Saturday, August 5, 2017

Assert your assumptions.

A lot of software bugs are caused by the following scenario: The programmer assumed a certain situation and wrote code that would rely on this assumption. Very often, these mental assumptions are never made explicit, and live in the programmer's mind only. A standard weapon against these bugs in the arsenal of the coder is the assertion.

When working in teams I would often argue for the use of asserts in production code, to the astonishment of my colleagues. The most frequent objection to this would be the performance impact. However, the 98% of your code that is high level will never show a perceivable performance impact from asserts. The remaining 2% you just disable the asserts in non-debug builds. The inner loop of the physics solver, the inner loop of the collider, the inner loop of the path planner. Sure, leave them out there. For all other code? Ship your product with asserts! You'll be amazed at what gets triggered.

To take the controversy a small step further: I also firmly believe in making the asserts user-facing. Let's take Android apps for instance... instead of having Android report "Foo stopped working" I prefer to show a dialog with the assertion that failed, the file name and line nr. By doing this you can leverage your customer base to improve your app's stability. A certain user may detect the pattern: "Every time I do X after doing Y, I get a crash with message Z." Exactly the information the developer needs to fix the bug!

And just for good measure, (or is it overkill?) I took it further again: and sometimes will even show callstacks to users. For instance, a steam user is quickly able to screencapture and post it on steam, or email it to the dev. So much valuable info.

So last month, I was noticing crashes reported by Google Play on my Android app. And not always will users contact you with the crash report, so I even with pulling in the user for crash reports, the right information would not always reach me. One solution is to adopt a heavyweight framework to do these reports for me. Link to an analytics framework. But it's possible to make a very lightweight solution yourself.

I was interested in the assert message only. No need for user info, device info, anything. Just the assert message with source code line number. And this is very small data. Smaller than an UDP packet!

So I coded a minimalistic assert report server to run on Amazon's EC2. Just a few lines of Python code, where a socketserver.UDPServer prints out the datagrams it receives.

Client side, it is a little bit more code. Mine is in C, as I make NDK based Android Apps, but in Java it is even simpler. This slightly verbose C code will do the trick to open a socket and send a single datagram to my assert report server.

I am happy to report the assert report server is doing its job, and helps me to get on top of the crashes in my game. For future improvement, I should probably add an app version number to the assert message, so I will know if it is an old issue or not. Thank you for reading, and let me know what you think of assertions, and if you run assertions in your production code or not.