merge revision(s) 66111: [Backport #14561]

	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/branches/ruby_2_5@66777 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

cont.c

@@ -1616,6 +1616,7 @@ 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

@@ -378,4 +378,14 @@ def test_to_s
     assert_match(/terminated/, f.to_s)
     assert_match(/resumed/, Fiber.current.to_s)
   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

version.h

@@ -1,10 +1,10 @@
 #define RUBY_VERSION "2.5.4"
-#define RUBY_RELEASE_DATE "2019-01-07"
-#define RUBY_PATCHLEVEL 125
+#define RUBY_RELEASE_DATE "2019-01-10"
+#define RUBY_PATCHLEVEL 126
 
 #define RUBY_RELEASE_YEAR 2019
 #define RUBY_RELEASE_MONTH 1
-#define RUBY_RELEASE_DAY 7
+#define RUBY_RELEASE_DAY 10
 
 #include "ruby/version.h"