCharBuffer dEFAULT_CHAR_BUFFER_SIZE state
 -- See [note Buffer Sizing], GHC.IO.Handle.Types
 ref <- newIORef buffer
 return (ref, NoBuffering)

-- -----------------------------------------------------------------------------
-- Flushing buffers

-- | syncs the file with the buffer, including moving the
-- file pointer backwards in the case of a read buffer. This can fail
-- on a non-seekable read Handle.
flushBuffer :: Handle__ -> IO ()
flushBuffer h_@Handle__{..} = do
 buf <- readIORef haCharBuffer
 case bufState buf of
 ReadBuffer -> do
 flushCharReadBuffer h_
 flushByteReadBuffer h_
 WriteBuffer -> do
 flushByteWriteBuffer h_

-- | flushes the Char buffer only. Works on all Handles.
flushCharBuffer :: Handle__ -> IO ()
flushCharBuffer h_@Handle__{..} = do
 cbuf <- readIORef haCharBuffer
 case bufState cbuf of
 ReadBuffer -> do
 flushCharReadBuffer h_
 WriteBuffer ->
 when (not (isEmptyBuffer cbuf)) $
 error "internal IO library error: Char buffer non-empty"

-- -----------------------------------------------------------------------------
-- Writing data (flushing write buffers)

-- flushWriteBuffer flushes the buffer iff it contains pending write
-- data. Flushes both the Char and the byte buffer, leaving both
-- empty.
flushWriteBuffer :: Handle__ -> IO ()
flushWriteBuffer h_@Handle__{..} = do
 buf <- readIORef haByteBuffer
 when (isWriteBuffer buf) $ flushByteWriteBuffer h_

flushByteWriteBuffer :: Handle__ -> IO ()
flushByteWriteBuffer h_@Handle__{..} = do
 bbuf <- readIORef haByteBuffer
 when (not (isEmptyBuffer bbuf)) $ do
 bbuf' <- Buffered.flushWriteBuffer haDevice bbuf
 writeIORef haByteBuffer bbuf'

-- write the contents of the CharBuffer to the Handle__.
-- The data will be encoded and pushed to the byte buffer,
-- flushing if the buffer becomes full.
writeCharBuffer :: Handle__ -> CharBuffer -> IO ()
writeCharBuffer h_@Handle__{..} !cbuf = do
 --
 bbuf <- readIORef haByteBuffer

 debugIO ("writeCharBuffer: cbuf=" ++ summaryBuffer cbuf ++
 " bbuf=" ++ summaryBuffer bbuf)

 (cbuf',bbuf') <- case haEncoder of
 Nothing -> latin1_encode cbuf bbuf
 Just encoder -> (streamEncode encoder) cbuf bbuf

 debugIO ("writeCharBuffer after encoding: cbuf=" ++ summaryBuffer cbuf' ++
 " bbuf=" ++ summaryBuffer bbuf')

 -- flush if the write buffer is full
 if isFullBuffer bbuf'
 -- or we made no progress
 || not (isEmptyBuffer cbuf') && bufL cbuf' == bufL cbuf
 -- or the byte buffer has more elements than the user wanted buffered
 || (case haBufferMode of
 BlockBuffering (Just s) -> bufferElems bbuf' >= s
 NoBuffering -> True
 _other -> False)
 then do
 bbuf'' <- Buffered.flushWriteBuffer haDevice bbuf'
 writeIORef haByteBuffer bbuf''
 else
 writeIORef haByteBuffer bbuf'

 if not (isEmptyBuffer cbuf')
 then writeCharBuffer h_ cbuf'
 else return ()

-- -----------------------------------------------------------------------------
-- Flushing read buffers

-- It is always possible to flush the Char buffer back to the byte buffer.
flushCharReadBuffer :: Handle__ -> IO ()
flushCharReadBuffer Handle__{..} = do
 cbuf <- readIORef haCharBuffer
 if isWriteBuffer cbuf || isEmptyBuffer cbuf then return () else do

