Consider the root Ruby thread to not be isRubyManagedThread() (since it exists before)

* Necessary for MiscTest#timeoutExecution() where the thread running
  finalizeContext() must not wait for the main Java thread.

Author: eregon

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

@@ -316,9 +316,8 @@ public void start(RubyFiber fiber, Thread javaThread) {
 
         if (Thread.currentThread() == javaThread) {
             context.getThreadManager().rubyFiber.set(fiber);
-        }
-        if (!threadManager.isRubyManagedThread(javaThread)) {
-            context.getThreadManager().rubyFiberForeignMap.put(javaThread, fiber);
+        } else {
+            context.getThreadManager().javaThreadToRubyFiber.put(javaThread, fiber);
         }
 
         fiber.thread = javaThread;
@@ -345,7 +344,7 @@ public void cleanup(RubyFiber fiber, Thread javaThread) {
         if (Thread.currentThread() == javaThread) {
             threadManager.rubyFiber.remove();
         }
-        threadManager.rubyFiberForeignMap.remove(javaThread);
+        threadManager.javaThreadToRubyFiber.remove(javaThread);
 
         fiber.finishedLatch.countDown();
     }

src/main/java/org/truffleruby/core/thread/ThreadManager.java

@@ -77,11 +77,12 @@ public class ThreadManager {
 
     private final Set<RubyThread> runningRubyThreads = Collections.newSetFromMap(new ConcurrentHashMap<>());
 
+    /** The set of Java threads TruffleRuby created, and is responsible to exit in {@link #killAndWaitOtherThreads()} */
     private final Set<Thread> rubyManagedThreads = Collections.newSetFromMap(new ConcurrentHashMap<>());
 
-    public final Map<Thread, RubyFiber> rubyFiberForeignMap = new ConcurrentHashMap<>();
+    public final Map<Thread, RubyFiber> javaThreadToRubyFiber = new ConcurrentHashMap<>();
     public final ThreadLocal<RubyFiber> rubyFiber = ThreadLocal
-            .withInitial(() -> rubyFiberForeignMap.get(Thread.currentThread()));
+            .withInitial(() -> javaThreadToRubyFiber.get(Thread.currentThread()));
 
     private boolean nativeInterrupt;
     private Timer nativeInterruptTimer;
@@ -116,13 +117,11 @@ public void dispose() {
 
     public void initializeMainThread(Thread mainJavaThread) {
         rootJavaThread = mainJavaThread;
-        rubyManagedThreads.add(rootJavaThread);
         start(rootThread, rootJavaThread);
     }
 
     public void resetMainThread() {
         cleanup(rootThread, rootJavaThread);
-        rubyManagedThreads.remove(rootJavaThread);
         rootJavaThread = null;
     }