How to force stop doParallel %dopar% calculations in shiny app - r

Question

My shiny app does clustering using %dopar% from doParallel library. There is one thing that bothers me: if an operation takes, for example, 10 minutes to calculate, but user closes the window of an app before it's done - calculations will continue to happen until they're finished.
What I'm trying to do is to force stop them on session end. I tried to do something like this
session$onSessionEnded(function() {
stopCluster(clu)
print("stopped")
})
to stop calculations on window closure, but it is evaluated after the calculations are done. Here is a test app i'm trying to get it to work on:
server.r
library(shiny)
library(doParallel)
shinyServer(function(input, output, session) {
wd <- getwd()
rv <- reactiveValues()
clu <- makeCluster(2)
registerDoParallel(clu)
observeEvent(input$loadDataButton, {
rv$listDF <- readRDS(paste0(wd, "/iris15k.RData"))
})
observeEvent(input$clusterButton, {
if (input$clusterButton >= 1) {
df <- rv$listDF
rv$resultTime <- system.time(
rv$r <- foreach(icount(100)) %dopar% {
table <- kmeans(as.data.frame(df), centers = 3)
}
)
}
})
observeEvent(rv$listDF, {
output$done <- renderText("Done loading!")
})
observeEvent(rv$resultTime, {
output$time <- renderText(c("Done caltulating, time:", rv$resultTime))
print(rv$resultTime)
})
session$onSessionEnded(function() {
stopCluster(clu)
print("stopped")
})
})
ui.r
library(shiny)
shinyUI(fluidPage(
actionButton(
inputId = "loadDataButton",
label = "load data"
),
actionButton(
inputId = "clusterButton",
label = "Do clustering"
),
textOutput("done"),
textOutput("time")
))
iris15k.RData is a simple iris dataset with replicated rows to make the operation longer.
There is a way to write a bash script that will do the job and kill the processes in very straight way, but I'd rather do it from R side as I know little of it and it seems to be right to get it work withing an app code.
Any suggestions?