 -- haLastDecode is the byte buffer just before we did our last batch of
 -- decoding. We're going to re-decode the bytes up to the current char,
 -- to find out where we should revert the byte buffer to.
 (codec_state, bbuf0) <- readIORef haLastDecode

 cbuf0 <- readIORef haCharBuffer
 writeIORef haCharBuffer cbuf0{ bufL=0, bufR=0 }

 -- if we haven't used any characters from the char buffer, then just
 -- re-install the old byte buffer.
 if bufL cbuf0 == 0
 then do writeIORef haByteBuffer bbuf0
 return ()
 else do

 case haDecoder of
 Nothing -> do
 writeIORef haByteBuffer bbuf0 { bufL = bufL bbuf0 + bufL cbuf0 }
 -- no decoder: the number of bytes to decode is the same as the
 -- number of chars we have used up.

 Just decoder -> do
 debugIO ("flushCharReadBuffer re-decode, bbuf=" ++ summaryBuffer bbuf0 ++
 " cbuf=" ++ summaryBuffer cbuf0)

 -- restore the codec state
 setState decoder codec_state
 
 (bbuf1,cbuf1) <- (streamEncode decoder) bbuf0
 cbuf0{ bufL=0, bufR=0, bufSize = bufL cbuf0 }
 
 debugIO ("finished, bbuf=" ++ summaryBuffer bbuf1 ++
 " cbuf=" ++ summaryBuffer cbuf1)

 writeIORef haByteBuffer bbuf1


-- When flushing the byte read buffer, we seek backwards by the number
-- of characters in the buffer. The file descriptor must therefore be
-- seekable: attempting to flush the read buffer on an unseekable
-- handle is not allowed.

flushByteReadBuffer :: Handle__ -> IO ()
flushByteReadBuffer h_@Handle__{..} = do
 bbuf <- readIORef haByteBuffer

 if isEmptyBuffer bbuf then return () else do

 seekable <- IODevice.isSeekable haDevice
 when (not seekable) $ ioe_cannotFlushNotSeekable

 let seek = negate (bufR bbuf - bufL bbuf)

 debugIO ("flushByteReadBuffer: new file offset = " ++ show seek)
 IODevice.seek haDevice RelativeSeek (fromIntegral seek)

 writeIORef haByteBuffer bbuf{ bufL=0, bufR=0 }

-- ----------------------------------------------------------------------------
-- Making Handles

mkHandle :: (IODevice dev, BufferedIO dev, Typeable dev) => dev
 -> FilePath
 -> HandleType
 -> Bool -- buffered?
 -> Maybe TextEncoding
 -> NewlineMode
 -> Maybe HandleFinalizer
 -> Maybe (MVar Handle__)
 -> IO Handle

mkHandle dev filepath ha_type buffered mb_codec nl finalizer other_side = do
 openTextEncoding mb_codec ha_type $ \ mb_encoder mb_decoder -> do

 let buf_state = initBufferState ha_type
 bbuf <- Buffered.newBuffer dev buf_state
 bbufref <- newIORef bbuf
 last_decode <- newIORef (error "codec_state", bbuf)

 (cbufref,bmode) <- 
 if buffered then getCharBuffer dev buf_state
 else mkUnBuffer buf_state

 spares <- newIORef BufferListNil
 newFileHandle filepath finalizer
 (Handle__ { haDevice = dev,
 haType = ha_type,
 haBufferMode = bmode,
 haByteBuffer = bbufref,
 haLastDecode = last_decode,
 haCharBuffer = cbufref,
 haBuffers = spares,
 haEncoder = mb_encoder,
 haDecoder = mb_decoder,
 haCodec = mb_codec,
 haInputNL = inputNL nl,
 haOutputNL = outputNL nl,
 haOtherSide = other_side
 })

