siyuan-note/siyuan · error

stdout pipe

Error message

stdout pipe: %w

What it means

connectStdio calls cmd.StdoutPipe() to obtain the read end of the child's stdout, which the MCP IOTransport reads JSON-RPC messages from. A failure creating the underlying pipe is wrapped with the "stdout pipe:" prefix. Like the stdin-pipe error, this indicates OS-level resource exhaustion rather than a configuration problem.

Solutions

1. Raise the process file-descriptor limit (ulimit -n, LimitNOFILE) and retry the connection
2. Inspect for descriptor leaks: list MCP child processes (ps/lsof) and terminate stale ones
3. Lower the number of concurrently configured stdio MCP servers
4. Restart the SiYuan kernel to reclaim leaked pipe descriptors

Example fix

// before: unbounded reconnect loop spawning pipes each retry
// after: reuse or close previous pipes/session before reconnecting
if prev != nil { prev.Close(); prev.Cmd.Process.Kill(); prev.Cmd.Wait() }

var r syscall.Rlimit
syscall.Getrlimit(syscall.RLIMIT_NOFILE, &r)
// ensure r.Cur comfortably exceeds 3 fds per configured stdio server

null

if err := connectStdio(ctx, client, server); err != nil {
    if strings.Contains(err.Error(), "stdout pipe") {
        // free descriptors (close stale sessions, kill children), then retry with backoff
        return retryAfterCleanup
    }
    return err
}

When it happens

Trigger: connectStdio calls cmd.StdoutPipe() and the underlying os.Pipe syscall fails, typically EMFILE/ENFILE from hitting the file-descriptor limit.

Common situations: Too many concurrent MCP child processes each holding stdin/stdout pipes, leaked descriptors from earlier crashed connections, or a low hard fd limit in containers/CI sandboxes.

Related errors

• stdin pipe
• command is required for stdio server
• connect
• environment
• start command

AI-assisted analysis of siyuan-note/siyuan@9f775e8a12 (2026-09-19). Data as JSON: /api/errors/7dd7ddd1434ffd89. Report an issue: GitHub.

Appendix: source

Thrown at kernel/mcp/client/mcp.go:472

	// stdio 环境变量插值不受密钥 AllowedHosts 约束：目标是本地子进程而非网络主机，管理员在 Env 中
	// 引用 {{secrets.NAME}} 本身就是对该服务器的显式授权，与直接写入明文属于同一信任级别。
	cmdEnv, err := buildStdioEnvironment(server, os.LookupEnv, func(value string) string {
		if model.Conf == nil {
			return value
		}
		return conf.ResolveSecretsVars(model.Conf.Secrets, model.Conf.Variables, value)
	}, runtime.GOOS)
	if err != nil {
		return nil, nil, fmt.Errorf("environment: %w", err)
	}
	cmd.Env = cmdEnv
	stdin, err := cmd.StdinPipe()
	if err != nil {
		return nil, nil, fmt.Errorf("stdin pipe: %w", err)
	}
	stdout, err := cmd.StdoutPipe()
	if err != nil {
		return nil, nil, fmt.Errorf("stdout pipe: %w", err)
	}
	cmd.Stderr = io.Discard

	if err := cmd.Start(); err != nil {
		return nil, nil, fmt.Errorf("start command: %w", err)
	}

	connectCtx, connectCancel := context.WithTimeout(ctx, serverTimeout(server))
	defer connectCancel()
	transport := &mcp.IOTransport{Reader: stdout, Writer: stdin}
	session, err := client.Connect(connectCtx, transport, nil)
	if err != nil {
		cmd.Process.Kill()
		cmd.Wait()
		return nil, cmd, fmt.Errorf("connect: %w", err)
	}

	return session, cmd, nil

View on GitHub (pinned to 9f775e8a12)