fix(cubejs): guard express error handler against already-sent responses

[acmeguy]

Summary

• Short-circuit the fallback error middleware with res.headersSent → next(err) in services/cubejs/index.js.
• Prevents the handler from calling res.status().send() on a response whose headers have already been flushed.

Context

Observed in production logs on synmetrix-cubejs:

Error: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader ...
    at ServerResponse.send ...
    at file:///app/index.js:112:33
    at handleErrorMiddleware (/app/node_modules/@cubejs-backend/api-gateway/src/gateway.ts:2287:5)

The upstream cube api-gateway error handler already sends its own response, then forwards via next(err). The trailing app.use((err, req, res, next) => ...) in index.js then tries to write again and throws. This masks the original error (the surfaced message was a hasura auth rejection, not a header issue) and adds log noise.

Not the cause of any crash — cube's orchestrator handles the re-throw — but it's obscuring real errors.

Test plan

• Trigger a 401 against /cubejs-api/v1/load with no auth, confirm response is still 403/401 (handled upstream) and no Cannot set headers line appears in logs.
• Trigger a generic error from a custom route in src/routes/, confirm it still reaches this fallback handler and returns 500 with err.message.

🤖 Generated with Claude Code