> The problem record and replay is expansions of languages and apis too. That is a good thing for some things but it needs to be reworded sometimes too and implementations of things aren't always newer versions of things either.

Changes to languages and APIs can be a problem to record/replay depending on exactly how they're implemented.

Undo's core tech, rr (and, arguably, GDB's built in record/replay) operate at the level of machine instructions and operating system calls, so changes to language and library behaviours don't generally affect us, outside of a few corner cases.

When you have that, you don't need to even know what the language is in order to operate - though if you want source-level debugging then it does matter as you have to be able to map from "your program counter is here" to "you're at this source line".

We occasionally need to add support for new system calls but an advantage of Linux is that the kernel ABI is very stable. New extensions to CPU instruction set also require work - these can be harder to support but they change more slowly.

Of course, operating at such a low level level isn't the only way to record/replay - there are distinct costs and benefits to operating at a higher level in the stack.