Austin Clements from the Go team just landed a series of 17 patches. If everything goes well, you'll never notice that anything changed.
That being said, watching this come together has been mind blowing. This is serious engineering (TM).
🧵 Thread ...
From a high level point of view, these patches are "just" a refactoring of the stack trace implementation in Go.
The gnarly old gentraceback() function has been replace with a new iterator API that is more flexible, yet easier to understand.
To get why this is amazing engineering, first consider the old code.
gentraceback was a 500+ LoC monster function that will make you weep if you try to read it. And that's even before you realize how incredibly load-bearing it is.
https://github.com/golang/go/blob/go1.20.1/src/runtime/traceback.go#L24-L563
When I say load-bearing, I mean that without it, the GC won't be able to scan the stacks of your goroutines, and your goroutines won't be able to grow their stacks (which needs pointer adjustments).
It's an absolutely vital organ of the Go runtime.
Any bug in this function has a chance to break all Go applications.
If you're doubtful, look no further than some recent issue that caused Go programs on arm64 to become completely unresponsive due to gentraceback getting stuck in an infinite loop.
Speaking of arm64, it's part of why this function is complicated. ARM is a so called link architecture which works very different from intel when it comes to function calls.
A register, rather than the stack, is updated when CALLing a function. More here
Now let's explore why it's so difficult to refactor this function. The first reason is that it's insanely difficult to debug this code when you break it.
It'll produce the most cryptic panics, crashes, corruptions and issues you can imagine.
Last but not least: Breaking up such a complex refactoring is necessary in order to give somebody a chance to review it.
And this makes everything harder. What if you're 10 patches in, and you notice a mistake in patch 3? It's possible, but very hard to make these kind of edits.
I've still skipped over many other aspects, such as designing a nice API, dealing with inlining, etc. - but hopefully you can see why this is really gnarly stuff to be hacking on.
So now look at these 17 patches again. It's a real work of art 🙇🏻♂️.
If you need a comparison (or laugh), feel free to look at my pathetic attempt from last year to do the same refactoring 🤣.
I spent a week on it and produced 54 chaotic patches that didn't even get me half way to a full iterator API.
So again, 👏🏻 to Austin Clements for this incredible work, but also
@prattmic for some fantastic review comments.
The people working on the Go runtime team are really incredible. ❤️
💐RIP gentraceback
2010-03-23 - 2023-03-10
Thank you for all the unwinding.
@felixge 🫡 there's no better attitude to replacing code than being grateful for what it's done. It's probably been executed billions (trillions?) of times.
Thanks for the thread btw, wasn't aware of this awesome work!
Felix Geisendörfer
Jan Lehnardt 
Javier Honduvilla Coto