feat(fast_io): BGID recycling observability (#2296)#4362
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Free-list-first allocation already exists in BgidAllocator::allocate (see docs/audits/bgid-lifecycle.md sections 2 and 7); this change adds two atomic counters so callers can verify the recycling policy is in effect and operators can monitor steady-state churn. - BGID_REUSED_COUNT: incremented when allocate pops from the free-list. - BGID_FRESH_COUNT: incremented when allocate mints a new id from NEXT_BGID. Exhaustion (BgidExhausted) does not advance either counter. - Public accessors BgidAllocator::bgid_reused_count and BgidAllocator::bgid_freshly_allocated_count expose the snapshot values via the existing fast_io re-export. - Stub parity in io_uring_stub/buffer_ring.rs returns 0 for both so cross-platform callers compile without cfg-gating. Tests cover the three policy assertions called out in the BGE-4 spec: ten alloc/free/alloc cycles count at least ten reuses; 100 fresh allocations leave reused at 0; failed exhausted allocations leave both counters untouched.
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Summary
BGID_REUSED_COUNTandBGID_FRESH_COUNTatomic counters that record whether eachBgidAllocator::allocatecall was satisfied from the free-list or by minting fromNEXT_BGID.BgidAllocator::bgid_reused_count()andBgidAllocator::bgid_freshly_allocated_count()accessors via the existingfast_iore-export so benchmarks and operators can read the recycle ratio without touching the free-list mutex.io_uring_stub/buffer_ring.rsso cross-platform callers compile withoutcfg-gating.The free-list-first allocation path itself was already in place (see
docs/audits/bgid-lifecycle.mdsections 2 and 7, lines 197-203 inbuffer_ring.rs); this change adds the observability hooks that BGE-4 calls for so the existing recycling policy is bench-verifiable.Test plan
BgidAllocatorallocates 10, deallocates all, allocates 10 again:bgid_reused_count() >= 10,bgid_freshly_allocated_count() == 10.BgidAllocatorallocates 100 fresh with no deallocs:bgid_reused_count() == 0,bgid_freshly_allocated_count() == 100.cargo fmt --all.