fix issue #1658

DESCRIPTION

@@ -2,7 +2,7 @@ Package: brms
 Encoding: UTF-8
 Type: Package
 Title: Bayesian Regression Models using 'Stan'
-Version: 2.21.4
+Version: 2.21.5
 Date: 2024-05-27
 Authors@R:
     c(person("Paul-Christian", "Bürkner", email = "[email protected]",

NEWS.md

@@ -8,6 +8,9 @@
 
 * Fix a bug that led to partially duplicated Stan code in multilevel terms
 thanks to Henrik Singmann. (#1651)
+* Fix problems with parallel executions of post-processing functions 
+sometimes leaving unused R instances behind. Thanks to Andrew Johnson,
+Aki Vehtari, and Noa Kallioinen. (#1658)
 
 ### Other Changes
 

R/misc.R

@@ -425,13 +425,16 @@ plapply <- function(X, FUN, cores = 1, ...) {
   if (cores == 1) {
     out <- lapply(X, FUN, ...)
   } else {
-    if (!os_is_windows()) {
-      out <- parallel::mclapply(X = X, FUN = FUN, mc.cores = cores, ...)
-    } else {
-      cl <- parallel::makePSOCKcluster(cores)
-      on.exit(parallel::stopCluster(cl))
-      out <- parallel::parLapply(cl = cl, X = X, fun = FUN, ...)
-    }
+    cl_type <- ifelse(os_is_windows(), "PSOCK", "FORK")
+    cl <- parallel::makeCluster(cores, type = cl_type)
+    # Register a cleanup for the cluster in case the function fails
+    # Need to wrap in a tryCatch to avoid error if cluster is already stopped
+    on.exit(tryCatch(
+      { parallel::stopCluster(cl) },
+      error = function(e) invisible(NULL)
+    ))
+    out <- parallel::parLapply(cl = cl, X = X, fun = FUN, ...)
+    parallel::stopCluster(cl)
   }
   out
 }