Implement zombie connection detection and cleanup in ConnectionManager; enhance tests for edge cases

readme.hints.md

@@ -856,4 +856,42 @@ The WrappedSocket class has been implemented as the foundation for PROXY protoco
 For detailed information about proxy protocol implementation and proxy chaining:
 - **[Proxy Protocol Guide](./readme.proxy-protocol.md)** - Complete implementation details and configuration
 - **[Proxy Protocol Examples](./readme.proxy-protocol-example.md)** - Code examples and conceptual implementation
-- **[Proxy Chain Summary](./readme.proxy-chain-summary.md)** - Quick reference for proxy chaining setup
+- **[Proxy Chain Summary](./readme.proxy-chain-summary.md)** - Quick reference for proxy chaining setup
+
+## Connection Cleanup Edge Cases Investigation (v19.5.20+)
+
+### Issue Discovered
+"Zombie connections" can occur when both sockets are destroyed but the connection record hasn't been cleaned up. This happens when sockets are destroyed without triggering their close/error event handlers.
+
+### Root Cause
+1. **Event Handler Bypass**: In edge cases (network failures, proxy chain failures, forced socket destruction), sockets can be destroyed without their event handlers being called
+2. **Cleanup Queue Delay**: The `initiateCleanupOnce` method adds connections to a cleanup queue (batch of 100 every 100ms), which may not process fast enough
+3. **Inactivity Check Limitation**: The periodic inactivity check only examines `lastActivity` timestamps, not actual socket states
+
+### Test Results
+Debug script (`connection-manager-direct-test.ts`) revealed:
+- **Normal cleanup works**: When socket events fire normally, cleanup is reliable
+- **Zombies ARE created**: Direct socket destruction creates zombies (destroyed sockets, connectionClosed=false)
+- **Manual cleanup works**: Calling `initiateCleanupOnce` on a zombie does clean it up
+- **Inactivity check misses zombies**: The check doesn't detect connections with destroyed sockets
+
+### Potential Solutions
+1. **Periodic Zombie Detection**: Add zombie detection to the inactivity check:
+   ```typescript
+   // In performOptimizedInactivityCheck
+   if (record.incoming?.destroyed && record.outgoing?.destroyed && !record.connectionClosed) {
+     this.cleanupConnection(record, 'zombie_detected');
+   }
+   ```
+
+2. **Socket State Monitoring**: Check socket states during connection operations
+3. **Defensive Socket Handling**: Always attach cleanup handlers before any operation that might destroy sockets
+4. **Immediate Cleanup Option**: For critical paths, use `cleanupConnection` instead of `initiateCleanupOnce`
+
+### Impact
+- Memory leaks in edge cases (network failures, proxy chain issues)
+- Connection count inaccuracy
+- Potential resource exhaustion over time
+
+### Test Files
+- `.nogit/debug/connection-manager-direct-test.ts` - Direct ConnectionManager testing showing zombie creation