Description
Currently, all stack walking logic is in one venerable, large, and very, very complicated function: runtime.gentraceback. This function has three distinct operating modes: printing, populating a PC buffer, or invoking a callback. And it has three different modes of unwinding: physical Go frames, inlined Go frames, and cgo frames. It also has several flags. All of this logic is very interwoven.
I would like to replace all of this with a caller-driven iterator-style interface. This is a tracking issue for that change.
An iterator API will consolidate the logic for unwinding and allow us to lift out printing and pcbuf populating into separate code, while replacing the callback mode with direct use of the new API. It will allow us to better layer the different modes of unwinding by creating separate iterator types for physical, inlined, and cgo frames, while keeping the interface ergonomic. This is also a good opportunity to generally clean up this code.
As a follow-on, I plan to dramatically simplify the defer implementation. Regabi enabled many simplifications to defer and we've implemented many of them already, but there are more aggressive simplifications we haven't tackled yet. Part of this is simplifying open-coded defers, and doing that efficiently requires being able to simultaneously walk the stack frames and the defer stack. An iterator API will make this much easier to do.
An alternative approach would be to use a callback interface rather than an iterator. This would be an improvement over the status quo and also be a simpler change, but I think it has two drawbacks: 1. It makes layering physical/inlined frame unwinding more awkward because you need multiple levels of callbacks. 2. It's poorly suited to parallel iteration like we need for the open-coded defer implementation. Long term, an iterator API probably makes it simpler to scan a goroutine stack while the goroutine is still running (reducing per-goroutine latency for goroutines with large stacks) because we can easily pause and resume unwinding, while a callback API doesn't easily afford this opportunity.