Fix handling output file in DOMjudge reactive problem

Previously, DOMJudgeReactiveRunner ignored output file because
reactive problems don't usually have output, and interactor don't
use it.

However, when packing zip to upload, diff file is mandatory, so
even though it just creates an empty file, DOMJudgeReactiveRunner
should make sure diff file is created so that later pack and upload
would succeed.

Fixes icpc-jag#102.

rime/plugins/judge_system/domjudge.py

@@ -6,7 +6,6 @@
 import shutil
 import signal
 import subprocess
-import tempfile
 import threading
 import time
 
@@ -213,17 +212,17 @@ def Run(self, reactive, args, cwd, input, output, timeout, precise):
         if os.path.exists(feedback_dir_name):
             shutil.rmtree(feedback_dir_name)
         os.makedirs(feedback_dir_name, exist_ok=True)
-        # 2nd argument is an "expected output" file, which is not supported
-        # in rime interactive for now.
-        # As a placeholder, using a temporary file.
-        with tempfile.NamedTemporaryFile() as tmpfile:
-            judge_args = reactive.run_args + \
-                (input, tmpfile.name, feedback_dir_name, )
-            solution_args = args
-            task = DOMJudgeReactiveTask(
-                judge_args, solution_args,
-                cwd=cwd, timeout=timeout, exclusive=precise)
-            (judge_proc, solution_proc) = yield task
+
+        # Makes sure output file exists.
+        open(output, 'w').close()
+
+        judge_args = reactive.run_args + \
+            (input, output, feedback_dir_name, )
+        solution_args = args
+        task = DOMJudgeReactiveTask(
+            judge_args, solution_args,
+            cwd=cwd, timeout=timeout, exclusive=precise)
+        (judge_proc, solution_proc) = yield task
 
         judge_code = judge_proc.returncode
         solution_code = solution_proc.returncode