This commit addresses 3 critical P0 race conditions and resource leaks in core infrastructure:
**P0-1: Discovery Service Goroutine Leak** (service.go:468, 488)
- **Problem**: ForceRefresh() and SetSubnet() spawned unbounded goroutines without checking if scan already in progress
- **Impact**: Rapid API calls create goroutine explosion, resource exhaustion
- **Fix**:
- ForceRefresh: Check isScanning before spawning goroutine (lines 470-476)
- SetSubnet: Check isScanning, defer scan if already running (lines 491-504)
- Both now log when skipping to aid debugging
**P0-2: Config Persistence Unlock/Relock Race** (persistence.go:1177-1206)
- **Problem**: LoadNodesConfig() unlocked RLock, called SaveNodesConfig (acquires Lock), then relocked
- **Impact**: Another goroutine could modify config between unlock/relock, causing migrated data loss
- **Fix**:
- Copy instance slices while holding RLock to ensure consistency (lines 1189-1194)
- Release lock, save copies, then return without relocking (lines 1196-1205)
- Prevents TOCTOU vulnerability where migrations could be overwritten
**P0-3: Config Watcher Channel Close Race** (watcher.go:19-178)
- **Problem**: Stop() used select-check-close pattern vulnerable to concurrent calls
- **Impact**: Multiple Stop() calls panic on double-close
- **Fix**:
- Added sync.Once field stopOnce to ConfigWatcher struct (line 26)
- Changed Stop() to use stopOnce.Do() ensuring single execution (lines 175-178)
- Removed racy select-based guard
All fixes maintain backwards compatibility and add defensive logging for operational visibility.
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