8023103: FJ parallelism zero

8020537: java/util/concurrent/forkjoin/ThrowingRunnable.java
Reviewed-by: chegar, mduigou, alanb

src/share/classes/java/util/concurrent/ForkJoinPool.java

@@ -144,7 +144,8 @@ import java.util.concurrent.TimeUnit;
  * Upon any error in establishing these settings, default parameters
  * are used. It is possible to disable or limit the use of threads in
  * the common pool by setting the parallelism property to zero, and/or
- * using a factory that may return {@code null}.
+ * using a factory that may return {@code null}. However doing so may
+ * cause unjoined tasks to never be executed.
  *
  * <p><b>Implementation notes</b>: This implementation restricts the
  * maximum number of running threads to 32767. Attempts to create
@@ -3303,8 +3304,8 @@ public class ForkJoinPool extends AbstractExecutorService {
         }
 
         if (parallelism < 0 && // default 1 less than #cores
-            (parallelism = Runtime.getRuntime().availableProcessors() - 1) < 0)
-            parallelism = 0;
+            (parallelism = Runtime.getRuntime().availableProcessors() - 1) <= 0)
+            parallelism = 1;
         if (parallelism > MAX_CAP)
             parallelism = MAX_CAP;
         return new ForkJoinPool(parallelism, factory, handler, LIFO_QUEUE,