Fix Mutex#unlock with a scheduler and thread contention

* It would hit "[BUG] unexpected THREAD_STOPPED" before.

test/fiber/test_mutex.rb

@@ -47,6 +47,29 @@ class TestFiberMutex < Test::Unit::TestCase
     thread.join
   end
 
+  def test_mutex_thread
+    mutex = Mutex.new
+    mutex.lock
+
+    thread = Thread.new do
+      scheduler = Scheduler.new
+      Thread.current.scheduler = scheduler
+
+      Fiber.schedule do
+        mutex.lock
+        sleep 0.1
+        mutex.unlock
+      end
+
+      scheduler.run
+    end
+
+    sleep 0.1
+    mutex.unlock
+
+    thread.join
+  end
+
   def test_condition_variable
     mutex = Mutex.new
     condition = ConditionVariable.new

thread_sync.c

@@ -402,20 +402,20 @@ rb_mutex_unlock_th(rb_mutex_t *mutex, rb_thread_t *th, rb_fiber_t *fiber)
 
             if (cur->th->scheduler != Qnil) {
                 rb_scheduler_unblock(cur->th->scheduler, cur->self, rb_fiberptr_self(cur->fiber));
+            } else {
+                switch (cur->th->status) {
+                  case THREAD_RUNNABLE: /* from someone else calling Thread#run */
+                  case THREAD_STOPPED_FOREVER: /* likely (rb_mutex_lock) */
+                    rb_threadptr_interrupt(cur->th);
+                    goto found;
+                  case THREAD_STOPPED: /* probably impossible */
+                    rb_bug("unexpected THREAD_STOPPED");
+                  case THREAD_KILLED:
+                    /* not sure about this, possible in exit GC? */
+                    rb_bug("unexpected THREAD_KILLED");
+                    continue;
+                }
             }
-
-	    switch (cur->th->status) {
-	      case THREAD_RUNNABLE: /* from someone else calling Thread#run */
-	      case THREAD_STOPPED_FOREVER: /* likely (rb_mutex_lock) */
-		rb_threadptr_interrupt(cur->th);
-		goto found;
-	      case THREAD_STOPPED: /* probably impossible */
-		rb_bug("unexpected THREAD_STOPPED");
-	      case THREAD_KILLED:
-                /* not sure about this, possible in exit GC? */
-		rb_bug("unexpected THREAD_KILLED");
-		continue;
-	    }
 	}
       found:
 	while (*th_mutex != mutex) {