mutagen VS rr

There are two testing techniques you didn't mention: Snapshot tests (which are greatly simplified using the insta crate and mutation testing (which can be done on nightly with my mutagen crate.

How does this compare to mutagen?

Do you mean as part of build.rs? Yes, that's certainly doable, and has been done in the past. You can use env!("OUT_DIR") for that. Examples you may want to refer to include my mutagen crate and criterion.

Would be great if this could include mutation testing.

I wrote a good number of macros though, both macro_rules! and various proc_macros. The latest iteration of overflower has both, for example. mutagen is a mutation testing tool built as a proc macro, and it's helper library has a bunch of macros, too. compact_arena uses macros to tie unique lifetime tags to arenas.

Also there are far more mutation testing frameworks. I maintain the rust-based mutagen one. There are also LLVM-based ones (etc. mull) that can cover multiple languages (but may yield mutations not expressible in your preferred one).

Cargo (and Rust) makes it so easy to write test cases that you should really use it to find these kinds of bugs. And there are other good test crates available: mutagen, quickcheck, etc.

I had a student completely reachitecture my mutagen tool, and saw some working on various clippy contributions.

I think this tool must share a lot techniques and use cases with rr. I wonder how it compares in various aspects.

https://rr-project.org/

rr "sells" as a "reversible debugger", but it obviously needs the determinism for its record and replay to work, and AFAIK it employs similar techniques regarding system call interception and serializing on a single CPU. The reversible debugger aspect is built on periodic snapshotting on top of it and replaying from those snapshots, AFAIK. They package it in a gdb compatible interface.

Hermit also lists record/replay as a motivation, although it doesn't list reversible debugging in general.

Interesting. Perhaps you can inspect the disassembly of the function in question when using Graal and HotSpot. It is likely related to that.

Another debugging technique we use for heisenbugs is to see if `rr` [1] can reproduce it. If it can then that's great as it allows you to go back in time to debug what may have caused the bug. But `rr` is often not great for concurrency bugs since it emulates a single-core machine. Though debugging a VM is generally a nightmare. What we desperately need is a debugger that can debug both the VM and the language running on top of it. Usually it's one or the other.

> In general I’d argue you haven’t fixed a bug unless you understand why it happened and why your fix worked, which makes this frustrating, since every indication is that the bug exists within proprietary code that is out of my reach.

Were you using Oracle GraalVM? GraalVM community edition is open source, so maybe it's worth checking if it is reproducible in that.

[1]: https://github.com/rr-debugger/rr

https://rr-project.org/ had the same problem. They use the retired conditional branch counter instead of instruction counter, and then instruction steeping until at the correct address.

That'll work great for your Distributed QSort Incorporated startup, where the only product is a sorting algorithm.

Formal software verification is very useful. But what can be usefully formalized is rather limited, and what can be formalized correctly in practice is even more limited. That means you need to restrict your scope to something sane and useful. As a result, in the real world running thousands of tests is practically useful. (Well, it depends on what those tests are; it's easy to write 1000s of tests that either test the same thing, or only test the things that will pass and not the things that would fail.) They are especially useful if running in a mode where the unexpected happens often, as it sounds like this system can do. (It's reminiscent of rr's chaos mode -- https://rr-project.org/ linking to https://robert.ocallahan.org/2016/02/introducing-rr-chaos-mo... )

The approach of check-pointing computation such that it is resumable and restartable sounds similar to a time-traveling debugger, like rr or WinDbg:

https://rr-project.org/

https://learn.microsoft.com/windows-hardware/drivers/debugge...

https://github.com/rr-debugger/rr?tab=readme-ov-file#system-...