fix(mcp): treat a stdin 'error' as shutdown so the server can't orphan/spin (#799) (#805)

A stdio MCP server's lifeline is stdin: when the host/client goes away,
stdin should end and the server should exit. The server paths listened
for stdin 'end'/'close' but NOT 'error'.

That gap bites with a socket-backed stdin — the shape VS Code / Claude
Code use (a socketpair, not a pipe). On client death the socket can
surface as an 'error' (ECONNRESET/hangup) instead of a clean 'close'.
Unhandled, it escalated to the process-wide uncaughtException handler,
which logs and keeps running — so the server orphaned instead of
exiting. On Linux a POLLHUP socket fd left registered in epoll then
wakes the event loop continuously, pinning a core at 100% CPU; once the
main thread spins, the setInterval PPID watchdog can't even fire, so the
orphan runs forever (the report's 28+ minutes).

Add treatStdinFailureAsShutdown(): listen for 'error' as well as
'end'/'close', and DESTROY the stdin stream on any terminal event so the
fd leaves epoll and can't churn, then run the path's shutdown. Wired into
the live paths — startDirect, the local-handshake proxy, and
StdioTransport — plus the legacy pipe proxy. Fires once (re-entry guard).

Note: this is hardening for a class of failure that matches every piece
of the report's evidence (socket stdin, userspace main-thread spin, high
involuntary context switches, watchdog never firing), but the exact 100%
CPU spin could not be reproduced in Docker (Linux) across /dev/null EOF,
socket peer-death (RST/FIN), the reporter's 0.9.7 bundle, and the npx
chain — all exited cleanly — so the trigger is environment-specific.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/mcp/index.ts

@@ -50,6 +50,7 @@ import {
 import { connectWithHello, runLocalHandshakeProxy } from './proxy';
 import { getDaemonSocketPath } from './daemon-paths';
 import { supervisionLostReason } from './ppid-watchdog';
+import { treatStdinFailureAsShutdown } from './stdin-teardown';
 import { HOST_PPID_ENV } from '../extraction/wasm-runtime-flags';
 
 /**
@@ -330,8 +331,10 @@ export class MCPServer {
     // Detect parent-process death — same logic as pre-refactor. When stdin
     // closes we go through StdioTransport's `process.exit(0)` already, but
     // SIGKILL of the parent doesn't reliably close stdin on Linux (#277).
-    process.stdin.on('end', () => this.stop());
-    process.stdin.on('close', () => this.stop());
+    // Also treat a stdin `'error'` (a socket-backed stdin can fail with
+    // ECONNRESET/hangup instead of a clean close) as shutdown, and destroy the
+    // stream so a hung fd can't busy-spin the event loop (#799).
+    treatStdinFailureAsShutdown(() => this.stop());
 
     this.mode = 'direct';
     this.installSignalHandlers();