Remove from waiter in Mutex#lock with ensure when calling rb_scheduler_block()

* Previously this could lead to an invalid waiter entry and then trying
  to wake up that waiter would result in various issues in rb_mutex_unlock_th().

test/fiber/test_mutex.rb

@@ -70,6 +70,38 @@ class TestFiberMutex < Test::Unit::TestCase
     thread.join
   end
 
+  def test_mutex_fiber_raise
+    mutex = Mutex.new
+    ran = false
+
+    main = Thread.new do
+      mutex.lock
+
+      thread = Thread.new do
+        scheduler = Scheduler.new
+        Thread.current.scheduler = scheduler
+
+        f = Fiber.schedule do
+          assert_raise_with_message(RuntimeError, "bye") do
+            assert_same scheduler, Thread.scheduler
+            mutex.lock
+          end
+          ran = true
+        end
+
+        Fiber.schedule do
+          f.raise "bye"
+        end
+      end
+
+      thread.join
+    end
+
+    main.join # causes mutex to be released
+    assert_equal false, mutex.locked?
+    assert_equal true, ran
+  end
+
   def test_condition_variable
     mutex = Mutex.new
     condition = ConditionVariable.new

thread_sync.c

@@ -214,18 +214,17 @@ VALUE
 rb_mutex_trylock(VALUE self)
 {
     rb_mutex_t *mutex = mutex_ptr(self);
-    VALUE locked = Qfalse;
 
     if (mutex->fiber == 0) {
 	rb_fiber_t *fiber = GET_EC()->fiber_ptr;
 	rb_thread_t *th = GET_THREAD();
 	mutex->fiber = fiber;
-	locked = Qtrue;
 
 	mutex_locked(th, self);
+	return Qtrue;
     }
 
-    return locked;
+    return Qfalse;
 }
 
 /*
@@ -246,6 +245,16 @@ mutex_owned_p(rb_fiber_t *fiber, rb_mutex_t *mutex)
     }
 }
 
+static VALUE call_rb_scheduler_block(VALUE mutex) {
+    return rb_scheduler_block(rb_thread_current_scheduler(), mutex);
+}
+
+static VALUE remove_from_mutex_lock_waiters(VALUE arg) {
+    struct list_node *node = (struct list_node*)arg;
+    list_del(node);
+    return Qnil;
+}
+
 static VALUE
 do_mutex_lock(VALUE self, int interruptible_p)
 {
@@ -276,9 +285,7 @@ do_mutex_lock(VALUE self, int interruptible_p)
             if (scheduler != Qnil) {
                 list_add_tail(&mutex->waitq, &w.node);
 
-                rb_scheduler_block(scheduler, self);
+                rb_ensure(call_rb_scheduler_block, self, remove_from_mutex_lock_waiters, (VALUE)&w.node);
-
-                list_del(&w.node);
 
                 if (!mutex->fiber) {
                     mutex->fiber = fiber;