Set the thread current fiber in initializeThread()

Author: eregon

src/main/java/org/truffleruby/RubyLanguage.java

@@ -630,8 +630,9 @@ public void initializeThread(RubyContext context, Thread thread) {
                             .shouldNotReachHere("Ruby threads should be initialized on their Java thread");
                 }
                 context.getThreadManager().start(rubyThread, thread);
-            } else {
-                // Fiber
+            } else { // (non-root) Fiber
+                var fiber = this.rubyFiber.get(thread);
+                rubyThread.setCurrentFiber(fiber);
             }
             return;
         }

src/main/java/org/truffleruby/core/fiber/FiberManager.java

@@ -136,9 +136,6 @@ private void beforeEnter(RubyFiber fiber, Node currentNode) {
         } catch (InterruptedException e) {
             throw CompilerDirectives.shouldNotReachHere("unexpected interrupt in Fiber beforeEnter()");
         }
-
-        // enter() polls so we need the current Fiber to be set before enter()
-        fiber.rubyThread.setCurrentFiber(fiber);
     }
 
     private void fiberMain(RubyContext context, RubyFiber fiber, RubyProc block, Node currentNode) {