Avoid GCing dead stack after switching away from a fiber

Fixes <https://bugs.ruby-lang.org/issues/14561> and discussed
<https://bugs.ruby-lang.org/issues/15362>.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66111 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

cont.c

@@ -1747,6 +1747,7 @@ fiber_store(rb_fiber_t *next_fib, rb_thread_t *th)
     return fib->cont.value;
 
 #else /* FIBER_USE_NATIVE */
+    fib->cont.saved_ec.machine.stack_end = NULL;
     if (ruby_setjmp(fib->cont.jmpbuf)) {
         /* restored */
         fib = th->ec->fiber_ptr;

test/ruby/test_fiber.rb

@@ -398,4 +398,14 @@ class TestFiber < Test::Unit::TestCase
     }.value
     assert_equal :ok, ret, '[Bug #14642]'
   end
+
+  def test_machine_stack_gc
+    assert_normal_exit <<-RUBY, '[Bug #14561]', timeout: 10
+      enum = Enumerator.new { |y| y << 1 }
+      thread = Thread.new { enum.peek }
+      thread.join
+      sleep 5     # pause until thread cache wait time runs out. Native thread exits.
+      GC.start
+    RUBY
+  end
 end