openFile can leak file descriptors

Summary

openFile' looks like this:

openFile' :: String -> IOMode -> Bool -> Bool -> IO Handle
openFile' filepath iomode binary non_blocking = do
  -- first open the file to get an FD
  (fd, fd_type) <- FD.openFile filepath iomode non_blocking

  mb_codec <- if binary then return Nothing else fmap Just getLocaleEncoding

  -- then use it to make a Handle
  mkHandleFromFD fd fd_type filepath iomode
                   False {- do not *set* non-blocking mode -}
                   mb_codec
            `onException` IODevice.close fd

If an asynchronous exception occurs after GHC.IO.FD.openFile opens the file but before installing the exception handler, then the file descriptor will remain open with no finalizer. Shouldn't we mask around this entire function?

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Expected behavior

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