Do not call quit() in worker teardown

Because it creates a race condition and the output
of the teardown function might be lost.

This means that covr probably does not work for these
workers, because there is no time for it to write out
the coverage counters.

Author: gaborcsardi

R/parallel.R

@@ -313,9 +313,10 @@ queue_teardown <- function(queue) {
   tasks <- queue$list_tasks()
   num <- nrow(tasks)
 
+  # calling quit() here creates a race condition, and the output of
+  # the deferred_run() might be lost.
   clean_fn <- function() {
     withr::deferred_run(.GlobalEnv)
-    quit(save = "no", status = 0L, runLast = TRUE)
   }
 
   topoll <- list()