"un-register" a doParallel cluster

Question

If I run foreach... %dopar% without registering a cluster, foreach raises a warning, and executes the code sequentially:

library("doParallel")
foreach(i=1:3) %dopar%
  sqrt(i)

Yields:

Warning message:
executing %dopar% sequentially: no parallel backend registered 

However, if I run this same code after starting, registering, and stopping a cluster, it fails:

cl <- makeCluster(2)
registerDoParallel(cl)
stopCluster(cl)
rm(cl)
foreach(i=1:3) %dopar%
  sqrt(i)

Yields:

Error in summary.connection(connection) : invalid connection

Is there an opposite of registerDoParallel() that cleans up the cluster registration? Or am I stuck with the ghost of the old cluster until I re-start my R session?

/edit: some googling reveals the bumphunter:::foreachCleanup() function in the bumphunter Biocondoctor package:

function () 
{
    if (exists(".revoDoParCluster", where = doParallel:::.options)) {
        if (!is.null(doParallel:::.options$.revoDoParCluster)) 
            stopCluster(doParallel:::.options$.revoDoParCluster)
        remove(".revoDoParCluster", envir = doParallel:::.options)
    }
}
<environment: namespace:bumphunter>

However, this function doesn't seem to fix the problem.

library(bumphunter)
cl <- makeCluster(2)
registerDoParallel(cl)
stopCluster(cl)
rm(cl)
bumphunter:::foreachCleanup()
foreach(i=1:3) %dopar%
  sqrt(i)

Where does foreach keep the information on the registered cluster?

Answer 1

The only official way to "unregister" a foreach backend is to register the sequential backend:

registerDoSEQ()

This makes sense to me because you're supposed to declare which backend to use, so I didn't see any point in providing a way to "undeclare" which backend to use. Instead, you declare that you want to use the sequential backend, which is the default.

I originally considered including an "unregister" function, but since I couldn't convince myself that it was useful, I decided to leave it out since it's much easier to add a function than to remove one.

That being said, I think all you need to do is to remove all of the variables from foreach:::.foreachGlobals which is where foreach keeps all of its state:

unregister <- function() {
  env <- foreach:::.foreachGlobals
  rm(list=ls(name=env), pos=env)
}

After calling this function, any parallel backend will be deregistered and the warning will be issued again if %dopar% is called.