Bound the ring walk of ParentConsistentHash::selectParent

[masaori335]

When every parent in a consistent_hash pool is down, selectParent walked the whole hash ring taking the global host_status_rwlock on every hop. The ring holds 1024 replica nodes per parent (num_parents * 1024 nodes) and the chash_lookup() gate withholds wrap_around until the ring is traversed twice, so one all-down selection cost ~2 * num_parents * 1024 HostStatus::getHostStatus() calls (~49k for 24 parents) -- inline ET_NET CPU that starved the loopback health probe and drove the VIP flap in inc-p1s2-260703.

Track the distinct parents examined on each ring: skip the locked getHostStatus read for a parent already seen, and force wrap_around once every distinct parent has been rejected. The expensive locked read is now paid at most once per parent (O(num_parents)); the ring still advances ~O(N*logN) cheap, lock-free hops to reach every distinct parent. Selection order and the retry-window logic are unchanged.

The seen-parent tracking is sized to num_parents (std::vector), not MAX_PARENTS: the parent.config parser does not cap num_parents at MAX_PARENTS, so a fixed [MAX_PARENTS] array would overflow the stack for pools larger than 64.

Add consistent_hash_ring_walk.test.py: an all-down 100-parent pool (marked down via HostStatus, >MAX_PARENTS on purpose) must report "getHostStatus calls: 100", proving the walk reads each parent once instead of walking the full ring.

[Copilot]

Pull request overview

This PR optimizes ParentConsistentHash::selectParent() for the “all parents down” case by bounding the consistent-hash ring walk so each distinct parent’s HostStatus is read at most once per selection, avoiding excessive contention on the global host-status lock in ET_NET threads.

Changes:

• Add per-selection tracking of “seen” parents to avoid repeated HostStatus::getHostStatus() calls while walking replica nodes on the hash ring.
• Force wrap-around once all distinct parents in the active ring have been rejected, preventing multi-pass full-ring scans.
• Add a gold test that constructs a >64-parent all-down consistent-hash pool and asserts bounded getHostStatus call counts.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/proxy/ParentConsistentHash.cc	Adds seen-parent tracking and bounded ring-walk logic; emits per-selection getHostStatus call count under parent_select debug.
tests/gold_tests/parent_proxy/consistent_hash_ring_walk.test.py	New regression test for all-down large pools; validates bounded getHostStatus calls and 502 behavior.

[masaori335]

[approve ci autest 2of4]