nbrOfFreeWorkers() attempts to free up one parallel worker, if possible none are available [#774]

Author: HenrikBengtsson

DESCRIPTION

@@ -1,5 +1,5 @@
 Package: future
-Version: 1.58.0-9016
+Version: 1.58.0-9017
 Title: Unified Parallel and Distributed Processing in R for Everyone
 Depends:
     R (>= 3.2.0)

NEWS.md

@@ -8,6 +8,11 @@
    large number of CPU cores (e.g. 192 and 256 cores) while leaving 16
    connection slots available for other needs.
 
+## New Features
+
+ * `nbrOfFreeWorkers()` attempts to free up one parallel worker in a
+   non-blocking way, if possible and if none are available.
+
 ## Performance
 
  * The pre-validation of the cluster worker allotted to a future when

R/utils_api-nbrOfWorkers.R

@@ -146,6 +146,18 @@ nbrOfFreeWorkers.NULL <- function(evaluator, background = FALSE, ...) {
   backend <- plan("backend")
   if (!inherits(backend, "FutureBackend")) backend <- plan("next")
   res <- nbrOfFreeWorkers(backend, background = background, ...)
+
+  ## Attempt to free up a working, if none are left and possible
+  if (res == 0L) {
+    reg <- backend[["reg"]]
+    if (!is.null(reg)) {
+      FutureRegistry(reg, action = "collect-first", earlySignal = FALSE)
+      res0 <- res
+      res <- nbrOfFreeWorkers(backend, background = background, ...)
+      if (debug) mdebugf("Freed up %d workers", res - res0)
+    }
+  }
+
   res
 }