-- | makes a new 'Handle'
mkFileHandle :: (IODevice dev, BufferedIO dev, Typeable dev)
 => dev -- ^ the underlying IO device, which must support 
 -- 'IODevice', 'BufferedIO' and 'Typeable'
 -> FilePath
 -- ^ a string describing the 'Handle', e.g. the file
 -- path for a file. Used in error messages.
 -> IOMode
 -- The mode in which the 'Handle' is to be used
 -> Maybe TextEncoding
 -- Create the 'Handle' with no text encoding?
 -> NewlineMode
 -- Translate newlines?
 -> IO Handle
mkFileHandle dev filepath iomode mb_codec tr_newlines = do
 mkHandle dev filepath (ioModeToHandleType iomode) True{-buffered-} mb_codec
 tr_newlines
 (Just handleFinalizer) Nothing{-other_side-}

-- | like 'mkFileHandle', except that a 'Handle' is created with two
-- independent buffers, one for reading and one for writing. Used for
-- full-duplex streams, such as network sockets.
mkDuplexHandle :: (IODevice dev, BufferedIO dev, Typeable dev) => dev
 -> FilePath -> Maybe TextEncoding -> NewlineMode -> IO Handle
mkDuplexHandle dev filepath mb_codec tr_newlines = do

 write_side@(FileHandle _ write_m) <- 
 mkHandle dev filepath WriteHandle True mb_codec
 tr_newlines
 (Just handleFinalizer)
 Nothing -- no othersie

 read_side@(FileHandle _ read_m) <- 
 mkHandle dev filepath ReadHandle True mb_codec
 tr_newlines
 Nothing -- no finalizer
 (Just write_m)

 return (DuplexHandle filepath read_m write_m)

ioModeToHandleType :: IOMode -> HandleType
ioModeToHandleType ReadMode = ReadHandle
ioModeToHandleType WriteMode = WriteHandle
ioModeToHandleType ReadWriteMode = ReadWriteHandle
ioModeToHandleType AppendMode = AppendHandle

initBufferState :: HandleType -> BufferState
initBufferState ReadHandle = ReadBuffer
initBufferState _ = WriteBuffer

openTextEncoding
 :: Maybe TextEncoding
 -> HandleType
 -> (forall es ds . Maybe (TextEncoder es) -> Maybe (TextDecoder ds) -> IO a)
 -> IO a

openTextEncoding Nothing ha_type cont = cont Nothing Nothing
openTextEncoding (Just TextEncoding{..}) ha_type cont = do
 mb_decoder <- if isReadableHandleType ha_type then do
 decoder <- mkTextDecoder
 return (Just decoder)
 else
 return Nothing
 mb_encoder <- if isWritableHandleType ha_type then do
 encoder <- mkTextEncoder
 return (Just encoder)
 else 
 return Nothing
 cont mb_encoder mb_decoder

closeTextCodecs :: Handle__ -> IO ()
closeTextCodecs Handle__{..} = do
 case haDecoder of Nothing -> return (); Just d -> Encoding.close d
 case haEncoder of Nothing -> return (); Just d -> Encoding.close d

-- ---------------------------------------------------------------------------
-- closing Handles

-- hClose_help is also called by lazyRead (in GHC.IO.Handle.Text) when
-- EOF is read or an IO error occurs on a lazy stream. The
-- semi-closed Handle is then closed immediately. We have to be
-- careful with DuplexHandles though: we have to leave the closing to
-- the finalizer in that case, because the write side may still be in
-- use.
hClose_help :: Handle__ -> IO (Handle__, Maybe SomeException)
hClose_help handle_ =
 case haType handle_ of 
 ClosedHandle -> return (handle_,Nothing)
 _ -> do mb_exc1 <- trymaybe $ flushWriteBuffer handle_ -- interruptible
 -- it is important that hClose doesn't fail and
 -- leave the Handle open (#3128), so we catch
 -- exceptions when flushing the buffer.
 (h_, mb_exc2) <- hClose_handle_ handle_
 return (h_, if isJust mb_exc1 then mb_exc1 else mb_exc2)


trymaybe :: IO () -> IO (Maybe SomeException)
trymaybe io = (do io; return Nothing) `catchException` \e -> return (Just e)

hClose_handle_ :: Handle__ -> IO (Handle__, Maybe SomeException)
hClose_handle_ h_@Handle__{..} = do

