Unit testing with `testthat` on functions that write/read files, or a Fortran error

What is the best way to make unit testing with testthat on functions that read and write files?

Apologies for asking a complicated question, but I am not sure what is wrong here.

I have implemented a number of functions in Fortran, that reads and writes files. They are compiled in an R package cf. Writing R Extension manual. My unit testing with testthat generates random content that is written to temporary files with tempfile(). Running R CMD check on the R package works on my local Windows machine.

Running with R-devel fails however because it cannot detect Rtools for R-3.5.0 (devel). So I submitted to win-builder.

http://win-builder.r-project.org/ however fails with the following error:

  At line 9 of file auxil.f95
Fortran runtime error: Actual string length is shorter than the declared one for dummy argument 'fn' (96/255)

with corresponding Fortran source:

subroutine get_nlines(fn, nlines, stat) !line 9
  implicit none

  !! Arguments
  character(255), intent(in) :: fn
  integer, intent(out) :: nlines, stat

  !! Local variables
  character(len=1) :: one

  nlines = 0
  open(40, file=fn, status='OLD')
  do
    read(40, *, iostat=stat) one
    if (stat /= 0) exit
    nlines = nlines + 1
  end do
  close(40)

end subroutine

The Fortran code is stored in the src subdirectory of the R package, and is called with

get_nlines <- function(fn) {
  stopifnot(file.exists(fn))
  res <- .Fortran('get_nlines', fn=as.character(fn), nlines=integer(1), stat=integer(1))
  if (res$nlines == 0 & res$stat != 0) {
    warning(paste0('get_nlines did not read lines; IOSTAT error ', res$stat, '.'))
    return(structure(NA, code=res$stat))
  }
  res$nlines
}

So there it is. I don't know if my Fortran code is wrong, or if it's something that occurs on the win-builder server.