rb_fiber_terminate must not return [Bug #18497]

In a forked process from a fiber, the fiber becomes the only
fiber, `fiber_switch` does nothing as there is no other fibers,
`rb_fiber_terminate` does not terminate the fiber.  In that case,
reaches the end of `fiber_entry` finaly, which is declared as
"COROUTINE" and should never return.

cont.c

@@ -2039,7 +2039,7 @@ rb_fiber_set_scheduler(VALUE klass, VALUE scheduler)
     return rb_fiber_scheduler_set(scheduler);
 }
 
-static void rb_fiber_terminate(rb_fiber_t *fiber, int need_interrupt, VALUE err);
+NORETURN(static void rb_fiber_terminate(rb_fiber_t *fiber, int need_interrupt, VALUE err));
 
 void
 rb_fiber_start(rb_fiber_t *fiber)
@@ -2408,6 +2408,7 @@ rb_fiber_terminate(rb_fiber_t *fiber, int need_interrupt, VALUE error)
         fiber_switch(next_fiber, -1, &error, RB_NO_KEYWORDS, NULL, false);
     else
         fiber_switch(next_fiber, 1, &value, RB_NO_KEYWORDS, NULL, false);
+    ruby_stop(0);
 }
 
 static VALUE

eval_intern.h

@@ -287,7 +287,7 @@ VALUE rb_make_exception(int argc, const VALUE *argv);
 
 NORETURN(void rb_method_name_error(VALUE, VALUE));
 
-void rb_fiber_start(rb_fiber_t*);
+NORETURN(void rb_fiber_start(rb_fiber_t*));
 
 NORETURN(void rb_print_undef(VALUE, ID, rb_method_visibility_t));
 NORETURN(void rb_print_undef_str(VALUE, VALUE));

test/fiber/test_process.rb

@@ -33,4 +33,19 @@ class TestFiberProcess < Test::Unit::TestCase
       end
     end.join
   end
+
+  def test_fork
+    omit 'fork not supported' unless Process.respond_to?(:fork)
+    Thread.new do
+      scheduler = Scheduler.new
+      Fiber.set_scheduler scheduler
+
+      Fiber.schedule do
+        pid = Process.fork {}
+        Process.wait(pid)
+
+        assert_predicate $?, :success?
+      end
+    end.join
+  end
 end

test/ruby/test_fiber.rb

@@ -407,6 +407,11 @@ class TestFiber < Test::Unit::TestCase
     pid, status = Process.waitpid2(pid)
     assert_not_predicate(status, :signaled?, bug5700)
     assert_predicate(status, :success?, bug5700)
+
+    pid = Fiber.new {fork}.resume
+    pid, status = Process.waitpid2(pid)
+    assert_not_predicate(status, :signaled?)
+    assert_predicate(status, :success?)
   end
 
   def test_exit_in_fiber