Being an engine programmer usually means being a bit of a jack of all trades. There’s always something weird going on and you have to be pretty familiar with a bunch of low level details that come in handy in unexpected ways. Recently I went down a somewhat unexpected rabbit hole where those skills came in extremely hand. In an effort to blog more and also because it seems like I was the first to run into this issue, I figured I should sit down and just write about it so future people can benefit from it.

X-Plane’s plugin systems allows authors to load models in two ways: Asynchronously and Synchronously. Most plugins tend to use synchronous loading, but since all plugins run on the main thread there is a need and desire for some to use asynchronous loading. Up until now, the latter was broken however, and plugin authors complained about invisible models. From what I understand Pilot Edge were the first to complain, but they want to use async loading to dynamically load in models of planes that flew in via multiplayer, which makes it non-trivial to debug. Luckily, the author of the fantastic Better Pushback also ran into the issue and his plugin is completely local, so it’s no problem to stop at a debugger for any amount of time without being worried a multiplayer socket closes and the whole setup has to be recreated.

This post is about a crash to desktop that I investigated in a popular plugin for X-Plane, X-Assign. This happened in my free time, although I had the advantage of having the X-Plane source code at hand.

Git bisect

My new favourite tool on earth is git bisect, which I used to find the offending commit rather quickly. At this point I wasn’t sure who was at fault for the crash, X-Plane or X-Assign, since the issue only showed up with the update to X-Plane 11.02 and it was working fine in previous versions. The offending commit however turned out to be rather boring, it simply changed the capacity of a couple of datarefs from 100 to 250. Two things about that were interesting though, first of all, the capacity of the underlying variable was already 250, a change introduced in X-Plane 11. Second of all, those datarefs were input related, namely sim/joystick/joystick_axis_values, sim/joystick/joystick_axis_assignments and sim/joystick/joystick_axis_reverse. So not unreasonable that they would be used by X-Assign. The change however, shouldn’t have really broke X-Assign in any way. To figure out what happened, I did was every reasonable person would do: I opened up the disassembler!