Allow disabling the use of vthread fibers

In case there's an issue with vthread fibers that we have not yet
seen, this allows disabling them and going back to normal native
thread fibers.

core/src/main/java/org/jruby/ext/fiber/ThreadFiber.java

@@ -19,6 +19,7 @@
 import org.jruby.runtime.ThreadContext;
 import org.jruby.runtime.Visibility;
 import org.jruby.runtime.builtin.IRubyObject;
+import org.jruby.util.cli.Options;
 import org.jruby.util.log.Logger;
 import org.jruby.util.log.LoggerFactory;
 
@@ -38,28 +39,29 @@ public class ThreadFiber extends RubyObject implements ExecutionContext {
     private static final MethodHandle VTHREAD_START_METHOD;
 
     static {
-        BiConsumer<Ruby, Runnable> fiberLauncher;
-        MethodHandle start;
+        BiConsumer<Ruby, Runnable> fiberLauncher = ThreadFiber::nativeThreadLauncher;
+        MethodHandle start = null;
 
-        try {
-            // test that API is available
-            Method ofVirtualMethod = Thread.class.getMethod("ofVirtual");
-            Object builder = ofVirtualMethod.invoke(null);
-            Method startMethod = Class.forName("java.lang.Thread$Builder").getMethod("start", Runnable.class);
+        if (Options.FIBER_VTHREADS.load()) {
+            try {
+                // test that API is available
+                Method ofVirtualMethod = Thread.class.getMethod("ofVirtual");
+                Object builder = ofVirtualMethod.invoke(null);
+                Method startMethod = Class.forName("java.lang.Thread$Builder").getMethod("start", Runnable.class);
 
-            start = MethodHandles.publicLookup().unreflect(startMethod).bindTo(builder);
+                start = MethodHandles.publicLookup().unreflect(startMethod).bindTo(builder);
 
-            fiberLauncher = new VirtualThreadLauncher();
-        } catch (Throwable t) {
-            start = null;
-            fiberLauncher = ThreadFiber::accept;
+                fiberLauncher = new VirtualThreadLauncher();
+            } catch (Throwable t) {
+                // default impl set below
+            }
         }
 
         VTHREAD_START_METHOD = start;
         FIBER_LAUNCHER = fiberLauncher;
     }
 
-    private static void accept(Ruby runtime, Runnable runnable) {
+    private static void nativeThreadLauncher(Ruby runtime, Runnable runnable) {
         runtime.getFiberExecutor().submit(runnable);
     }
 

core/src/main/java/org/jruby/util/cli/Category.java

@@ -40,7 +40,7 @@ public enum Category {
     JIT("jit"),
     IR("intermediate representation"),
     NATIVE("native"),
-    THREADPOOL("thread pooling"),
+    THREADPOOL("thread pooling and launching"),
     MISCELLANEOUS("miscellaneous"),
     DEBUG("debugging and logging"),
     JAVA_INTEGRATION("java integration"),

core/src/main/java/org/jruby/util/cli/Options.java

@@ -129,6 +129,7 @@ public class Options {
     public static final Option<Integer> THREADPOOL_MAX = integer(THREADPOOL, "thread.pool.max", Integer.MAX_VALUE, "The maximum number of threads to allow in the pool.");
     public static final Option<Integer> THREADPOOL_TTL = integer(THREADPOOL, "thread.pool.ttl", 60, "The maximum number of seconds to keep alive an idle thread.");
     public static final Option<Integer> FIBER_THREADPOOL_TTL = integer(THREADPOOL, "fiber.thread.pool.ttl", 60, "The maximum number of seconds to keep alive a pooled fiber thread.");
+    public static final Option<Boolean> FIBER_VTHREADS = bool(THREADPOOL, "fiber.vthread", true, "Use JDK virtual threads for Ruby Fibers.");
 
     public static final Option<Boolean> CLASSLOADER_DELEGATE = bool(MISCELLANEOUS, "classloader.delegate", true, "In some cases of classloader conflicts it might help not to delegate first to the parent classloader but to load first from the jruby-classloader.");
     public static final Option<Boolean> OBJECTSPACE_ENABLED = bool(MISCELLANEOUS, "objectspace.enabled", false, "Enable or disable ObjectSpace.each_object.");