foreach with serial and parallel backend giving different results - r

Question

I have a strange case where using foreach with a serial and parallel backend gives different results the first time I call but then later on both results matches. I used RNG to make the results reproducible for the same seed
Below is a sample function to explain the scenario :
func <- function(ncores = NULL, seed = 1234){
if (!is.null(ncores)){ # this block registers for parallel backend
cl <- makeCluster(ncores)
registerDoParallel(cl)
registerDoRNG(seed, once = TRUE)
on.exit(stopCluster(cl))
} else { # this block registers for serial computation
registerDoSEQ()
registerDoRNG(seed, once = TRUE)
}
w = foreach(i = 1:10, .combine = 'c') %dorng% {
mean(sample(1:100, 50, replace = TRUE))
}
attr(w, "rng") <- NULL
return(w)
}
# first time running below 2 lines
# case 1 : serial
w1 <- func(ncores = NULL)
# Case 2 : parallel
w2 <- func(ncores= 5)
identical(w1, w2)
# second time running below 2 lines
# case 1: serial
w3 <- func(ncores = NULL)
# case 2: parallel
w4 <- func(ncores= 5)
identical(w1, w2)
# [1] FALSE
identical(w3, w4)
# [1] TRUE
Am i missing something while registering sequential process ?

Answer 1

The solution is to use the following expression:
w = foreach(i = 1:10, .combine = 'c', .options.RNG=seed) %dorng% {
mean(sample(1:100, 50, replace = TRUE))}
You can find an explanation at the vignette here.
So your function looks like this:
func <- function(ncores = NULL, seed = 1234){
if (!is.null(ncores)){ # this block registers for parallel backend
cl <- makeCluster(ncores)
registerDoParallel(cl)
on.exit(stopCluster(cl))
} else { # this block registers for serial computation
registerDoSEQ()
}
w = foreach(i = 1:10, .combine = 'c', .options.RNG=seed) %dorng% {
mean(sample(1:100, 50, replace = TRUE))
}
attr(w, "rng") <- NULL
return(w)
}