fix: prevent FD exhaustion from WebSocket reconnect loop#112
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CoderGamester merged 1 commit intoCoderGamester:mainfrom Feb 21, 2026
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…er#110) The Node.js MCP client used ws.close() for graceful WebSocket shutdown, but close() leaves the socket alive during the TCP close handshake. When reconnection fires immediately after, the old and new sockets overlap, accumulating file descriptors on the Unity side. Since websocket-sharp uses Mono's IOSelector (select()), FD values exceeding ~1024 crash Unity with "System.NotSupportedException: Could not register to wait for file descriptor N". Three-layer fix: 1. **Node.js — Always terminate():** Replace ws.close() with ws.terminate() in closeWebSocket(). This sends TCP RST and releases the FD immediately, preventing overlap with the next connection. Null the ws reference and strip all handlers before terminating to prevent stale event callbacks. 2. **Node.js — Cap reconnect attempts:** Change maxReconnectAttempts from -1 (unlimited) to 50. Even with terminate(), a permanently unavailable server would retry forever; 50 attempts with exponential backoff covers ~14 hours of downtime. 3. **Unity C# — Close stale sessions on connect:** In McpUnitySocketHandler.OnOpen(), enumerate existing sessions and close any that aren't the new connection. This cleans up zombie sessions that survived a Node.js process restart. Tested locally: 20+ rapid MCP tool calls over 3 minutes, FD count stayed at exactly 280 (zero growth). Unity logs confirmed stale connection cleanup: "Closed 1 stale connection(s) to accept new client".
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Summary
Fixes #110 — Mono IOSelector FD exhaustion crash caused by WebSocket reconnect loop.
The Node.js MCP client used
ws.close()for graceful shutdown, butclose()leaves the socket alive during the TCP close handshake. When reconnection fires immediately, old and new sockets overlap, accumulating file descriptors on the Unity side. Since websocket-sharp uses Mono'sIOSelector(select()), FD values exceeding ~1024 crash Unity withSystem.NotSupportedException: Could not register to wait for file descriptor N.Changes (3 layers of defense)
terminate(): Replacews.close()withws.terminate()incloseWebSocket(). Sends TCP RST, releases FD immediately, prevents overlap. Nullswsreference and strips handlers before terminating.maxReconnectAttemptsfrom-1(unlimited) →50. With exponential backoff this covers ~14 hours of downtime before giving up.McpUnitySocketHandler.OnOpen()now enumerates existing sessions and closes any that aren't the new connection, cleaning up zombies from Node.js restarts.Test Evidence
Deployed fix locally (Unity 6.3 LTS, macOS, Ultraleap project) and ran 20+ rapid MCP tool calls over 3 minutes while monitoring Unity's FD count:
FD monitor log (sampled every 25s):
Unity console confirmed stale connection cleanup working:
All 96 existing tests pass (
npm test).Related