Phase 5 Slice 4 (backlog#939): move the S3 region — a write-once identity
scalar — out of the GLOBAL_REGION process static into the per-instance
InstanceContext, so two instances can serve different regions.
- InstanceContext gains `region: OnceLock<Region>` with `set_region()` /
`region()`. `set_region` keeps the write-once fail-fast contract: a second
write panics, exactly as the process global did (not downgraded to a warn).
- global.rs `set_global_region` / `get_global_region` keep their signatures and
forward to the current instance's context; the GLOBAL_REGION static is
removed. Single-instance: startup writes the bootstrap context (which the
ECStore adopts), so reads are unchanged.
Tests: set/get round-trip, two contexts hold distinct regions, and a second
set_region panics (fail-fast preserved).
Verification: cargo test -p rustfs-ecstore (9 instance-context tests green),
cargo clippy -p rustfs-ecstore --all-targets (clean), make pre-commit (pass).
Refs: backlog#939 (Phase 5, Slice 4). Stacked on Slice 3 (#4417).
Zhengchao An
c138265440