{-# LANGUAGE CPP #-}
-- | Compatibility layer for the @unix@ package to provide a @fileSetCaching@ function.
--
-- @unix >= 2.8.7@ defines a @fileSetCaching@ function, but @unix < 2.8.7@ does not. This module defines the function for @unix@ versions @< 2.8.7@. The implementation is adapted from https://github.com/haskell/unix/blob/v2.8.8.0/System/Posix/Fcntl.hsc#L116-L182.
--
-- NOTE: in the future if we no longer support @unix@ versions @< 2.8.7@, then this module can be removed.
module System.FS.BlockIO.Internal.Fcntl (fileSetCaching) where

#if MIN_VERSION_unix(2,8,7)

import System.Posix.Fcntl (fileSetCaching)

#else

-- hsc2hs does not define _GNU_SOURCE, so a .hsc file must define it explicitly
-- or O_DIRECT stays hidden. The unix package doesn’t define it in source, but
-- its configure script calls AC_USE_SYSTEM_EXTENSIONS, which adds -D_GNU_SOURCE
-- to the build CFLAGS, and those flags are passed on to hsc2hs through the
-- generated `config.mk`.
#define _GNU_SOURCE

#include <fcntl.h>

import           Data.Bits (complement, (.&.), (.|.))
import           Foreign.C (throwErrnoIfMinus1, throwErrnoIfMinus1_)
import           System.Posix.Internals
import           System.Posix.Types (Fd (Fd))

-- | For simplification, we considered that Linux !HAS_F_NOCACHE and HAS_O_DIRECT
fileSetCaching :: Fd -> Bool -> IO ()
fileSetCaching (Fd fd) val = do
    r <- throwErrnoIfMinus1 "fileSetCaching" (c_fcntl_read fd #{const F_GETFL})
    let r' | val       = fromIntegral r .&. complement opt_val
           | otherwise = fromIntegral r .|. opt_val
    throwErrnoIfMinus1_ "fileSetCaching" (c_fcntl_write fd #{const F_SETFL} r')
  where
    opt_val = #{const O_DIRECT}
#endif