 -- close the file descriptor, but not when this is the read
 -- side of a duplex handle.
 -- If an exception is raised by the close(), we want to continue
 -- to close the handle and release the lock if it has one, then 
 -- we return the exception to the caller of hClose_help which can
 -- raise it if necessary.
 maybe_exception <- 
 case haOtherSide of
 Nothing -> trymaybe $ IODevice.close haDevice
 Just _ -> return Nothing

 -- free the spare buffers
 writeIORef haBuffers BufferListNil
 writeIORef haCharBuffer noCharBuffer
 writeIORef haByteBuffer noByteBuffer
 
 -- release our encoder/decoder
 closeTextCodecs h_

 -- we must set the fd to -1, because the finalizer is going
 -- to run eventually and try to close/unlock it.
 -- ToDo: necessary? the handle will be marked ClosedHandle
 -- XXX GHC won't let us use record update here, hence wildcards
 return (Handle__{ haType = ClosedHandle, .. }, maybe_exception)

{-# NOINLINE noCharBuffer #-}
noCharBuffer :: CharBuffer
noCharBuffer = unsafePerformIO $ newCharBuffer 1 ReadBuffer

{-# NOINLINE noByteBuffer #-}
noByteBuffer :: Buffer Word8
noByteBuffer = unsafePerformIO $ newByteBuffer 1 ReadBuffer

-- ---------------------------------------------------------------------------
-- Looking ahead

hLookAhead_ :: Handle__ -> IO Char
hLookAhead_ handle_@Handle__{..} = do
 buf <- readIORef haCharBuffer
 
 -- fill up the read buffer if necessary
 new_buf <- if isEmptyBuffer buf
 then readTextDevice handle_ buf
 else return buf
 writeIORef haCharBuffer new_buf
 
 peekCharBuf (bufRaw buf) (bufL buf)

-- ---------------------------------------------------------------------------
-- debugging

debugIO :: String -> IO ()
debugIO s
 | c_DEBUG_DUMP
 = do _ <- withCStringLen (s ++ "\n") $
 \(p, len) -> c_write 1 (castPtr p) (fromIntegral len)
 return ()
 | otherwise = return ()

-- ----------------------------------------------------------------------------
-- Text input/output

-- Read characters into the provided buffer. Return when any
-- characters are available; raise an exception if the end of 
-- file is reached.
readTextDevice :: Handle__ -> CharBuffer -> IO CharBuffer
readTextDevice h_@Handle__{..} cbuf = do
 --
 bbuf0 <- readIORef haByteBuffer

 debugIO ("readTextDevice: cbuf=" ++ summaryBuffer cbuf ++ 
 " bbuf=" ++ summaryBuffer bbuf0)

 bbuf1 <- if not (isEmptyBuffer bbuf0)
 then return bbuf0
 else do
 (r,bbuf1) <- Buffered.fillReadBuffer haDevice bbuf0
 if r == 0 then ioe_EOF else do -- raise EOF
 return bbuf1

 debugIO ("readTextDevice after reading: bbuf=" ++ summaryBuffer bbuf1)

