init_background_expiry resolved its worker count through three env vars, none documented, none set in any known deployment: RUSTFS_MAX_EXPIRY_WORKERS, silently overridden by the underscore-prefixed _RUSTFS_ILM_EXPIRATION_WORKERS (a MinIO fossil, comment included), with a zero value then falling through to RUSTFS_DEFAULT_EXPIRY_WORKERS.
RUSTFS_MAX_EXPIRY_WORKERS stays as the canonical name per the rc constraint (no new env var names): the count is now resolved once — a set, parseable, non-zero value wins, anything else falls back to min(cpus, 16). The constant moves to rustfs-config's runtime constants alongside ENV_TRANSITION_WORKERS, the two dead names are gone repo-wide (rg-verified), and a serial four-state unit test (unset/zero/valid/garbage) pins the resolution, modeled on the transition-worker env harness.
Ref rustfs/backlog#1832 (PR2).
Zhengchao An
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