[maleadt]

Yggdrasil rebuilds every platform of a recipe whenever the recipe changes. For the LLVM *_unified JLLs, which carry all LLVM versions in one package, bumping one LLVM version rebuilt all 968 tarballs; 824 of them came out identical (same tree hash) to the previous release. That was ~35 of the 51 agent-hours of JuliaPackaging/Yggdrasil#15144.

The plan is for Yggdrasil's CI to schedule only the platforms whose inputs changed, and to register the new JLL version with the unchanged platforms pointing at the tarballs of the earlier release. This PR adds what BinaryBuilder needs for that:

• --meta-json records the build_tarballs keyword arguments that can affect the tarballs (preferred_gcc_version, compilers, skip_audit, ...) as build_options. Without them, two recipe versions with equal sources, script and dependencies could still produce different tarballs. Only explicitly passed options are recorded, and the repr-based values are only meant to be compared under the same Julia and BinaryBuilder versions.
• rebuild_jll_package accepts reused_urls: tarballs that are not rebuilt and stay where an earlier release published them. Their Artifacts.toml entries keep that URL, and their hashes and product paths come from the build metadata sidecar (read_build_meta(...; verify=false)), so they are neither downloaded nor re-uploaded. A platform that is both rebuilt and reused, or reused twice, is an error.

Registration of Clang_unified currently downloads and re-uploads 39 GB (25 min); with reuse that drops to the ~7 GB that actually changed.

[giordano]

In https://github.com/JuliaPackaging/Yggdrasil/blob/910714b8c451f9d9058b57fe98e99dbc49848bdf/.ci/bb1_project/register_package.jl we already allow reusing binaries from previous releases with [skip build], but my understanding is that this is for a more granular approach were only part of the platforms exists already, is that correct?

[maleadt]

Right. [skip build] is all or nothing, with this JLLs like the unified LLVM ones or the CUDA Runtime typically only rebuild a single version, so they can reuse the others. See the Yggdrasil PR.