[Bug #19624] Clean up backquote IO

It should not be hidden, since it can be grabbed by a fiber scheduler.

io.c

@@ -5574,20 +5574,24 @@ clear_codeconv(rb_io_t *fptr)
     clear_writeconv(fptr);
 }
 
-void
-rb_io_fptr_finalize_internal(void *ptr)
+static void
+rb_io_fptr_cleanup_all(rb_io_t *fptr)
 {
-    rb_io_t *fptr = ptr;
-
-    if (!ptr) return;
     fptr->pathv = Qnil;
     if (0 <= fptr->fd)
         rb_io_fptr_cleanup(fptr, TRUE);
     fptr->write_lock = Qnil;
     free_io_buffer(&fptr->rbuf);
     free_io_buffer(&fptr->wbuf);
     clear_codeconv(fptr);
-    free(fptr);
+}
+
+void
+rb_io_fptr_finalize_internal(void *ptr)
+{
+    if (!ptr) return;
+    rb_io_fptr_cleanup_all(ptr);
+    free(ptr);
 }
 
 #undef rb_io_fptr_finalize
@@ -10467,11 +10471,9 @@ rb_f_backquote(VALUE obj, VALUE str)
     if (NIL_P(port)) return rb_str_new(0,0);
 
     GetOpenFile(port, fptr);
-    rb_obj_hide(port);
     result = read_all(fptr, remain_size(fptr), Qnil);
     rb_io_close(port);
-    RFILE(port)->fptr = NULL;
-    rb_io_fptr_finalize(fptr);
+    rb_io_fptr_cleanup_all(fptr);
     RB_GC_GUARD(port);
 
     return result;

test/fiber/test_io.rb

@@ -219,4 +219,19 @@ def test_io_select
       assert_equal [[r], [w], []], result
     end
   end
+
+  def test_backquote
+    result = nil
+
+    thread = Thread.new do
+      scheduler = Scheduler.new
+      Fiber.set_scheduler scheduler
+      Fiber.schedule do
+        result = `#{EnvUtil.rubybin} -e "sleep 0.1;puts %[ok]"`
+      end
+    end
+    thread.join
+
+    assert_equal "ok\n", result
+  end
 end