sin-ack's writings

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2024-07-07 09:00:04

I got the chance to investigate an interesting bug in SerenityOS this week. It was related to the decoding of JPG images in the operating system. For some reason, when a JPG image is viewed, it comes out like this:

And I remembered very well that JPG images worked just fine about a week or two ago, as I had set a JPG image as my background and would’ve noticed if it looked wrong.

Well, time to bisect! I didn’t know when to start, so I picked the last 1000 commits (where images showed up correctly), and started bisecting.

SerenityOS, being an operating system project that focuses on doing its own thing, also has its own standard library called AK (which stands for Agnostic Kit ). This library is analogous to C++’s STL, but is more readable due to not having to support a myriad of different operating systems and not having to contort oneself to conform to hideous coding standards.

One of the nice things about having the standard library in the same repository as its users is that making changes is very easy as the change propagates to everyone who pulls from master. However, this is a double edged sword when it comes to C++; because everyone includes the standard library (even if you don’t include it, your includes will), and because C++’s template system means that everything that’s templated has to include the definitions in the header as well, this means that anytime someone touches AK in a commit, the entire operating system has to be rebuilt (~3400 files at the time of writing). ccache, while being useful in many situations, cannot handle this case. Additionally, due to the breakneck pace of the SerenityOS project, someone ends up touching AK at least once every 100 commits or so.