 (bbuf2,cbuf') <- 
 case haDecoder of
 Nothing -> do
 writeIORef haLastDecode (error "codec_state", bbuf1)
 latin1_decode bbuf1 cbuf
 Just decoder -> do
 state <- getState decoder
 writeIORef haLastDecode (state, bbuf1)
 (streamEncode decoder) bbuf1 cbuf

 debugIO ("readTextDevice after decoding: cbuf=" ++ summaryBuffer cbuf' ++ 
 " bbuf=" ++ summaryBuffer bbuf2)

 writeIORef haByteBuffer bbuf2
 if bufR cbuf' == bufR cbuf -- no new characters
 then readTextDevice' h_ bbuf2 cbuf -- we need more bytes to make a Char
 else return cbuf'

-- we have an incomplete byte sequence at the end of the buffer: try to
-- read more bytes.
readTextDevice' :: Handle__ -> Buffer Word8 -> CharBuffer -> IO CharBuffer
readTextDevice' h_@Handle__{..} bbuf0 cbuf0 = do
 --
 -- copy the partial sequence to the beginning of the buffer, so we have
 -- room to read more bytes.
 bbuf1 <- slideContents bbuf0

 -- readTextDevice only calls us if we got some bytes but not some characters.
 -- This can't occur if haDecoder is Nothing because latin1_decode accepts all bytes.
 let Just decoder = haDecoder
 
 (r,bbuf2) <- Buffered.fillReadBuffer haDevice bbuf1
 if r == 0
 then do
 (bbuf3, cbuf1) <- recover decoder bbuf2 cbuf0
 writeIORef haByteBuffer bbuf3
 -- We should recursively invoke readTextDevice after recovery,
 -- if recovery did not add at least one new character to the buffer:
 -- 1. If we were using IgnoreCodingFailure it might be the case that
 -- cbuf1 is the same length as cbuf0 and we need to raise ioe_EOF
 -- 2. If we were using TransliterateCodingFailure we might have *mutated*
 -- the byte buffer without changing the pointers into either buffer.
 -- We need to try and decode it again - it might just go through this time.
 if bufR cbuf1 == bufR cbuf0
 then readTextDevice h_ cbuf1
 else return cbuf1
 else do
 debugIO ("readTextDevice' after reading: bbuf=" ++ summaryBuffer bbuf2)
 
 (bbuf3,cbuf1) <- do
 state <- getState decoder
 writeIORef haLastDecode (state, bbuf2)
 (streamEncode decoder) bbuf2 cbuf0
 
 debugIO ("readTextDevice' after decoding: cbuf=" ++ summaryBuffer cbuf1 ++ 
 " bbuf=" ++ summaryBuffer bbuf3)
 
 writeIORef haByteBuffer bbuf3
 if bufR cbuf0 == bufR cbuf1
 then readTextDevice' h_ bbuf3 cbuf1
 else return cbuf1

-- Read characters into the provided buffer. Do not block;
-- return zero characters instead. Raises an exception on end-of-file.
readTextDeviceNonBlocking :: Handle__ -> CharBuffer -> IO CharBuffer
readTextDeviceNonBlocking h_@Handle__{..} cbuf = do
 --
 bbuf0 <- readIORef haByteBuffer
 when (isEmptyBuffer bbuf0) $ do
 (r,bbuf1) <- Buffered.fillReadBuffer0 haDevice bbuf0
 if isNothing r then ioe_EOF else do -- raise EOF
 writeIORef haByteBuffer bbuf1

 decodeByteBuf h_ cbuf

-- Decode bytes from the byte buffer into the supplied CharBuffer.
decodeByteBuf :: Handle__ -> CharBuffer -> IO CharBuffer
decodeByteBuf h_@Handle__{..} cbuf = do
 --
 bbuf0 <- readIORef haByteBuffer

 (bbuf2,cbuf') <-
 case haDecoder of
 Nothing -> do
 writeIORef haLastDecode (error "codec_state", bbuf0)
 latin1_decode bbuf0 cbuf
 Just decoder -> do
 state <- getState decoder
 writeIORef haLastDecode (state, bbuf0)
 (streamEncode decoder) bbuf0 cbuf

 writeIORef haByteBuffer bbuf2
 return cbuf'