WISH: Make it possible do undo a registerNnn() call

[HenrikBengtsson]

Background

Some package register dopar adaptors internally, e.g.

pkg_fcn <- function() {
  registerDoMC(4)
  ...
}

This, rather common, design pattern breaks whatever the user has previously set for whatever foreach purposes they have. For example, the following will not do what the user expects:

registerDoRedis()
y <- pkg_fcn()  ## <= silently registers another dopar adaptor
z <- foreach(x = 1:3) %dopar% { sqrt(x) }

Sometimes, these re-registrations happens deep down in the package dependency graphs making them really hard to locate.

Suggestion

Provide a way for package developers to temporarily register a dopar adapter using:

pkg_fcn <- function() {
  oldDoPar <- registerDoMC(4)
  on.exit(foreach::setDoPar(oldDoPar))
  ...
}

[HenrikBengtsson]

I've created PR #22 for this.