api: Update concurrency

* R/api.R(concurrency): Updated. Add comments and allow the plan to be
  set from outside shinyscreen (in accordance with the policy of the
  future package that users should have the control). Also, make
  debugging slightly easier by substituting future::future call with
  identity when debugging.

R/api.R

@@ -172,10 +172,33 @@ verify_data <- function(conf,all_sets) {
 
 ## @export
 concurrency <- function(m) {
-    m$conf$workers <- if (!is.null(m$conf$workers)) m$conf$workers else NO_WORKERS
-    
-    future::plan("multiprocess",workers=m$conf$workers)
-    message("workers: ",m$conf$workers)
+    ## Reads the concurrency entry in the config. It is optional, if
+    ## not given, then it is up to the user to define the plan of the
+    ## futures package. If present, it contains at least the `plan'
+    ## specification. It can also contain `workers` entry specifying
+    ## the number of workers. If that entry is absent, the default
+    ## number of workers is NO_WORKERS from the resources.R.
+    workers <- m$conf$concurrency$workers
+    plan <- m$conf$concurrency$plan
+    if (!is.null(plan)) {
+        n <- if (!is.null(workers)) workers else NO_WORKERS
+        if (!is.na(n)) future::plan(plan,workers=workers) else future::plan(plan)
+        m$conf$concurrency$workers <- n
+
+    } else {
+        m$conf$concurrency$workers <- NA
+        m$conf$concurrency$plan <- "user"
+    }
+    message("plan: ",m$conf$concurrency$plan)
+    message("workers: ",m$conf$concurrency$workers)
+
+    ## So we can actually debug.
+    m$future <- if (!m$conf$debug)
+                    future::future
+                else {
+                    message("Debug: futures evaluate as identity")
+                    function(x,...) identity(x)
+                }
     m
 }