Wake up join list within thread EC context. (#4471)

* Wake up join list within thread EC context.

* Consume items from join list so that they are not re-executed.

If `rb_fiber_scheduler_unblock` raises an exception, it can result in a
segfault if `rb_threadptr_join_list_wakeup` is not within a valid EC. This
change moves `rb_threadptr_join_list_wakeup` into the thread's top level EC
which initially caused an infinite loop because on exception will retry. We
explicitly remove items from the thread's join list to avoid this situation.

* Verify the required scheduler interface.

* Test several scheduler hooks methods with broken `unblock` implementation.

1 files changed, 16 insertions, 2 deletions

diff --git a/test/fiber/test_scheduler.rb b/test/fiber/test_scheduler.rb
index eeb0d67ec5..f0f5b79f36 100644
--- a/test/fiber/test_scheduler.rb
+++ b/test/fiber/test_scheduler.rb
@@ -66,9 +66,23 @@ class TestFiberScheduler < Test::Unit::TestCase
     RUBY
   end
 
-  def test_optional_close
+  def test_minimal_interface
+    scheduler = Object.new
+
+    def scheduler.block
+    end
+
+    def scheduler.unblock
+    end
+
+    def scheduler.io_wait
+    end
+
+    def scheduler.kernel_sleep
+    end
+
     thread = Thread.new do
-      Fiber.set_scheduler Object.new
+      Fiber.set_scheduler scheduler
     end
 
     thread.join