Capture unexpected future-launch errors

Author: HenrikBengtsson

Diff for: DESCRIPTION

@@ -1,5 +1,5 @@
 Package: future
-Version: 1.40.0-9002
+Version: 1.40.0-9003
 Title: Unified Parallel and Distributed Processing in R for Everyone
 Depends:
     R (>= 3.2.0)

Diff for: NAMESPACE

@@ -138,6 +138,7 @@ export(FutureError)
 export(FutureGlobals)
 export(FutureInterruptError)
 export(FutureJournalCondition)
+export(FutureLaunchError)
 export(FutureMessage)
 export(FutureResult)
 export(FutureWarning)

Diff for: NEWS.md

@@ -4,6 +4,16 @@
 
  * Now `print()` for `Future` reports also on the current state of the
    future, e.g. 'created', 'running', 'finished', and 'interrupted'.
+
+## Bug Fixes
+
+ * In rare cases, a future backend might fails to launch a future and
+   at the same time fail to handle such errors. That would result in
+   hard-to-understand, obscure errors. Now these errors are detected
+   by the **future** package and resignaled as informative errors of
+   class `FutureLaunchError`. This also allows the future that failed
+   to be launched to be reset and relaunched again, possible on
+   another future backend.
 
 
 # Version 1.40.0 [2025-04-10]

Diff for: R/backend_api-Future-class.R

@@ -443,7 +443,18 @@ run.Future <- function(future, ...) {
     if (debug) mdebug_push("Launching futures ...")
     future[["backend"]] <- backend
     future[["start"]] <- proc.time()[[3]]
-    future2 <- launchFuture(backend, future = future)
+    future2 <- tryCatch(
+      launchFuture(backend, future = future)
+    , FutureError = function(ex) {
+      ## Known error caught by the future backend
+      stop(ex)
+    }, error = function(ex) {
+      ## Unexpected error
+      msg <- conditionMessage(ex)
+      label <- sQuoteLabel(future[["label"]])
+      msg <- sprintf("Caught an unexpected error of class %s when trying to launch future (%s) on backend of class %s. The reason was: %s", class(ex)[1], label, class(backend)[1], msg)
+      stop(FutureLaunchError(msg, future = future))
+    })
     if (debug) mdebug_pop("Launching futures ... done")
     if (debug) mdebug("Future launched: ", commaq(class(future2)))
     stop_if_not(inherits(future2, "Future"))

Diff for: R/protected_api-FutureCondition-class.R

@@ -315,11 +315,21 @@ DeviceMisuseFutureError <- function(...) {
 }
 
 
+
+#' @rdname FutureCondition
+#' @export
+FutureLaunchError <- function(..., future = NULL) {
+  cond <- FutureError(..., future = future)
+  class <- c("FutureLaunch", class(cond))
+  class(cond) <- class[!duplicated(class, fromLast = TRUE)]
+  cond
+}
+
 #' @rdname FutureCondition
 #' @export
 FutureInterruptError <- function(..., future = NULL) {
   cond <- FutureError(..., future = future)
-  class <- c("FutureInterruptError", "FutureError", class(cond))
+  class <- c("FutureInterruptError", class(cond))
   class(cond) <- class[!duplicated(class, fromLast = TRUE)]
   cond
 }
@@ -329,7 +339,7 @@ FutureInterruptError <- function(..., future = NULL) {
 #' @export
 FutureDroppedError <- function(..., future = NULL) {
   cond <- FutureError(..., future = future)
-  class <- c("FutureDroppedError", "FutureError", class(cond))
+  class <- c("FutureDroppedError", class(cond))
   class(cond) <- class[!duplicated(class, fromLast = TRUE)]
   cond
 }