Bug: switching LBUG_LINUX_VARIANT keeps using the previously cached archive #36

Ladybug version: lbug crate 0.19.1 with engine v0.20.1 (LBUG_VERSION=0.20.1). build.rs on main still has the same logic, so I'd expect 0.20.4 to behave the same.

OS: Ubuntu 24.04.5 LTS, x86_64, Rust 1.83.0

What happened?

I was comparing the compat and perf Linux builds of the engine, so I set LBUG_LINUX_VARIANT=perf on a project that had already built once with compat. The build went through without any complaints, but it was still linking the compat archive. I only noticed because the linked liblbug.a hash didn't change.

It looks like the variant isn't part of the prebuilt cache key. prebuilt_cache_key() builds the key from LBUG_PRECOMPILED_RUN_ID or LBUG_VERSION and nothing else, and try_download_prebuilt_lbug() skips the download whenever liblbug.a already exists at that path. So switching the variant does re-run the build script (thanks to rerun-if-env-changed=LBUG_LINUX_VARIANT), but it finds the old archive in the same directory and uses it again.

I expected changing the variant to either download the other archive, or at least fail loudly rather than silently giving me the one I just asked not to use.

This one is easy to miss. If you A/B the two variants you end up measuring compat twice and conclude the variant doesn't matter. With LBUG_PREBUILT_CACHE_DIR pointing at a shared cache it can also spill across projects.

Steps to reproduce

1. On Linux x86_64, build any crate that depends on lbug with the default variant, e.g. LBUG_VERSION=0.20.1 cargo build --release.
2. Note the hash of the cached archive (.../lbug-prebuilt/version-0.20.1/lib/liblbug.a).
3. Rebuild with LBUG_VERSION=0.20.1 LBUG_LINUX_VARIANT=perf cargo build --release.
4. The build script re-runs, nothing is downloaded, and the archive hash is exactly the same as before.

Another quick check is counting AVX register uses in the final binary, e.g. objdump -d --no-show-raw-insn <bin> | grep -coE '%ymm[0-9]+'. For me the count came out noticeably different between the two variants when each was built from a clean cache, but it didn't change at all after just switching the env var.

Possible fix

Probably a small one: add the variant to the key in prebuilt_cache_key() (something like version-0.20.1-perf) on Linux. LBUG_LIB_KIND might need the same treatment, but I've only actually run into the variant case.

For now I'm working around it by clearing the cached lbug-prebuilt/<key>/ directory (and running cargo clean -p lbug) whenever I switch variants, then checking the archive hash against the published release.

I've opened #37 with that change. Happy to adjust it if you'd rather fix it a different way.