file_coverage should trace code run using parallel::mc*

[akersting]

I currently use this slightly adapted version of file_coverage to also trace code run using mclapply, mcparallel, etc.:

file_coverage <- function(source_files, test_files, line_exclusions = NULL,
                          function_exclusions = NULL, parent_env = parent.frame()) {
  tdir <- tempfile("covr_traces_")
  dir.create(tdir)
  fix_mcexit(paste0("'", tdir, "'"))

  env <- new.env(parent = parent_env)
  withr::with_options(c(keep.parse.data.pkgs = TRUE), {
    lapply(source_files, sys.source, keep.source = TRUE,
           envir = env)
  })
  trace_environment(env)
  on.exit({
    reset_traces()
    clear_counters()
  })
  lapply(test_files, sys.source, keep.source = TRUE, envir = env)

  save_trace(tdir)
  coverage <- structure(merge_coverage(list.files(tdir, full.names = TRUE)),
                        class = "coverage")
  exclude(coverage, line_exclusions = line_exclusions, function_exclusions = function_exclusions,
          path = NULL)
}

Maybe you want to consider adding this to covr? I guess, one would certainly want to make patching parallel:::mcexit optional, just as it is for package_coverage.

[jimhester]

Possibly, though I am unlikely to work on this myself, if you want this feature I would suggest you open a PR to add it.