ADR-0010: A cache salt per measured cell

Status: accepted Date: 2026-09-20

Context

The results matrix must have cold cells that are really cold. With buck2 clean between cells the local state is gone, but the shared Buildbarn cache is not, and an earlier cell's uploads would make the next one warm. Fresh REAPI instance names were the obvious fix: in Buildbarn's local storage they do nothing, because the storage ignores the instance name.

Decision

Each measured cell gets a unique cache salt (variant-mode-timestamp), passed as br2.cache_salt and added to every action's environment. A different environment is a different action key, so the whole graph misses once; the warm rerun inside the cell reuses the same salt and hits. Individual packages can also take a salts entry in project.json to force just that package to rebuild, and it is added only when non-empty so an unused salt does not change any key.

Consequences

• Easier: cold means cold, provably: the cold run's cached count is 0.
• Easier: measurements are repeatable without wiping the cache between cells.
• Harder: cells do not share work, so a full matrix costs one full build per cold cell. For a project that builds for two hours, a full matrix is a day of compute.