Enable CLOEXEC on the open file descriptor

This prevents child processes from inheriting an open file lock, which
can cause locking mechanisms to misbehave in serious ways. Ideally, we
would open the file descriptor with CLOEXEC enabled, but since unix 2.8
hasn't been released yet and we want backwards compatibility with older
releases, this commit sets CLOEXEC after opening the file descriptor.
This may seem like a race condition at first. However, since the lock is
always taken after CLOEXEC is set, the worst that can happen is that a
child process inherits the open FD in an _unlocked_ state. While
non-ideal from a performance standpoint, it doesn't introduce any
locking bugs.

System/FileLock/Internal/Flock.hsc

@@ -12,7 +12,7 @@ import Data.Bits
 import Foreign.C.Error
 import Foreign.C.Types
 import System.Posix.Files
-import System.Posix.IO (openFd, closeFd, defaultFileFlags, OpenMode(..))
+import System.Posix.IO (openFd, closeFd, defaultFileFlags, OpenMode(..), setFdOption, FdOption(..))
 import System.Posix.Types
 import Prelude
 
@@ -38,7 +38,10 @@ unlock :: Lock -> IO ()
 unlock fd = closeFd fd
 
 open :: FilePath -> IO Fd
-open path = openFd path WriteOnly (Just stdFileMode) defaultFileFlags
+open path = do
+  fd <- openFd path WriteOnly (Just stdFileMode) defaultFileFlags
+  setFdOption fd CloseOnExec True
+  return fd
 
 flock :: Fd -> Bool -> Bool -> IO Bool
 flock (Fd fd) exclusive block = do