SelectByKey and SelectExcludingByKey previously had a RLock→RUnlock→
rebuildCircle(Lock)→RLock pattern when the hash ring was empty. Under
cold-start concurrency, multiple goroutines could trigger simultaneous
rebuild attempts.
Add atomic.Bool 'rebuilt' flag with ensureRebuilt() check before any
RLock acquisition:
- Fast path: atomic load returns true → skip rebuild, proceed to RLock
- Cold start: first caller rebuilds and sets flag, subsequent callers
see the flag and skip rebuild
- Rebuild() explicitly resets the flag for explicit ring invalidation
Eliminates the RLock→Unlock→Lock→RLock transition entirely. The ring
is guaranteed ready before RLock is acquired.
xfy
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