Backend hook function for shutting down/cleaning up backend

[HenrikBengtsson]

Currently, changing plan as in:

plan(multisession, workers = 2)
plan(sequential)

will trigger a call to an internal plan_cleanup() function:

  plan_cleanup <- function() {
    ClusterRegistry(action = "stop")
  }

As we can see, this function will stop any cluster futures. That is, it works for plan(cluster, ...) and plan(multisession, ...) and derivatives, but that's it.

We could add support for custom cleanup "hook" functions so we can support shutdown of all backends. One approach could be to designate an special attribute for the "evaluator" function, e.g.

attr(cluster, "cleanup") <- function() {
  future::ClusterRegistry(action = "stop")
}

attr(multisession, "cleanup") <- function() {
  future::ClusterRegistry(action = "stop")
}

Then, we could do something like:

  plan_cleanup <- function() {
    evaluator <- oldStack[[1L]]
    cleanup <- attr(evaluator, "cleanup", exact = TRUE)
    if (is.function(cleanup)) {
      cleanup()
    }
  }

[HenrikBengtsson]

Completed. Added R option future.plan.cleanup.legacy (default FALSE) that can be set to TRUE to force the past behavior of calling ClusterRegistry(action = "stop") regardless of future plan currently set.