Dynamic Type System Trouble

This week I really learned to appreciate my Java compiler. I learned it the hard way – by not using it. In the last game that we released (Battlefield 4) I have implemented a feature for our players which suggests 3 game items to progress on, i.e. a weapon to unlock, an assignment that should be finished etc. Our internal name for this feature is “Suggestions”. A player would not only see these 3 items but also see his own progress towards reaching the suggestion goal of each item. The code that calculates the 3 items has become quite complex since there are a lot of different item types that we can pick from and we need to match each player individually. The code is written in Python, my favorite language at this point, which uses a dynamic type system.

The “Suggestions” feature was tested thoroughly and worked quite well in production. I implemented some additional functionality on top. Players now also had the opportunity to manually pick individual items so they could see their progress in the game and on our companion website Battlelog. Unfortunately after a few weeks players complained about strange problems. These players would see completely random items being suggested to them – even with the progression totally being off. In some cases, players got items suggested that they had completed or unlocked. These errors happened completely random. Not able to reproduce in any of our test systems. But it was happening mostly to players that played the game a lot. So I started to investigate.

No unit test was broken and also a long code review did not surface any problems. Fortunately we have very short release cycles. So I added some additional logging to this functionality, which was released to production earlier this week. This finally got me something! I could see that in some rare cases the function, which calculates the suggested items of a player, returns not just 3 but more: 4, 5, 6 sometimes 9 items! I am posting you a ridiculous simplified version of the code below. Try to spot the problem.

I should also tell you, that an instance of the SuggestionService is shared. The Service is used in an application which uses gevent. There are many Greenlets (lightweight Threads) which call the suggest method simultaneously. Ring ring – multithreading issue! The problem is in Line 10, where two parentheses are missing. Instead of creating an instance of the ProgressSuggestions class every time the suggest method is called, the code gets a reference to the ProgressSuggestions class and assigns it to a variable called progress. Then, on the first invocation, it dynamically adds a suggestions class field to that class. Something that would neither be possible nor compile in a statically typed language like Java. All Greenlets modify the same class instance, so player’s suggestions can overwrite each other. The simple fix is to create an instance of the ProgressSuggestions class as it was intended. I am surprised that this bug could live so long. In a real multithreaded application this would have affected much more players. Greenlets are only semi parallel. They must yield at a bad time to trigger this problem. Here is the correct version.