修复竞态条件导致的计数器错误

judge_dispatcher/models.py

@@ -16,16 +16,20 @@ class JudgeServer(models.Model):
     status = models.BooleanField(default=True)
 
     def use_judge_instance(self):
-        self.left_instance_number -= 1
-        self.workload = 100 - int(float(self.left_instance_number) / self.max_instance_number * 100)
-        print self.left_instance_number, self.workload
-        self.save()
+        # 因为use 和 release 中间是判题时间，可能这个 model 的数据已经被修改了，所以不能直接使用self.xxx，否则取到的是旧数据
+        server = JudgeServer.objects.select_for_update().get(id=self.id)
+        server.left_instance_number -= 1
+        server.workload = 100 - int(float(server.left_instance_number) / server.max_instance_number * 100)
+        print "\n ---- use", server.left_instance_number, server.workload
+        server.save()
 
     def release_judge_instance(self):
-        self.left_instance_number += 1
-        self.workload = 100 - int(float(self.left_instance_number) / self.max_instance_number * 100)
-        print self.left_instance_number, self.workload
-        self.save()
+        # 使用原子操作
+        server = JudgeServer.objects.select_for_update().get(id=self.id)
+        server.left_instance_number += 1
+        server.workload = 100 - int(float(server.left_instance_number) / server.max_instance_number * 100)
+        print "\n ---- release", server.left_instance_number, server.workload
+        server.save()
 
     class Meta:
         db_table = "judge_server"